CA1111775A - Loudspeaker system with improved lc crossover network - Google Patents
Loudspeaker system with improved lc crossover networkInfo
- Publication number
- CA1111775A CA1111775A CA373,128A CA373128A CA1111775A CA 1111775 A CA1111775 A CA 1111775A CA 373128 A CA373128 A CA 373128A CA 1111775 A CA1111775 A CA 1111775A
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- loudspeaker
- frequency
- crossover
- loudspeakers
- loudspeaker system
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Abstract
ABSTRACT OF THE DISCLOSURE
A loudspeaker system including a low frequency loudspeaker as well as midrange and high frequency loud-speakers and an LC crossover network, is disclosed. The LC crossover network includes an auto-transformer which not only serves as a component to determine a crossover freqeuncy but which also boosts the electrical signal that is input to the electroacoustic transducer of a less efficient loudspeaker. The autotransformer increases the output of the less efficient loudspeaker and accommodates its use with more efficient loud-speakers so that the overall loudspeaker system operates at optimum efficiency. An illustrative embodiment of the loud-speaker system affords 108 dB SPL output at one meter with one watt input which corresponds to about 20% overall efficiency.
The smoothness of amplitude response over the range of audible frequencies that is necessary for high fidelity sound repro-duction is improved by inclusion of peaking circuits in the LC crossover network of the loudspeaker system to enhance amplitude response in the regions of crossover frequencies.
A loudspeaker system including a low frequency loudspeaker as well as midrange and high frequency loud-speakers and an LC crossover network, is disclosed. The LC crossover network includes an auto-transformer which not only serves as a component to determine a crossover freqeuncy but which also boosts the electrical signal that is input to the electroacoustic transducer of a less efficient loudspeaker. The autotransformer increases the output of the less efficient loudspeaker and accommodates its use with more efficient loud-speakers so that the overall loudspeaker system operates at optimum efficiency. An illustrative embodiment of the loud-speaker system affords 108 dB SPL output at one meter with one watt input which corresponds to about 20% overall efficiency.
The smoothness of amplitude response over the range of audible frequencies that is necessary for high fidelity sound repro-duction is improved by inclusion of peaking circuits in the LC crossover network of the loudspeaker system to enhance amplitude response in the regions of crossover frequencies.
Description
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~hi~s is a division oE Can;ldian ~pplica-~ion Serial No. 30~,59~3 Eiled June 1, 1~7~ and assiyned ~o tlle assignee of tlle prese~t application.
The invention o~ the above-reEerenced parent application relates to an electroacoustical device and, more particularly, to a loudspeaker for reproduction oE low audible Erequencies~ Specifically, this invention relates -to a small dimension low Erequency Eolded exponential horn loudspeaker which has a unitary sound path for direction of acoustical waves from an electroacoustic transducer to a volume into which the acoustical waves are radiated. On the other hand, the invention oE the present application relates to a loudspeaker system, including a lo~
frequency loudspeaker and midrange and high frequency loudspeakers and an LC crossover network, which operates at optimum efficiency and which has a smooth amplitude response over the range of audible frequencies that is necessary for high fidelity sound reproduction.
BACKGROUND OF THE INVENTION
High fidelity sound reproduction requires reproduction of low audible frequencies. W~ B. Snow, "Audible Frequency Ranges of Music, Speech, and Noise", Jour. Acous. Soc. ~m., Vol. 3, July, 1931, p. 155, for example, indicates that high fidelity sound reproduction of orchestral music requires that the frequency band should extend to as low as ~0 Hz~
- It is well established that loudspeakers, in order to reproduce a giveD ~requency range, must have dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn loudspeaker, for example, the area of the exponential horn mouth is de-termlned on the basis of the wavelength of the lowest frequency to be reproduced.
`~ At an early date, to obtain high fidelity sound reproduction ~ with exponential horn loudspeakers, and, in particular, the inclusion of .::
low audible frequencies~ large exponential horn loudspeakers were constructed.
~l0~ For example, theater loudspeakers : :
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~s l~r(~ or ~ (3~r t~ rl c~i(]~ (?n(JL~ our ~ t l~y ~our Ecet ;n trallsverse dimensiolls werc built in orcler to o}~t~in reproductioll of low au~lible ~îectuencies. I.ater, -the outsidc dimellsiolls o~ ~-he exponentiaL horns ~lere reduced by fo]din{3, but eVell -tllen th~ dimellsiolls of khe mouths were large for re-production of Jo~ audihle frequencies. ~fore recently, fol~ed exponential horn loudspeakers wi~h reduced mou-th dimensions have been used in proximity to boundary surfaces, such as a Eloor, a ceiling, and!or ~alls of a room, to increase the effecti~e mouth area so -that low audible frequencies are reproduced while at the same time the dimensions of the low frequency loudspeakers are minimized. See, fox example, Sandeman, U. S. Patent No.
lt984,550, Klipsch, U. S. Patent Nos. 2,310,243 and 2,373,692, and Klipsch, "La Scala", Audio k'ngineeriny Society Preprint ~o.
372, April, 1965.
Prior art low frequency folded exponential horn loud-speakers, such as those which are disclosed in the above-cited references, have small dimensions ~nd, when their mouths are located proximate planar surfaces, enable reproduction of low audlble frequencies. However, each of these prior art low frequency folded expoJlential horn loudspeakers is structurally complex due to the $tructure of the folded exponential horn wllich defines the sound path from the electroacoustic transducer to- the volume into which sound is radiated. Perhaps the simplest construction appears in the above-cited Audio Engineering Society .
publicat`ion. In that construction~ the folded exponential horn is bifurcated to define a double sound path.
Due to the complex structure, the production of high .
fidelity, small dimension, low frequency ~olded exponential ho2-~30 loudspeakers has required considerable craftsmanship. ~ligh ' .~ .
quali.ty control in manufacture has been necessary to assure that the cons-truction meets specificatlons. Consequenlly, the cost of low frequency Eolded exponential horn loudspeakers has been high. Furthermore, the amount of material which has been used in some structurally complex prior ar-t low frequency folded exponential horn loudspeakers has meant that -these loudspeakers do not have the high degree of portability which is required by traveling musicians.
One objective o~ this invention is to provide a loudspeaker system, including a low frequency folded exponenti.al horn loudspeaker and midrange and high f:requency loudspeakers and an LC crossover network, for reproduction of audible frequencies of the acoustical spectrum without harmonic distortion.
A~otle/ objective ~t this invention is to pro-ide a ~ ' sd/P ~
77~i loudspeaker syst~m whi.ch operates at optirnum efficiency~
Anotller objective of this invention :is to provicle a loudspeaker system which has a smooth ampli.tude response over the ran~e of audible frequencies -that is necessary for high fidelity sound reproduction.
A :Eurther objec-tive of -this invention is to enhance -the overall smoo-thness of the ampli-tude response of a loud-speaker system over a range of audible frecluencies.
An additional objective of the prese.nt invention is to provide a loudspeaker system which can be used for radiation of sound into either a ~ solid angle or a ~/2 solicl angle without deterioration of smoothness of amplitude response.
SUMMARY OF THE INVENTION
The invention of the above-referenced paren-t application provides a simplified structure for a high fidelity, small dimension, low frequency folded exponential ~ horn loudspeakerO The low frequency loudspeaker has a folded ; exponential horn which provides a unitary curved sound path from an electroacoustic transducer at the tilroat of the .~
horn to a volume into which sound is radiated a-t the mouth of ~ ::
the horn. The length of ~he horn is such that, at an exponen-tial rate of expansion between the throat and the mouth, the mouth, when it is bounded by at least one planar surface t such as a : floor, a ceiling~ and/or walls oE a room, has adequate area to enable reproduction of low audible frequencies. An .
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illustrative ~mbodiment of the low frequency loudspeaker of ::
the 1nvention of the parent application has a low end cut-o~E frequency below 70 Hz. Advantageously, the low frequency loudspeaker has.hbgh output capacity and efficiency. The ,:
simplified structure of the folded exponential horn facllitates constructions and reduces the weight as well as lowers the cost of production.
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. : -4-Broa~.ly speaking, the invention of the above-referenced parent application provides in a louclspealcer for opera-~ion il~ a low audible fre~uency ran~e, w~le.re;.n -the ~.ouc~sp~aker lncludes an electroacous,tic trarlsducer, wh~:ch is immersed .in a back a.ir ch~mber and which radiates sound ~aves throu~h an exponential horn having a -throat and a mouth inko a volume of air, the improvement in sai,d horn comprising: structure de~ining a regi:on for acoustically couplin~ the electroacous-tic transducer at the throat to a volume of air at the mouth, the structure including: (.al a first element having an inner surface bordering the region and having an aperture forming the throat; (,b) a second element having an inner sur~ace ~ordering the region and connected to the first element such. tha~ the f;rst element inner surfac~ and the second element inner surface form an angle grea-ter than ~' , . 180 ; (:cl a third element h.aving an inner surface border iny the region and connected to the first element near the ~ :~
-throat; (d) a fourth element having an inner surface bordering the reglon and connected to ~he third element such that the th.ird element inner sur~ace and the fourth element ;nner surface form an angle less than 180; the .first and second elements being oriented with.respec-t to the th;rd ~nd fourth.' elements such that the distance ; there~etween increases at an exponential rate from the throat; and side wall means having an inner surface and ~.
~connected to the. elements for enclosin~ the region from ~.
:~ the throat to the mouth; the region beiny curved to min.i.-' mi~e the siæe of the loudspeaker and to provide a len~th such that, at an exponen~ial rate of e*pansion between the throat and the mouth, the mouth, when located proxi~
~: ~ mate at least one boundary surface r has ade~uate area ~or high fidelity sound reproduction to below a preselected low end cut-off frequency~
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The folded exponential horn of the low frequency loudspeaker of -the invention of -the above-reFerenced parellt applicatioll inc~udes a panel with an aper-ture proximate the disphra~m of a low frequency elec-troacoustic transducer. The panel is connected to a vertical back wall by an upper support baffle, such that the panel is spaced apark from and acutely ang:Led with respect to the vertical back wall. The diaphragm of the low frequency transducer is acoustically coupled ~o the space between the panel and the back wall by the aperture.
A horizon-tal lower wall is connected by a lower support baffle to the back wall and extends to the plane of a vertical opening. A front support baffle, which is spaced above the lower wall and acutely angled upwardly, is connected to the panel and extends to the plane of the opening.
Two vertical side walls are oppositely disposed in spaced planes which are perpendicular to the line which is formed by the intersection of the planes of the back and lower walls. The two side walls abut opposite edges of the panel, the upper, lower, and front support baffles, and the back and lower walls.
The panel, baffles, and walls define a unitary curved sound path from the aperture~ or throat, to -the opening, or mouth. These members may have exponen-tially curved surfaces but preferably are flat surface approximations to e~ponentially curved surfaces to facilitate construction.
The present invention, on the other hand, may be used in a loudspeaker system which includes a low frequency folded ~ exponential horn loudspeaker such as that defined in the above-; referenced parent application and additionally includes midrange `-30 and high frequency straight exponential horn loudspeakers and an LC crossover network. An autotransformer in the ~C cross-~over network boosts the electrical signal that is input tothe electroacoustic transducer of a less efficient loudspeaker.
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In an illustra-tive embodimen-t of the loudspeaker system of the present invention, Eor example, the high frequency loud-speaker is less efficient than the low Erequency and midrange frequency loudspeakers, which operate with comparab]e efEiciency.
Since the electrical signal to the electroacous-tic transducer of the less efEicient loudspeaker is boosted, the output of the less efficient loudspeaker is increased. This accommodates use of the less efEicent loudspeaker with the more efficient loudspeakers and enables the overall loudspeaker system to operate with optimum efficiency.
The loudspeaker system of the present invention has a smooth frequency response characteristic over the range of audible frequencies that is necessary for high fidelity sound xeproduction. The smoothness of frequency response is enhanced by inclusion in -the I.C crossover network of "peaking" circuits which are effective to increase the amplitude of the response at frequencies near lower and upper crossover frequencies.
; Si~e wings may be included to eliminate cavities a-t the sides `:
of the loudspeaker system of the present invention to prevent deterioration of smoothness of amplitude response.
Thus the present invention may be seen as providing in a loudspeaker system having low, mldrange, and high frequency loudspeakers and an LC crossover network, wherein at least one . of the loudspeakers is less efficient than other of the loud-.:
speakers, the improvement in the LC crossover network, compris-in~: an auto~ransformer included in the LC crossover network , and~connected between an amplifier and the at least one less efficient loudspeaker for boosting the audio frequency input to the at least one less efficient loudspeaker, whereby the .-. : ~ ~
; ~ output of the at least one less efficient loudspeaker is increased to enable use of the at least one less efficient loudspeaker with the other loudspeakers and whereby the overall loudspeaker system operates at optimum efficiency.
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The presen-t inventioll also provides in a loudspeaker system havlng at least n loudspeakers, wherein n is greater than one, and an LC crossover ne-twork dividing the audio fre~uency output of an ampli:Eier into n fre~uency bands, each frequency band driving a separa-te one of the loudspeakers, -thereby being n~l crossover f.requencies, the improvement in the LC c.rossover network, comprising: at least one peaking circui-t means included in the LC crossover network and resonant near a-t least one crossover frequency for enhancing the amplitude .response of the loudspeaker system, whereby the loudspeaker system has a smooth amplitude response characteristic in the region of ~ -the at least one crossover frequency.
The ob~ectives of the present invention will become better understood and the advantages of the present invention will become clear to those of skill in the art by a consideration of the de-tailed description of a low frequency loudspeaker and : a loudspeaker system in accordance with illus-trative embodiments of the present invention in connection with the drawing in which:
Fig. 1 is a perspective view of a loudspeaker system in accordance with the present invention; -~ Fig. 2 is a sectional view along line 2-2 of Fig. l;
- Fig. 3 is a front view of the loudspeaker system of ~'"
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sd/f~- -8-77~i ,iFig. l;
~ig. 4 is a scc~ional view aloncJ line 4-4 of Fic3. l;
.. Fig. 5 is a schematic diagram of an LC crossover network with values shown for implementat:ion of an illustrative embodiment of a loudspeaker system in accordance with the present invention; and Fig. 6 is a performance curve, which shows the ampli-; tude response at various frequencies, for an illustrative em-bodiment of a loudspeaker system in accordance with the present " invention.
With reference to Fig. 2, a low frequency loudspeaker, . which is designated generally by the numeral 12, includes a low frequency electroacoustic transducer l3 which has an elec-tro-magnet 14 that is responsive to an electrical signal to vibrate a diaphragm 15. The electroacoustic transducer 13 vibrates . air such that the electrical signal is converted to acoustical, .~ or sound, waves. The magnitude of vibration of the diaphragm : ` 15 by the electromagnet 14 at a particular frequency is propor-tional to the amplitude of the component at that frequency in the 20 . electrical signal. The electroacoustic transducer 13 is conven-. tional in design and may be, for example, a l5-inch cone-type 1l .
diaphragm electroacoustic transducer.
: . The electroacoustic transducer 13 is secured, for ~: 1 example, by means of screws (not shown) to a panel 16 which has : , an aperture that forms the throat T of a folded expon~n-tial horn 17. In accordance with the present invention, the folded , exponential horn 17 defines a unitary curved sound path, which ~ . lS indicated by the dashed line 3, to interconnect the throat T
; ~ of the exponential horn 17 to the mouth M of the exponential horn 170 The mouth M provides an openinc3 into a volume o~ air such _ g _ as a room, audltorium, theater, etc.
When an electrical signal is input to electromagnet 14, therefore, diaphragm 15 vibrates, and ~coustical, or soun~, waves propagate through -the throat T, along the unitary sound path 3, and through the mouth M into the volume of air. Hence, a listener who is positioned within -the volume of air hears the sound.
With reference to ~igs. 2 and 3, the structure of the i exponential horn 17 will now be described. As best shown in Fig. 2, the exponential horn 17 of the low frequency loudspeaker I of the present invention includes a panel 16 with an aperture T, ~l which comprlses the throat of the exponential horn 17 and which is proximate the diaphragm lS of the electroacoustic transducer 13. The panel 16 is connected to a vertical back wall 8 by an upper support baffle 18, such that the panel 16 is spaced apart -from and acutely angled with respect to the vertical back wall 8. The diaphragm 15 of the electroacoustic transducer 13 is acoustically coùpled to the space be-tween the panel 16 and the ;`
back wall 8 by the throat T.
A horizontal lower wall 9 is connected by a lower support baffle 19 to the back wall 8 and extends to the plane of `
a vertical opening M which comprises the mouth of the exponential ; horn 17. A front support baffle 5, which is spaced above the ; lower wall 9 and acutely angled upwardly, is connected to the panel 16 and extends to the plane of the mouth M.
,, .
j I As best shown in Fig. 3, two vertical side walls 21 and 22 are oppositely disposed in spaced planes which are per-pendicular to the line formed by the intersection of the planes ~ j~of the back wall 3 and the lower wall 9. The two side walls 21 and 22 abut opposite edges of the panel 16, the uppe:r, lower, .
and front support baffles 1~', 19, 5,and the back and lower walls 8, 9.
Where the low frequency loudspeaker 12 is intended j to be portable, the lower wall 9 not only serves as a sound baffle but also adds strength to the overall structure. However, if the ~low frequency loudspeaker 12 is to sit on a smoo-th floor or to be mounted to a smooth ceiling of sound reflective material, the floor or ceiling may constit~te the lower wall 9.
The structure of the exponential horn 17 preferably has , elements which have flat surfaces that approximate exponentially curved surfaces rather than surfaces which are curved in accord-ance with the exponential function. It has been found tilat the use of flat elements in a low frequency loudspeaker of the ex~
ponential horn type does not greatly detract from high fidelity reproduction of low audible frequencies. The use of flat elements, rather than exponentially curved elements, is demon-strated by Olson and Massa, "A Compound Horn I,oudspeakeri', Jour.
Acous. Soc. Am., July, 1936, pp.48-52, wherein Fig. 6 shows horns with true exponentially curved surfaces and horns with flat surfaces that approximate exponentially curved surfaces.
These authors state that comparison demonstrates very little difference for operation at frequencies below 300 H~. The use of flat surfaces instead of exponentially curved surfaces in the , illustrative embodiment of the present invention facilitates constructio~ of the exponential horn 17~ The present invention, however, contemplates the use of elements with exponentially curved surfaces as well as flat surfaces~
With reference to Figs. 2 and 3, panel 16, top wall 6, ~; front wall 7, ~ront support baffle S, side wall 21, and, finally,side~wall 2? form a back air chamber 20 for the electroacoustic ~ ' : ~ '' '. '' ~ .:
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transducer l~n Thc back ail chamber 20 has two purposcs~ (l) to , ,neutralize the inductive reactance of the throat impedance of the exponential horn 17 in the low frequency pass band and (2) to act as an element of a high pass filter which i5 effective in the lower cut-off region to increase the reactive load Oll the diaphragm 15 to limit unwanted vibration which would otherwise ' cause modulation distortion. The back air chamber 20 must be substantially airtight. Otherwise, the back air charnber 20 will appear as a combination of acoustic resistance and inductive reactance instead of pure acoustic capacitive reactance as desired when the electroacoustic transducer 13 is operative in the low frequency pass band.
In summary, the low frequency loudspeaker 12 of the present invention includes an electroacoustic transducer 13 with , a substantially airtight back air chamber 20. The low frequency ; loudspeaker 12 further includes an exponential horn 17 which is preferably constructed with flat elements that approximate ex-ponentially curved surfaces to facilitate construction. The low frequency loudspeaker 12 has relati~ely small dimensions since the exponential horn 17 is folded as sho~n by the curved sound . path 3 which extends from the electroacoustic transducer 13 at the throat ~ to the volume into which sound is radiated at the mouth M. The low frequency loudspeaker 12 has a simplified structure since the folded exponential horn defines a unitary i ~ curved sound path 3.
As an examyle of a low frequency loudspeaker in accordance with the present inventlon, a specific construction , , .
, will now be described for radiation of sound into a ~/~ solid ~ i angle. The specific construction relates a low frequency loud- ~`
;30 speaker of the type which has been described above that is _ . .... . _ _ . _ _ _ _ _ . . ... . .. . . .
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intended for the traveling musician who needs portable sound reinforcement for the low vocal r~nge ~nd the upper bass range of some musi.cal instruments. The lowest significant fundamental of the male voice, for example, is about 100 HZ., and typical accompanying in-struments generally have significant output down to about 70 Hz~ Accordingly a specific construction will be given for a low frequency loudspeaker which has a low end cut-off frequency below 70 Hz.
; 10 Selection of the electroacoustic transducer 13 is based primarily upon.power re~uirements and frequency response in the desired frequency range of from below : 70 Hz. to at least 40Q Hzo The low end of the frequency range derives from the fact that typical accompanying in-struments for tra~eling musicians have significant output down to about 70 Hz. The h.igh end of th.e fre~uency range derives from the use in th.e illustrative embodiment of flat surface approximations to exponentially curved sur-faces to facilitate horn construction as. indicated previous-ly. Accordingly, a ~LIPSCH K.33E 15-inch cone-type diaphragm ~: electroacoustic transducer may be used for the electro-~: acoust.lc transducer 13.
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Once the electroacoustic transducer 13 has been selected, the characteristics of the selected electro- -.j .
acoustic transducer which are publ1shed by the manufacturer can be used to determine the area for the throat T o~ the .
~ exponential horn 17 in accordance with the article by : ~:
~ Wente and Thuras, "Auditory Perspective-Loud Speakers and :, : Uicrophones", Trans~ A.I.E.E., Jan., 1934, pp. 19-20. The throat area thus determined provide~ maximum power transfer, or eficiency, for ~he selected electroacoustic transducer. ~-~
: Por~:the:KLIPSCH X33E, the throat area must be approximately 78 square inches, or 503 : ~ ~ : : ,.
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~ ~ ~ Pg/~cl - 13 -77~i : ::
I square centimeters.
Since a response bclow 70 Hz. is desired, a low end cut-off frequency of 63.4 Hz. may be selected. With 63.4 Hz. as the selected low end cut-off frequency, the wavelength which corresponds to this frequency can be determined, since the wave-length is equal to the velocity of sound divided by the frequency.
Since the velocity of sound in air is approximately 13,550 inches a second, the wavelength which corresponcls to the selected low r end cut-off frequency of 63.4 Hz. is approximately 213 inches, or 543 centimeters.
; The diameter (equivalent circle) of the mouth of a horn must be a substantial fraction of a wavelength if the impedance at the throat of the horn is not to vary appreciably with frequency. Kellogg, "Means for Radiating Large Amounts of Low Frequency Sound", Jour. Acous. Soc. Am., July, 1931, p. 105, indicates that the mouth diameter (equivalent circle) of an exponential horn should at least be a half wavelength (and preferably more~ at the lowest frequency to be reproduced but may be safely reduced to a third wavelength if the mouth is ~0 surrounded by~a plane baffle of infinite extent as in the case where the exponential horn ends in a hole in a large baffle or ~` wall for a loudspeaker to radiate sound into a 2~r solid angle, or hemisphere.
A~lthough Kellogg expounds the general rule, Wente and ; ~ , Thuras in their above-cited article indicate that the effect of variations in impedance on the sound output which results from the use of a horn with a mouth diameter (equivalent circ~e) less than one-half wavelength can be kept down to a relatively small value if the electroacoustic transducer is properly ~30~ selected. ;Consequently, Wente and Thuras have used a mouth 14- ;
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diameter (equivalent circle) o approximately one-six-th wave-length of the low end cut-off frequency for a loudspeaker to radiate sound into a 2~r solid angle.
The mouth diarneter (equivalent circle) may be one-half as great for a low frequency loudspeaker that is construc-ted preferably for radiation into a ~/2 solid angle, that is, for a low frequency loudspeaker which is placed, for example, in the corner of a room. Consequently, the diameter of the mouth M
~equivalent circle) may be one-twelfth wavelength of the selec-ted low end cut-off frequency of 63.4 Hz. This translates to a mouth diameter ~equivalent circle) of approximately 17.8 inches, or 45 centimeters, and a mouth area of a~proximately 249 square inches, or 1,607 square centimeters.
As shown in Fig. 3, the mouth of the illustrative ; embodiment of the low frequency loudspeaker in accordance with the present invention is rectangular. An 18 inch by 14 inch -~ rectangular mouth results in a 252 square inch, or 1,626 square centimeter, mouth area and exceeds the 249 square inch, or 1,607 ~ ; square centimeter, minimum value established by the criterion of Wente and Thuras.
; The rate of expansion of an exponential horn must not .
exceed that in which the cross section of the exponential horn , increases in the ratio ~ or approximately 2.7183 in an axial length of 1/4rl times the wavelength of the lowest frequency to be reproduced. Stated differently, the cross-sectional area of an exponential horn may no more than double in approximately l/18.1 times the wavelen~th of the lowest frequency to be re-produced, as indicated by Kellogg in his above-cited article.
Consequently, because the wavelength at 63.4 Hz. is approximately ;
213 inches, the cross-sectional area of the exponential horn of ~' :: ' :
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the illustrative embodiment of the low frcquellcy loudspeaker of the present invention may no more than double approximately every 11.8 inches, or 30 centimeters. Si.nce the e~ponential horn 17 must expand from 78 square lnches at the throat to 252 square inches at the mouth, the mean length of t:he sound path 3 in Fig. 2 is established a-t approximately 20 i.nches, or 50.8 centi-meters.
Kellogg is his above-cited article indicates that folds may be made in an exponential horn, that is, the exponential horn may be bent without seriously altering the operation of the exponential horn, provided that the difference between the shortest and longest sound path is less than a half wavelength.
Given this criterion and the throat area, mean sound path length, and mouth area, the exponential horn 17 of the illustrative embodiment of the present invention which appears in Figs. 2 , and 3 can be constructed. As pointed out above, to facilitate construction, elements with flat surfaces which approximate exponent}ally curved surfaces are used. However, exponentially curved surfaces may be used and would preferably be used in a 20 - low frequency loudspeaker which operates in a range that extends significantly above 300 Hz.
Klipsch, "A Low Frequency Horn of Small Dimensions", Jour.~Acous. Soc. ~m., Vol. 13, No. 2, 1941, pp. 137-144, derives ~ .
the anal~tical expression for the volume of a back air chamber. .:
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Theoretically, the back air chamber 20 should be about 10-20%
, larger to compensate for the flexure of the suspended diaphragm ~ .
15 and for the immersed volume of the electromagnet 14 of the .!
electxoacoustic transducer 13 in Fig. 2. Since a 20%lchange in " ~.
' the volume of a back air chamber has been found to produce less i than approxi.mately one decibel of response error and since error -16- .
..__ 7~5 , toward a smaller back air c~amber results in less modulation `I distortion due to subsonic inputs, the bac]c alr chamber 20 pre-ferably has a volume of 2,730 cubic inches, or 44.74 liters, so that the volume is only 2%, rather than 10-~0%, larger than the analytical value.
Mathematically, the use of the back air chamber 20 with a volume of 2% larger than the analytical value raises the effective low end cut-off frequency between 5 and 10%, from 63.4 Hz. to between 66 and 70 Hz~ Since in the illustrative embodiment of the low frequency loudspeaker o.f the present invention a target low end cut-off frequency of approximately . ,j 70 Hz. has been selected, the increase in the low end cut-off frequency, due to the use of the back air chamber 20 with a volume only 2% larger than the analytical value, is acceptable.
The values for the various parameters for a specific construction of a low frequency loudspeaker in accordance with the illustrative embodiment of the present invention in Figs.
~hi~s is a division oE Can;ldian ~pplica-~ion Serial No. 30~,59~3 Eiled June 1, 1~7~ and assiyned ~o tlle assignee of tlle prese~t application.
The invention o~ the above-reEerenced parent application relates to an electroacoustical device and, more particularly, to a loudspeaker for reproduction oE low audible Erequencies~ Specifically, this invention relates -to a small dimension low Erequency Eolded exponential horn loudspeaker which has a unitary sound path for direction of acoustical waves from an electroacoustic transducer to a volume into which the acoustical waves are radiated. On the other hand, the invention oE the present application relates to a loudspeaker system, including a lo~
frequency loudspeaker and midrange and high frequency loudspeakers and an LC crossover network, which operates at optimum efficiency and which has a smooth amplitude response over the range of audible frequencies that is necessary for high fidelity sound reproduction.
BACKGROUND OF THE INVENTION
High fidelity sound reproduction requires reproduction of low audible frequencies. W~ B. Snow, "Audible Frequency Ranges of Music, Speech, and Noise", Jour. Acous. Soc. ~m., Vol. 3, July, 1931, p. 155, for example, indicates that high fidelity sound reproduction of orchestral music requires that the frequency band should extend to as low as ~0 Hz~
- It is well established that loudspeakers, in order to reproduce a giveD ~requency range, must have dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn loudspeaker, for example, the area of the exponential horn mouth is de-termlned on the basis of the wavelength of the lowest frequency to be reproduced.
`~ At an early date, to obtain high fidelity sound reproduction ~ with exponential horn loudspeakers, and, in particular, the inclusion of .::
low audible frequencies~ large exponential horn loudspeakers were constructed.
~l0~ For example, theater loudspeakers : :
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~s l~r(~ or ~ (3~r t~ rl c~i(]~ (?n(JL~ our ~ t l~y ~our Ecet ;n trallsverse dimensiolls werc built in orcler to o}~t~in reproductioll of low au~lible ~îectuencies. I.ater, -the outsidc dimellsiolls o~ ~-he exponentiaL horns ~lere reduced by fo]din{3, but eVell -tllen th~ dimellsiolls of khe mouths were large for re-production of Jo~ audihle frequencies. ~fore recently, fol~ed exponential horn loudspeakers wi~h reduced mou-th dimensions have been used in proximity to boundary surfaces, such as a Eloor, a ceiling, and!or ~alls of a room, to increase the effecti~e mouth area so -that low audible frequencies are reproduced while at the same time the dimensions of the low frequency loudspeakers are minimized. See, fox example, Sandeman, U. S. Patent No.
lt984,550, Klipsch, U. S. Patent Nos. 2,310,243 and 2,373,692, and Klipsch, "La Scala", Audio k'ngineeriny Society Preprint ~o.
372, April, 1965.
Prior art low frequency folded exponential horn loud-speakers, such as those which are disclosed in the above-cited references, have small dimensions ~nd, when their mouths are located proximate planar surfaces, enable reproduction of low audlble frequencies. However, each of these prior art low frequency folded expoJlential horn loudspeakers is structurally complex due to the $tructure of the folded exponential horn wllich defines the sound path from the electroacoustic transducer to- the volume into which sound is radiated. Perhaps the simplest construction appears in the above-cited Audio Engineering Society .
publicat`ion. In that construction~ the folded exponential horn is bifurcated to define a double sound path.
Due to the complex structure, the production of high .
fidelity, small dimension, low frequency ~olded exponential ho2-~30 loudspeakers has required considerable craftsmanship. ~ligh ' .~ .
quali.ty control in manufacture has been necessary to assure that the cons-truction meets specificatlons. Consequenlly, the cost of low frequency Eolded exponential horn loudspeakers has been high. Furthermore, the amount of material which has been used in some structurally complex prior ar-t low frequency folded exponential horn loudspeakers has meant that -these loudspeakers do not have the high degree of portability which is required by traveling musicians.
One objective o~ this invention is to provide a loudspeaker system, including a low frequency folded exponenti.al horn loudspeaker and midrange and high f:requency loudspeakers and an LC crossover network, for reproduction of audible frequencies of the acoustical spectrum without harmonic distortion.
A~otle/ objective ~t this invention is to pro-ide a ~ ' sd/P ~
77~i loudspeaker syst~m whi.ch operates at optirnum efficiency~
Anotller objective of this invention :is to provicle a loudspeaker system which has a smooth ampli.tude response over the ran~e of audible frequencies -that is necessary for high fidelity sound reproduction.
A :Eurther objec-tive of -this invention is to enhance -the overall smoo-thness of the ampli-tude response of a loud-speaker system over a range of audible frecluencies.
An additional objective of the prese.nt invention is to provide a loudspeaker system which can be used for radiation of sound into either a ~ solid angle or a ~/2 solicl angle without deterioration of smoothness of amplitude response.
SUMMARY OF THE INVENTION
The invention of the above-referenced paren-t application provides a simplified structure for a high fidelity, small dimension, low frequency folded exponential ~ horn loudspeakerO The low frequency loudspeaker has a folded ; exponential horn which provides a unitary curved sound path from an electroacoustic transducer at the tilroat of the .~
horn to a volume into which sound is radiated a-t the mouth of ~ ::
the horn. The length of ~he horn is such that, at an exponen-tial rate of expansion between the throat and the mouth, the mouth, when it is bounded by at least one planar surface t such as a : floor, a ceiling~ and/or walls oE a room, has adequate area to enable reproduction of low audible frequencies. An .
: , .
illustrative ~mbodiment of the low frequency loudspeaker of ::
the 1nvention of the parent application has a low end cut-o~E frequency below 70 Hz. Advantageously, the low frequency loudspeaker has.hbgh output capacity and efficiency. The ,:
simplified structure of the folded exponential horn facllitates constructions and reduces the weight as well as lowers the cost of production.
-' .
. : -4-Broa~.ly speaking, the invention of the above-referenced parent application provides in a louclspealcer for opera-~ion il~ a low audible fre~uency ran~e, w~le.re;.n -the ~.ouc~sp~aker lncludes an electroacous,tic trarlsducer, wh~:ch is immersed .in a back a.ir ch~mber and which radiates sound ~aves throu~h an exponential horn having a -throat and a mouth inko a volume of air, the improvement in sai,d horn comprising: structure de~ining a regi:on for acoustically couplin~ the electroacous-tic transducer at the throat to a volume of air at the mouth, the structure including: (.al a first element having an inner surface bordering the region and having an aperture forming the throat; (,b) a second element having an inner sur~ace ~ordering the region and connected to the first element such. tha~ the f;rst element inner surfac~ and the second element inner surface form an angle grea-ter than ~' , . 180 ; (:cl a third element h.aving an inner surface border iny the region and connected to the first element near the ~ :~
-throat; (d) a fourth element having an inner surface bordering the reglon and connected to ~he third element such that the th.ird element inner sur~ace and the fourth element ;nner surface form an angle less than 180; the .first and second elements being oriented with.respec-t to the th;rd ~nd fourth.' elements such that the distance ; there~etween increases at an exponential rate from the throat; and side wall means having an inner surface and ~.
~connected to the. elements for enclosin~ the region from ~.
:~ the throat to the mouth; the region beiny curved to min.i.-' mi~e the siæe of the loudspeaker and to provide a len~th such that, at an exponen~ial rate of e*pansion between the throat and the mouth, the mouth, when located proxi~
~: ~ mate at least one boundary surface r has ade~uate area ~or high fidelity sound reproduction to below a preselected low end cut-off frequency~
: ~ -5-7~
The folded exponential horn of the low frequency loudspeaker of -the invention of -the above-reFerenced parellt applicatioll inc~udes a panel with an aper-ture proximate the disphra~m of a low frequency elec-troacoustic transducer. The panel is connected to a vertical back wall by an upper support baffle, such that the panel is spaced apark from and acutely ang:Led with respect to the vertical back wall. The diaphragm of the low frequency transducer is acoustically coupled ~o the space between the panel and the back wall by the aperture.
A horizon-tal lower wall is connected by a lower support baffle to the back wall and extends to the plane of a vertical opening. A front support baffle, which is spaced above the lower wall and acutely angled upwardly, is connected to the panel and extends to the plane of the opening.
Two vertical side walls are oppositely disposed in spaced planes which are perpendicular to the line which is formed by the intersection of the planes of the back and lower walls. The two side walls abut opposite edges of the panel, the upper, lower, and front support baffles, and the back and lower walls.
The panel, baffles, and walls define a unitary curved sound path from the aperture~ or throat, to -the opening, or mouth. These members may have exponen-tially curved surfaces but preferably are flat surface approximations to e~ponentially curved surfaces to facilitate construction.
The present invention, on the other hand, may be used in a loudspeaker system which includes a low frequency folded ~ exponential horn loudspeaker such as that defined in the above-; referenced parent application and additionally includes midrange `-30 and high frequency straight exponential horn loudspeakers and an LC crossover network. An autotransformer in the ~C cross-~over network boosts the electrical signal that is input tothe electroacoustic transducer of a less efficient loudspeaker.
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In an illustra-tive embodimen-t of the loudspeaker system of the present invention, Eor example, the high frequency loud-speaker is less efficient than the low Erequency and midrange frequency loudspeakers, which operate with comparab]e efEiciency.
Since the electrical signal to the electroacous-tic transducer of the less efEicient loudspeaker is boosted, the output of the less efficient loudspeaker is increased. This accommodates use of the less efEicent loudspeaker with the more efficient loudspeakers and enables the overall loudspeaker system to operate with optimum efficiency.
The loudspeaker system of the present invention has a smooth frequency response characteristic over the range of audible frequencies that is necessary for high fidelity sound xeproduction. The smoothness of frequency response is enhanced by inclusion in -the I.C crossover network of "peaking" circuits which are effective to increase the amplitude of the response at frequencies near lower and upper crossover frequencies.
; Si~e wings may be included to eliminate cavities a-t the sides `:
of the loudspeaker system of the present invention to prevent deterioration of smoothness of amplitude response.
Thus the present invention may be seen as providing in a loudspeaker system having low, mldrange, and high frequency loudspeakers and an LC crossover network, wherein at least one . of the loudspeakers is less efficient than other of the loud-.:
speakers, the improvement in the LC crossover network, compris-in~: an auto~ransformer included in the LC crossover network , and~connected between an amplifier and the at least one less efficient loudspeaker for boosting the audio frequency input to the at least one less efficient loudspeaker, whereby the .-. : ~ ~
; ~ output of the at least one less efficient loudspeaker is increased to enable use of the at least one less efficient loudspeaker with the other loudspeakers and whereby the overall loudspeaker system operates at optimum efficiency.
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The presen-t inventioll also provides in a loudspeaker system havlng at least n loudspeakers, wherein n is greater than one, and an LC crossover ne-twork dividing the audio fre~uency output of an ampli:Eier into n fre~uency bands, each frequency band driving a separa-te one of the loudspeakers, -thereby being n~l crossover f.requencies, the improvement in the LC c.rossover network, comprising: at least one peaking circui-t means included in the LC crossover network and resonant near a-t least one crossover frequency for enhancing the amplitude .response of the loudspeaker system, whereby the loudspeaker system has a smooth amplitude response characteristic in the region of ~ -the at least one crossover frequency.
The ob~ectives of the present invention will become better understood and the advantages of the present invention will become clear to those of skill in the art by a consideration of the de-tailed description of a low frequency loudspeaker and : a loudspeaker system in accordance with illus-trative embodiments of the present invention in connection with the drawing in which:
Fig. 1 is a perspective view of a loudspeaker system in accordance with the present invention; -~ Fig. 2 is a sectional view along line 2-2 of Fig. l;
- Fig. 3 is a front view of the loudspeaker system of ~'"
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~ig. 4 is a scc~ional view aloncJ line 4-4 of Fic3. l;
.. Fig. 5 is a schematic diagram of an LC crossover network with values shown for implementat:ion of an illustrative embodiment of a loudspeaker system in accordance with the present invention; and Fig. 6 is a performance curve, which shows the ampli-; tude response at various frequencies, for an illustrative em-bodiment of a loudspeaker system in accordance with the present " invention.
With reference to Fig. 2, a low frequency loudspeaker, . which is designated generally by the numeral 12, includes a low frequency electroacoustic transducer l3 which has an elec-tro-magnet 14 that is responsive to an electrical signal to vibrate a diaphragm 15. The electroacoustic transducer 13 vibrates . air such that the electrical signal is converted to acoustical, .~ or sound, waves. The magnitude of vibration of the diaphragm : ` 15 by the electromagnet 14 at a particular frequency is propor-tional to the amplitude of the component at that frequency in the 20 . electrical signal. The electroacoustic transducer 13 is conven-. tional in design and may be, for example, a l5-inch cone-type 1l .
diaphragm electroacoustic transducer.
: . The electroacoustic transducer 13 is secured, for ~: 1 example, by means of screws (not shown) to a panel 16 which has : , an aperture that forms the throat T of a folded expon~n-tial horn 17. In accordance with the present invention, the folded , exponential horn 17 defines a unitary curved sound path, which ~ . lS indicated by the dashed line 3, to interconnect the throat T
; ~ of the exponential horn 17 to the mouth M of the exponential horn 170 The mouth M provides an openinc3 into a volume o~ air such _ g _ as a room, audltorium, theater, etc.
When an electrical signal is input to electromagnet 14, therefore, diaphragm 15 vibrates, and ~coustical, or soun~, waves propagate through -the throat T, along the unitary sound path 3, and through the mouth M into the volume of air. Hence, a listener who is positioned within -the volume of air hears the sound.
With reference to ~igs. 2 and 3, the structure of the i exponential horn 17 will now be described. As best shown in Fig. 2, the exponential horn 17 of the low frequency loudspeaker I of the present invention includes a panel 16 with an aperture T, ~l which comprlses the throat of the exponential horn 17 and which is proximate the diaphragm lS of the electroacoustic transducer 13. The panel 16 is connected to a vertical back wall 8 by an upper support baffle 18, such that the panel 16 is spaced apart -from and acutely angled with respect to the vertical back wall 8. The diaphragm 15 of the electroacoustic transducer 13 is acoustically coùpled to the space be-tween the panel 16 and the ;`
back wall 8 by the throat T.
A horizontal lower wall 9 is connected by a lower support baffle 19 to the back wall 8 and extends to the plane of `
a vertical opening M which comprises the mouth of the exponential ; horn 17. A front support baffle 5, which is spaced above the ; lower wall 9 and acutely angled upwardly, is connected to the panel 16 and extends to the plane of the mouth M.
,, .
j I As best shown in Fig. 3, two vertical side walls 21 and 22 are oppositely disposed in spaced planes which are per-pendicular to the line formed by the intersection of the planes ~ j~of the back wall 3 and the lower wall 9. The two side walls 21 and 22 abut opposite edges of the panel 16, the uppe:r, lower, .
and front support baffles 1~', 19, 5,and the back and lower walls 8, 9.
Where the low frequency loudspeaker 12 is intended j to be portable, the lower wall 9 not only serves as a sound baffle but also adds strength to the overall structure. However, if the ~low frequency loudspeaker 12 is to sit on a smoo-th floor or to be mounted to a smooth ceiling of sound reflective material, the floor or ceiling may constit~te the lower wall 9.
The structure of the exponential horn 17 preferably has , elements which have flat surfaces that approximate exponentially curved surfaces rather than surfaces which are curved in accord-ance with the exponential function. It has been found tilat the use of flat elements in a low frequency loudspeaker of the ex~
ponential horn type does not greatly detract from high fidelity reproduction of low audible frequencies. The use of flat elements, rather than exponentially curved elements, is demon-strated by Olson and Massa, "A Compound Horn I,oudspeakeri', Jour.
Acous. Soc. Am., July, 1936, pp.48-52, wherein Fig. 6 shows horns with true exponentially curved surfaces and horns with flat surfaces that approximate exponentially curved surfaces.
These authors state that comparison demonstrates very little difference for operation at frequencies below 300 H~. The use of flat surfaces instead of exponentially curved surfaces in the , illustrative embodiment of the present invention facilitates constructio~ of the exponential horn 17~ The present invention, however, contemplates the use of elements with exponentially curved surfaces as well as flat surfaces~
With reference to Figs. 2 and 3, panel 16, top wall 6, ~; front wall 7, ~ront support baffle S, side wall 21, and, finally,side~wall 2? form a back air chamber 20 for the electroacoustic ~ ' : ~ '' '. '' ~ .:
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transducer l~n Thc back ail chamber 20 has two purposcs~ (l) to , ,neutralize the inductive reactance of the throat impedance of the exponential horn 17 in the low frequency pass band and (2) to act as an element of a high pass filter which i5 effective in the lower cut-off region to increase the reactive load Oll the diaphragm 15 to limit unwanted vibration which would otherwise ' cause modulation distortion. The back air chamber 20 must be substantially airtight. Otherwise, the back air charnber 20 will appear as a combination of acoustic resistance and inductive reactance instead of pure acoustic capacitive reactance as desired when the electroacoustic transducer 13 is operative in the low frequency pass band.
In summary, the low frequency loudspeaker 12 of the present invention includes an electroacoustic transducer 13 with , a substantially airtight back air chamber 20. The low frequency ; loudspeaker 12 further includes an exponential horn 17 which is preferably constructed with flat elements that approximate ex-ponentially curved surfaces to facilitate construction. The low frequency loudspeaker 12 has relati~ely small dimensions since the exponential horn 17 is folded as sho~n by the curved sound . path 3 which extends from the electroacoustic transducer 13 at the throat ~ to the volume into which sound is radiated at the mouth M. The low frequency loudspeaker 12 has a simplified structure since the folded exponential horn defines a unitary i ~ curved sound path 3.
As an examyle of a low frequency loudspeaker in accordance with the present inventlon, a specific construction , , .
, will now be described for radiation of sound into a ~/~ solid ~ i angle. The specific construction relates a low frequency loud- ~`
;30 speaker of the type which has been described above that is _ . .... . _ _ . _ _ _ _ _ . . ... . .. . . .
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intended for the traveling musician who needs portable sound reinforcement for the low vocal r~nge ~nd the upper bass range of some musi.cal instruments. The lowest significant fundamental of the male voice, for example, is about 100 HZ., and typical accompanying in-struments generally have significant output down to about 70 Hz~ Accordingly a specific construction will be given for a low frequency loudspeaker which has a low end cut-off frequency below 70 Hz.
; 10 Selection of the electroacoustic transducer 13 is based primarily upon.power re~uirements and frequency response in the desired frequency range of from below : 70 Hz. to at least 40Q Hzo The low end of the frequency range derives from the fact that typical accompanying in-struments for tra~eling musicians have significant output down to about 70 Hz. The h.igh end of th.e fre~uency range derives from the use in th.e illustrative embodiment of flat surface approximations to exponentially curved sur-faces to facilitate horn construction as. indicated previous-ly. Accordingly, a ~LIPSCH K.33E 15-inch cone-type diaphragm ~: electroacoustic transducer may be used for the electro-~: acoust.lc transducer 13.
.
Once the electroacoustic transducer 13 has been selected, the characteristics of the selected electro- -.j .
acoustic transducer which are publ1shed by the manufacturer can be used to determine the area for the throat T o~ the .
~ exponential horn 17 in accordance with the article by : ~:
~ Wente and Thuras, "Auditory Perspective-Loud Speakers and :, : Uicrophones", Trans~ A.I.E.E., Jan., 1934, pp. 19-20. The throat area thus determined provide~ maximum power transfer, or eficiency, for ~he selected electroacoustic transducer. ~-~
: Por~:the:KLIPSCH X33E, the throat area must be approximately 78 square inches, or 503 : ~ ~ : : ,.
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~ ~ ~ Pg/~cl - 13 -77~i : ::
I square centimeters.
Since a response bclow 70 Hz. is desired, a low end cut-off frequency of 63.4 Hz. may be selected. With 63.4 Hz. as the selected low end cut-off frequency, the wavelength which corresponds to this frequency can be determined, since the wave-length is equal to the velocity of sound divided by the frequency.
Since the velocity of sound in air is approximately 13,550 inches a second, the wavelength which corresponcls to the selected low r end cut-off frequency of 63.4 Hz. is approximately 213 inches, or 543 centimeters.
; The diameter (equivalent circle) of the mouth of a horn must be a substantial fraction of a wavelength if the impedance at the throat of the horn is not to vary appreciably with frequency. Kellogg, "Means for Radiating Large Amounts of Low Frequency Sound", Jour. Acous. Soc. Am., July, 1931, p. 105, indicates that the mouth diameter (equivalent circle) of an exponential horn should at least be a half wavelength (and preferably more~ at the lowest frequency to be reproduced but may be safely reduced to a third wavelength if the mouth is ~0 surrounded by~a plane baffle of infinite extent as in the case where the exponential horn ends in a hole in a large baffle or ~` wall for a loudspeaker to radiate sound into a 2~r solid angle, or hemisphere.
A~lthough Kellogg expounds the general rule, Wente and ; ~ , Thuras in their above-cited article indicate that the effect of variations in impedance on the sound output which results from the use of a horn with a mouth diameter (equivalent circ~e) less than one-half wavelength can be kept down to a relatively small value if the electroacoustic transducer is properly ~30~ selected. ;Consequently, Wente and Thuras have used a mouth 14- ;
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diameter (equivalent circle) o approximately one-six-th wave-length of the low end cut-off frequency for a loudspeaker to radiate sound into a 2~r solid angle.
The mouth diarneter (equivalent circle) may be one-half as great for a low frequency loudspeaker that is construc-ted preferably for radiation into a ~/2 solid angle, that is, for a low frequency loudspeaker which is placed, for example, in the corner of a room. Consequently, the diameter of the mouth M
~equivalent circle) may be one-twelfth wavelength of the selec-ted low end cut-off frequency of 63.4 Hz. This translates to a mouth diameter ~equivalent circle) of approximately 17.8 inches, or 45 centimeters, and a mouth area of a~proximately 249 square inches, or 1,607 square centimeters.
As shown in Fig. 3, the mouth of the illustrative ; embodiment of the low frequency loudspeaker in accordance with the present invention is rectangular. An 18 inch by 14 inch -~ rectangular mouth results in a 252 square inch, or 1,626 square centimeter, mouth area and exceeds the 249 square inch, or 1,607 ~ ; square centimeter, minimum value established by the criterion of Wente and Thuras.
; The rate of expansion of an exponential horn must not .
exceed that in which the cross section of the exponential horn , increases in the ratio ~ or approximately 2.7183 in an axial length of 1/4rl times the wavelength of the lowest frequency to be reproduced. Stated differently, the cross-sectional area of an exponential horn may no more than double in approximately l/18.1 times the wavelen~th of the lowest frequency to be re-produced, as indicated by Kellogg in his above-cited article.
Consequently, because the wavelength at 63.4 Hz. is approximately ;
213 inches, the cross-sectional area of the exponential horn of ~' :: ' :
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the illustrative embodiment of the low frcquellcy loudspeaker of the present invention may no more than double approximately every 11.8 inches, or 30 centimeters. Si.nce the e~ponential horn 17 must expand from 78 square lnches at the throat to 252 square inches at the mouth, the mean length of t:he sound path 3 in Fig. 2 is established a-t approximately 20 i.nches, or 50.8 centi-meters.
Kellogg is his above-cited article indicates that folds may be made in an exponential horn, that is, the exponential horn may be bent without seriously altering the operation of the exponential horn, provided that the difference between the shortest and longest sound path is less than a half wavelength.
Given this criterion and the throat area, mean sound path length, and mouth area, the exponential horn 17 of the illustrative embodiment of the present invention which appears in Figs. 2 , and 3 can be constructed. As pointed out above, to facilitate construction, elements with flat surfaces which approximate exponent}ally curved surfaces are used. However, exponentially curved surfaces may be used and would preferably be used in a 20 - low frequency loudspeaker which operates in a range that extends significantly above 300 Hz.
Klipsch, "A Low Frequency Horn of Small Dimensions", Jour.~Acous. Soc. ~m., Vol. 13, No. 2, 1941, pp. 137-144, derives ~ .
the anal~tical expression for the volume of a back air chamber. .:
.
Theoretically, the back air chamber 20 should be about 10-20%
, larger to compensate for the flexure of the suspended diaphragm ~ .
15 and for the immersed volume of the electromagnet 14 of the .!
electxoacoustic transducer 13 in Fig. 2. Since a 20%lchange in " ~.
' the volume of a back air chamber has been found to produce less i than approxi.mately one decibel of response error and since error -16- .
..__ 7~5 , toward a smaller back air c~amber results in less modulation `I distortion due to subsonic inputs, the bac]c alr chamber 20 pre-ferably has a volume of 2,730 cubic inches, or 44.74 liters, so that the volume is only 2%, rather than 10-~0%, larger than the analytical value.
Mathematically, the use of the back air chamber 20 with a volume of 2% larger than the analytical value raises the effective low end cut-off frequency between 5 and 10%, from 63.4 Hz. to between 66 and 70 Hz~ Since in the illustrative embodiment of the low frequency loudspeaker o.f the present invention a target low end cut-off frequency of approximately . ,j 70 Hz. has been selected, the increase in the low end cut-off frequency, due to the use of the back air chamber 20 with a volume only 2% larger than the analytical value, is acceptable.
The values for the various parameters for a specific construction of a low frequency loudspeaker in accordance with the illustrative embodiment of the present invention in Figs.
2 and 3 are su~.rized in TaLle I.
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TABLE I
LOW FREQUENCY LOUDSPEAKE:R
Electroacoustic = KLIPSCH
Transducer K33E
Analytical Low End = 66-70 Hz.
Cut-off Frequency Throat Area = 78 Square Inches Mouth. Area = 252 Square Inches Rate of Expansion = Cross-sectional ..
of Horn Area Doubles Every ll.8 :
Inches . .
Mean Sound Path = 20 Inches : Length Volume of Back Air = 2,730 Cubic Chamber Inches ' ; :
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The low Erequency loudspeaker of the present invention ~may be incorporated into a loudspeaker system which further includes a midrange frequenc~ loudspeaker and a high fre~uency loudspeaker together with an LC crossover network which estab-lishes the frequency ranges for the various loudspeakers.
I' A straight axis exponential horn may be used in 'connection with an appropriate electroacoustic transducer to , form the midrange frequency loudspeaker 10 as shown in Figs.
1-4. A midranye frequency loudspeaker similar to the one which 'is described in Klipsch, "A New High Frequency Horn", I.R.E.
Trans. on Audio, Vol. AU-ll, No. 6, Nov.-Dec., 1963, pp. 20~-206, ' with a low end cut-off frequency of 375 Hz., for example, may be used.
An illustrative embodiment :Eor the midrange frequency ! loudspeaker 10 includes a KLIPSCH K55V electroacoustic trans-ducer. For maximum power transfer, or efficiency, with the KLIPSCH K55V electroacoustic transdùcer, the throat area of the midrange frequency loudspeaker exponential horn must be approxi-mately ~4 s~uare inch, or 2.6 square centimeters. Since the midrange frequency loudspeaker 10 due to its position in the iloudspeaker system of the present invention effectively radiates ;isound into a 2 ~ solid angle, the diameter (equivalent circle) of the mouth must be at least one-sixth wavelength of the 375 ,i Hz. low end cut-of~ frequency in accordance with the criterion ;j of Wente and Thuras in their above-cited article. Based on the dimensions of the specific construction for the low frequency " j .:
loudspeaker in accordance with an illustrative embodiment of the present invention, an exponential horn for the midrange fre-quency loudspeaker 10 with a mouth area of 46 square inches, 30 ~ ! or 297 square centimeters, may be conveniently used. This ' i ` ' !
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!' translates to a mouth which has a diameter ~equivalent circle) o l/9.7 wavelength of the low end cut-ofE frequency of 375 E~z.
~which exceeds the minimum value established by the cri-terion of Wente and Thuras. In accordancé with the above-ci-ted Kellogg article, the cross-sectional area of the exponential horn for the midrange Erequency loudspeaker lO must no-t double in less than 2 inches. Given the throat area, mouth area, and rate of expansion for the midrange frequency loudspeaker straight ex- ;
ponential horn, a mean sound path length of approxima-tely 15 'inches, or 97 centimeters, is established. The back air chamber for the midrange frequency loudspeaker lO requires a volume of 2.55 cubic inches, or 41~B cubic centimeters, or equivalent combined air chamber and diaphragm suspension compliance. The , data for the midrange frequency loudspeaker 10 for the illustra-tive embodiment of a loudspeaker system in accordance with the present invention are tabulated in Table II.
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,I T~BLE II
IDRANGE FREQ~ENCY LOUDSPEAKER
I! , Electroacoustic = KLIPSCH
jj Transducer K55V
'i Analytical Low End = 375 Hz. I :
. Cut-off Frequency Throat Area = 0.4 Square ~ -~
, Inch ' Mouth Area = 46 Square I Inches .
l Rate of Expansion = Cross-Sectional i! f Horn Area Doubles ~ j Every 2 : ~ Inches - -:
: j Mean Sound Path = 15 Inches Length Volume of Back = 2.55 Cubic ¦ ~ -Air Chamber Inches or equi- I :
. Il valent combined air chamber and diaphragm sus~
pension com~
: pliance , 7~
A KLIPSCH K-77 may be used or the high frequency loudspeaker 11 of the illustrative embodimen-t of the loudspeaker Isystem of the present invention. Although tlle eEficierlcy o~
~this high frequency loudspeaker is lower than either the low or midrange frequency loudspeakers, the power demand ln the high audible frequency range, that is, in the range of 6,000-15,000 Hz.,l is small. Consequently, an autotransformer may be incorporated I :
linto an LC crossover network as described below so as to permi-t - use of the KLIPSCH K-77.
) The figures indicate that the midrange frequency loud- i 'speaker 10 and the high frequency loudspeaker 11 are mounted in iclose proximity to the low frequency loudspeaker 12 to minimize ,the size of the loudspeaker system of the present invention. An ,;LC crossover network 25 interconnects the three loudspeaker sections to an amplifier ~not shown) which drives the electro-,~acoustic transducers that are associated with the loudspeaker 'system.
i~ ' Certain principles must be observed if good overall sound quality is to be obtained: (1) the exponential horns of ) 'the midrange and high frequency loudspeakers must have straight, that is, not folded or reflexed, axes since folding would result in severe variations in amplitude response; (2) sound locali-i.~ation must be considered in connection with ~he midrange and Ihigh frequency loudspeakers; (3) the outputs of the loudspeakers ~ -. 1i , .
; must be balanced; and (4) the human ear is most sensitive to ¦ audible frequencies in the range of 300-6,000 Hz., and, conse-. I ~quently, the overall loudspeaker system must operate with high ;~: fidelity in this range. The configuration of the loudspeakers I :~
;!shown in Figs. 1-4 indicates application of the first two : ~ ~
principles since the midrange and high frequency loudspeakers :::
!
, I 1 1 :
-2~- . } :
.
7~5 have straight exponential horns and are located above the low frequency loudspeaker so as to be positioned above the floor, for example, to reduce reflection and provide a better locali-zation effect for the listener. The second two principles will now be discussed in conjunction with an LC crossover network in accordance with the present invention.
As shown in Fig. 5, the LC crossover network 25 Eor the loudspeaker system of the present invention comprises two-pole passive networks for each of the low and midrange frequency loudspeakers and a three-pole passive network for the high fre ~uency loudspeaker. An autotransformer 26 is incorporated into the LC crossover network 25 of the present invention to accommo- -!date the use of a high frequency loudspeaker which is less efficient than the low and midrange frequency loudspeakers. In contradistinction to known prior art techniques, the electrical signals that are input to the more efficient loudspeakers are not reduced so as to accommodate use of a less efficient loud-speaker. Instead, the~electrical signal to the less efficient '~ loudspeaker is boosted so that the output of the less efficient .~ .
~`20 loudspeaker is in balance with the outputs of the more efficient loudspeakers. This accommodates use of the less efficient loud~
, speaker with the more efficient loudspeakers and enables the loudspeaker system of the present invention to operate at optimum ,efficiency.
i The l,C crossover network 25 of the present invention 'is a selective network to divide the audio frequency output of :. .
an amplifier (not shown), which drives -the electroacoustic ~ -transducers, into three bands of frequencies. The fre~uency 'Iseparation is employed to feed the three electroacoustic ~rans~
! duoers~so that each operates in a restricted ~requency band ` ~`
: , : ~ , ~ , -23-:
.. . . ~
and thereby operates more eJficiently and with less distortion.
In the loudspeaker system o~ the present invention, the LC
crossover network 25 has been selected for crossover between the low and midrange frequency loudspeakers at a frequency of approximately 400 Hzo and between the midranc3e and high frequency loudspeakers at a frequency of approximately 6,000 Hz.
Wi-th reference to Fig. 5, the LC crossover network ; 25 incorporates inductors and capacitors connected to provide a low pass filter for feeding the low frequency electroacoustic , transducer that is connected across the LOW terminals; a band I pass filter for feeding the midrange frequency electroacoustic transducer that is connected across the MIDRANGE terminals; and a high pass filter for feeding the high frequency electroacoustic ! transducer that is connected across the HIGH terminals. At the 400 Hz. crossover frequency, the power output of the low pass filter for the low frequency loudspeaker and the power output of the band pass filter for the midrange fxequency loudspeaker are approximately equal. At 6,000 Hz., the power output of the band pass filter for the midrange frequency loudspeaker and the power output of the high pass ~ilter for the high fre(~u~ cy loudspeaker are approximately equal. At these frequencies, the outputs of the various filters are approximately 3 decibels -; ~ (dB) below pea~ amplitude response level.
! The LC crossover network 25 of the present invention ; includes "peaking" circuits to achieve slight improvements in smoothness o~ amplitude response near the selected crossover, or transitional, frequencies. ~ccordingly, with re~erence to Fig. 5, Ll, Cl produce a three dB rise at approximately 350 Hz~
L2l, C21 produce a similar rise at approximately ~50 Hz. L22, -~
C22 produce a rise at appro~imately 5,500 Hz. Similarly, - 2 4- . :
' ' ~ ' , ~ , ~ ' ' ' .
,,!l 7Si ~utotran former 26 and cap~citors C31, C3~ produce ~ rise at approximately 6,500 Hz.
The values for the various components in one imple-mentation of the LC crossover network 25 of the present inven-tion are indicated in Fig. 5. The autotI.ansformer 26 may be a KLIPSCH T-2-A autotransformer with iron r.emoved. L21 may be a KLIPSCH T-2-A autotra.nsformer with "E" iron only, the "I" iron having been removed~ Ll and L22 may have iron, as shown in the case of Ll, or air core, as shown in the case of L22. When iron is present in any of the inductive components, however, an air gap is pre~erably provided so that saturation does not occur during operation. Consequently, the inductive reactance of these inductive components remains constant. This in turn means that the frequency separation which is provided by the ~ 1~ crossover network 25 remains fixed and that each electro-: acoustic transducer is driven in a restricted frequency band where each electroacoustic transducer operates more efficiently and with less distortion. :~
The peaking circuits are resonant near the indicated ~:-:20 I frequencies so as to increase the amplitude of the response of the various filters near these frequencies. Consequently, the : ;:
amplitude response of the loudspeaker system o~ the present ~::
. invention, which is shown in Fig. 6, is more.smooth in the area :of the 400 Hz. and 6,000 Hz. crossover frequencies.
The peaking effect derives from the fact that the LC
crossover network has lower input, or driving point, impedance near the resonant frequencies of the various peaking circuits.
At these frequencies, the input impedanoe drops from 8 or 16 : ohms to as low as 4 ohms. Consequently, conventional solid :
state amplifiers, which are characteristically designed to ~:
~: : , .':' , ~ , ~ -~5- . 1 ~
1~ . ~
~v~ , 77~
deliver their maximum output into a 4 oh~ load, pro~uce a hi~h output near the crosscver frequencies of 400 and 6,000 Hz. where the outputs of the _oudspeakers are drooping.
In the practice of the present invention, the value of the inductors and capacitors which are used in the peaking circuit5 can be derived aralytically based on the selected crossover frequencies. Slight adjustments by means of a variable inductor and/or capacitor then pro-duce the lowest peak-trough ratio so that a smooth amplitllde response curve results. Values may be "tailored" to modify response.
The prese~ce of cavities at the sides of a loudspeaker system causes deterioration in the smoothness of amplitude response. It is desirable, therefore, that the mouths of the loudspeakers are bounded by a large baffle to avoid cavities at the sides of the loudspeaker system. See, Klipsch, "Eight Cardinal Points in Loudspeakers for Sound Reproduction", I.R.E. Trans, on Audio, Vol. AU-9, No. 6, Nov.-Dec., 1961, pp. 204-209 i In practical use, the loudspeaker system of the present inven-` ~ tion will be positioned, for example, against the floor or ceiling of a roo~ or auditorium or on the platform of a stage for r~d;ation of sound into a ~ solid angle or in a corner bounded by a floor or ceiling and two intersectmg walls for radiation of sound into a ~/2 solid angle.
If the loudspeaker system of the present invention, which in general shape is a rectangular structure, is placed in a corner, for example, cavities at the sides would cause deterioration of s~oothness of amplitude resp~nse.
In the illustrative embodLment of the loudspeaker system of the present invention, therefore, side wings 23 and 24 are used to ellminate - ~ cavities at the sides of the loudspeaker , ~ pg~C~ ~ 26 ;
77~ l 'l I
system as shown in Figs. 1 and 4. Side wings 23 and 24 may be attached by means of hinges -to side walls 21 and 22, respcctive ly, if desired, as s}lown in Figs. 1 and 4.
Il Fig. 6 shows the amplitude response characteristic of ,la specific construction for the loudspeaker system in accordance with the illustrative embodiment of the present invention. The ,amplitude response characteristic in Fig. 6 was obtained by ,means of three microphones in a typica] listening room as de-scribed by W. B. Snow, "Loudspeaker Testing in Rooms"/ Jour.
iAudio Eng~ Soc., Vol. 9, No. 1, Jan., 1961, pp. 54-60.
As described above, the analytical value of the low end cut-off frequency was selected to be below 70 Hz. Fig. 6 indicates that the efective low end cut-off frequency is approxi-mately 55 Hz.~ which is the point where the amplitude response is l 10 dB below peak amplitude response. This is in accordance with Klipsch, "A Note on Acoustic Horns", Proc. I.R.E., Vol. 33, No. 7,l ~ -! July, 1945, pp. 447-449, whereln the author indicates that there is not a sharp cut-off at the analytical cut-oEf frequency.
As shown in Fig. ~, the amplitude response is relatively smooth over the operating range from approximately 55 Hz. to ,, 15,000 Hz. The amplitude response curve in Fig. 6 shows a peak-trou~h ratio o~ less than 10 dB over the most significant part of this operating rangeO
The loudspeaker system afEords approximately lOg dB SPL
,l output at one meter with an input of one watt of power, which corresponds to an efficiency oE approximately 20~. At an input , capacity of 200 peak watts (100 watts amplifier rating) -the loudspeaker system also affords over 80 dB SPL at 30 meters (100 feet) outdoors.
' Having described my invention, I claim:
;. ' 1, .
li -27- , !l ~ ' ' ' '' ' ' ' ..
,' ' ~ ' ~;
~ .
' ' : ':
~; ~ : .' ' _ . .. ... . . . .. .. _ _ _ _ _ . . . . . _ __ _~_ ... . . .
TABLE I
LOW FREQUENCY LOUDSPEAKE:R
Electroacoustic = KLIPSCH
Transducer K33E
Analytical Low End = 66-70 Hz.
Cut-off Frequency Throat Area = 78 Square Inches Mouth. Area = 252 Square Inches Rate of Expansion = Cross-sectional ..
of Horn Area Doubles Every ll.8 :
Inches . .
Mean Sound Path = 20 Inches : Length Volume of Back Air = 2,730 Cubic Chamber Inches ' ; :
: ' , ~ ~
' ' ` ',:
' : ~ :
:~': :,`' ~ ~ Pg/ ~ ~ - 18 -:,~: ` ~ ".
The low Erequency loudspeaker of the present invention ~may be incorporated into a loudspeaker system which further includes a midrange frequenc~ loudspeaker and a high fre~uency loudspeaker together with an LC crossover network which estab-lishes the frequency ranges for the various loudspeakers.
I' A straight axis exponential horn may be used in 'connection with an appropriate electroacoustic transducer to , form the midrange frequency loudspeaker 10 as shown in Figs.
1-4. A midranye frequency loudspeaker similar to the one which 'is described in Klipsch, "A New High Frequency Horn", I.R.E.
Trans. on Audio, Vol. AU-ll, No. 6, Nov.-Dec., 1963, pp. 20~-206, ' with a low end cut-off frequency of 375 Hz., for example, may be used.
An illustrative embodiment :Eor the midrange frequency ! loudspeaker 10 includes a KLIPSCH K55V electroacoustic trans-ducer. For maximum power transfer, or efficiency, with the KLIPSCH K55V electroacoustic transdùcer, the throat area of the midrange frequency loudspeaker exponential horn must be approxi-mately ~4 s~uare inch, or 2.6 square centimeters. Since the midrange frequency loudspeaker 10 due to its position in the iloudspeaker system of the present invention effectively radiates ;isound into a 2 ~ solid angle, the diameter (equivalent circle) of the mouth must be at least one-sixth wavelength of the 375 ,i Hz. low end cut-of~ frequency in accordance with the criterion ;j of Wente and Thuras in their above-cited article. Based on the dimensions of the specific construction for the low frequency " j .:
loudspeaker in accordance with an illustrative embodiment of the present invention, an exponential horn for the midrange fre-quency loudspeaker 10 with a mouth area of 46 square inches, 30 ~ ! or 297 square centimeters, may be conveniently used. This ' i ` ' !
_ ~ . . . . . . ... .. . _ . .. . ...
~ . . . ..
7~ i ;
!' translates to a mouth which has a diameter ~equivalent circle) o l/9.7 wavelength of the low end cut-ofE frequency of 375 E~z.
~which exceeds the minimum value established by the cri-terion of Wente and Thuras. In accordancé with the above-ci-ted Kellogg article, the cross-sectional area of the exponential horn for the midrange Erequency loudspeaker lO must no-t double in less than 2 inches. Given the throat area, mouth area, and rate of expansion for the midrange frequency loudspeaker straight ex- ;
ponential horn, a mean sound path length of approxima-tely 15 'inches, or 97 centimeters, is established. The back air chamber for the midrange frequency loudspeaker lO requires a volume of 2.55 cubic inches, or 41~B cubic centimeters, or equivalent combined air chamber and diaphragm suspension compliance. The , data for the midrange frequency loudspeaker 10 for the illustra-tive embodiment of a loudspeaker system in accordance with the present invention are tabulated in Table II.
.
' .
~ , ~
!
l i : ' ~ ! ~
~1 -20-_ . . ..... _. ... .. .. . . . . .. . . .. .
. . . .
" ` 1~3~3l775 !
,I T~BLE II
IDRANGE FREQ~ENCY LOUDSPEAKER
I! , Electroacoustic = KLIPSCH
jj Transducer K55V
'i Analytical Low End = 375 Hz. I :
. Cut-off Frequency Throat Area = 0.4 Square ~ -~
, Inch ' Mouth Area = 46 Square I Inches .
l Rate of Expansion = Cross-Sectional i! f Horn Area Doubles ~ j Every 2 : ~ Inches - -:
: j Mean Sound Path = 15 Inches Length Volume of Back = 2.55 Cubic ¦ ~ -Air Chamber Inches or equi- I :
. Il valent combined air chamber and diaphragm sus~
pension com~
: pliance , 7~
A KLIPSCH K-77 may be used or the high frequency loudspeaker 11 of the illustrative embodimen-t of the loudspeaker Isystem of the present invention. Although tlle eEficierlcy o~
~this high frequency loudspeaker is lower than either the low or midrange frequency loudspeakers, the power demand ln the high audible frequency range, that is, in the range of 6,000-15,000 Hz.,l is small. Consequently, an autotransformer may be incorporated I :
linto an LC crossover network as described below so as to permi-t - use of the KLIPSCH K-77.
) The figures indicate that the midrange frequency loud- i 'speaker 10 and the high frequency loudspeaker 11 are mounted in iclose proximity to the low frequency loudspeaker 12 to minimize ,the size of the loudspeaker system of the present invention. An ,;LC crossover network 25 interconnects the three loudspeaker sections to an amplifier ~not shown) which drives the electro-,~acoustic transducers that are associated with the loudspeaker 'system.
i~ ' Certain principles must be observed if good overall sound quality is to be obtained: (1) the exponential horns of ) 'the midrange and high frequency loudspeakers must have straight, that is, not folded or reflexed, axes since folding would result in severe variations in amplitude response; (2) sound locali-i.~ation must be considered in connection with ~he midrange and Ihigh frequency loudspeakers; (3) the outputs of the loudspeakers ~ -. 1i , .
; must be balanced; and (4) the human ear is most sensitive to ¦ audible frequencies in the range of 300-6,000 Hz., and, conse-. I ~quently, the overall loudspeaker system must operate with high ;~: fidelity in this range. The configuration of the loudspeakers I :~
;!shown in Figs. 1-4 indicates application of the first two : ~ ~
principles since the midrange and high frequency loudspeakers :::
!
, I 1 1 :
-2~- . } :
.
7~5 have straight exponential horns and are located above the low frequency loudspeaker so as to be positioned above the floor, for example, to reduce reflection and provide a better locali-zation effect for the listener. The second two principles will now be discussed in conjunction with an LC crossover network in accordance with the present invention.
As shown in Fig. 5, the LC crossover network 25 Eor the loudspeaker system of the present invention comprises two-pole passive networks for each of the low and midrange frequency loudspeakers and a three-pole passive network for the high fre ~uency loudspeaker. An autotransformer 26 is incorporated into the LC crossover network 25 of the present invention to accommo- -!date the use of a high frequency loudspeaker which is less efficient than the low and midrange frequency loudspeakers. In contradistinction to known prior art techniques, the electrical signals that are input to the more efficient loudspeakers are not reduced so as to accommodate use of a less efficient loud-speaker. Instead, the~electrical signal to the less efficient '~ loudspeaker is boosted so that the output of the less efficient .~ .
~`20 loudspeaker is in balance with the outputs of the more efficient loudspeakers. This accommodates use of the less efficient loud~
, speaker with the more efficient loudspeakers and enables the loudspeaker system of the present invention to operate at optimum ,efficiency.
i The l,C crossover network 25 of the present invention 'is a selective network to divide the audio frequency output of :. .
an amplifier (not shown), which drives -the electroacoustic ~ -transducers, into three bands of frequencies. The fre~uency 'Iseparation is employed to feed the three electroacoustic ~rans~
! duoers~so that each operates in a restricted ~requency band ` ~`
: , : ~ , ~ , -23-:
.. . . ~
and thereby operates more eJficiently and with less distortion.
In the loudspeaker system o~ the present invention, the LC
crossover network 25 has been selected for crossover between the low and midrange frequency loudspeakers at a frequency of approximately 400 Hzo and between the midranc3e and high frequency loudspeakers at a frequency of approximately 6,000 Hz.
Wi-th reference to Fig. 5, the LC crossover network ; 25 incorporates inductors and capacitors connected to provide a low pass filter for feeding the low frequency electroacoustic , transducer that is connected across the LOW terminals; a band I pass filter for feeding the midrange frequency electroacoustic transducer that is connected across the MIDRANGE terminals; and a high pass filter for feeding the high frequency electroacoustic ! transducer that is connected across the HIGH terminals. At the 400 Hz. crossover frequency, the power output of the low pass filter for the low frequency loudspeaker and the power output of the band pass filter for the midrange fxequency loudspeaker are approximately equal. At 6,000 Hz., the power output of the band pass filter for the midrange frequency loudspeaker and the power output of the high pass ~ilter for the high fre(~u~ cy loudspeaker are approximately equal. At these frequencies, the outputs of the various filters are approximately 3 decibels -; ~ (dB) below pea~ amplitude response level.
! The LC crossover network 25 of the present invention ; includes "peaking" circuits to achieve slight improvements in smoothness o~ amplitude response near the selected crossover, or transitional, frequencies. ~ccordingly, with re~erence to Fig. 5, Ll, Cl produce a three dB rise at approximately 350 Hz~
L2l, C21 produce a similar rise at approximately ~50 Hz. L22, -~
C22 produce a rise at appro~imately 5,500 Hz. Similarly, - 2 4- . :
' ' ~ ' , ~ , ~ ' ' ' .
,,!l 7Si ~utotran former 26 and cap~citors C31, C3~ produce ~ rise at approximately 6,500 Hz.
The values for the various components in one imple-mentation of the LC crossover network 25 of the present inven-tion are indicated in Fig. 5. The autotI.ansformer 26 may be a KLIPSCH T-2-A autotransformer with iron r.emoved. L21 may be a KLIPSCH T-2-A autotra.nsformer with "E" iron only, the "I" iron having been removed~ Ll and L22 may have iron, as shown in the case of Ll, or air core, as shown in the case of L22. When iron is present in any of the inductive components, however, an air gap is pre~erably provided so that saturation does not occur during operation. Consequently, the inductive reactance of these inductive components remains constant. This in turn means that the frequency separation which is provided by the ~ 1~ crossover network 25 remains fixed and that each electro-: acoustic transducer is driven in a restricted frequency band where each electroacoustic transducer operates more efficiently and with less distortion. :~
The peaking circuits are resonant near the indicated ~:-:20 I frequencies so as to increase the amplitude of the response of the various filters near these frequencies. Consequently, the : ;:
amplitude response of the loudspeaker system o~ the present ~::
. invention, which is shown in Fig. 6, is more.smooth in the area :of the 400 Hz. and 6,000 Hz. crossover frequencies.
The peaking effect derives from the fact that the LC
crossover network has lower input, or driving point, impedance near the resonant frequencies of the various peaking circuits.
At these frequencies, the input impedanoe drops from 8 or 16 : ohms to as low as 4 ohms. Consequently, conventional solid :
state amplifiers, which are characteristically designed to ~:
~: : , .':' , ~ , ~ -~5- . 1 ~
1~ . ~
~v~ , 77~
deliver their maximum output into a 4 oh~ load, pro~uce a hi~h output near the crosscver frequencies of 400 and 6,000 Hz. where the outputs of the _oudspeakers are drooping.
In the practice of the present invention, the value of the inductors and capacitors which are used in the peaking circuit5 can be derived aralytically based on the selected crossover frequencies. Slight adjustments by means of a variable inductor and/or capacitor then pro-duce the lowest peak-trough ratio so that a smooth amplitllde response curve results. Values may be "tailored" to modify response.
The prese~ce of cavities at the sides of a loudspeaker system causes deterioration in the smoothness of amplitude response. It is desirable, therefore, that the mouths of the loudspeakers are bounded by a large baffle to avoid cavities at the sides of the loudspeaker system. See, Klipsch, "Eight Cardinal Points in Loudspeakers for Sound Reproduction", I.R.E. Trans, on Audio, Vol. AU-9, No. 6, Nov.-Dec., 1961, pp. 204-209 i In practical use, the loudspeaker system of the present inven-` ~ tion will be positioned, for example, against the floor or ceiling of a roo~ or auditorium or on the platform of a stage for r~d;ation of sound into a ~ solid angle or in a corner bounded by a floor or ceiling and two intersectmg walls for radiation of sound into a ~/2 solid angle.
If the loudspeaker system of the present invention, which in general shape is a rectangular structure, is placed in a corner, for example, cavities at the sides would cause deterioration of s~oothness of amplitude resp~nse.
In the illustrative embodLment of the loudspeaker system of the present invention, therefore, side wings 23 and 24 are used to ellminate - ~ cavities at the sides of the loudspeaker , ~ pg~C~ ~ 26 ;
77~ l 'l I
system as shown in Figs. 1 and 4. Side wings 23 and 24 may be attached by means of hinges -to side walls 21 and 22, respcctive ly, if desired, as s}lown in Figs. 1 and 4.
Il Fig. 6 shows the amplitude response characteristic of ,la specific construction for the loudspeaker system in accordance with the illustrative embodiment of the present invention. The ,amplitude response characteristic in Fig. 6 was obtained by ,means of three microphones in a typica] listening room as de-scribed by W. B. Snow, "Loudspeaker Testing in Rooms"/ Jour.
iAudio Eng~ Soc., Vol. 9, No. 1, Jan., 1961, pp. 54-60.
As described above, the analytical value of the low end cut-off frequency was selected to be below 70 Hz. Fig. 6 indicates that the efective low end cut-off frequency is approxi-mately 55 Hz.~ which is the point where the amplitude response is l 10 dB below peak amplitude response. This is in accordance with Klipsch, "A Note on Acoustic Horns", Proc. I.R.E., Vol. 33, No. 7,l ~ -! July, 1945, pp. 447-449, whereln the author indicates that there is not a sharp cut-off at the analytical cut-oEf frequency.
As shown in Fig. ~, the amplitude response is relatively smooth over the operating range from approximately 55 Hz. to ,, 15,000 Hz. The amplitude response curve in Fig. 6 shows a peak-trou~h ratio o~ less than 10 dB over the most significant part of this operating rangeO
The loudspeaker system afEords approximately lOg dB SPL
,l output at one meter with an input of one watt of power, which corresponds to an efficiency oE approximately 20~. At an input , capacity of 200 peak watts (100 watts amplifier rating) -the loudspeaker system also affords over 80 dB SPL at 30 meters (100 feet) outdoors.
' Having described my invention, I claim:
;. ' 1, .
li -27- , !l ~ ' ' ' '' ' ' ' ..
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a loudspeaker system having low, midrange, and high frequency loudspeakers and an LC crossover network, wherein at least one of said loudspeakers is less efficient than other of said loudspeakers, the improvement in said LC crossover network, comprising:
an autotransformer included in said LC crossover network and connected between an amplifier and said at least one less efficient loudspeaker for boosting the audio frequency input to said at least one less efficient loudspeaker, whereby the output of said at least one less efficient loudspeaker is increased to enable use of said at least one less efficient loudspeaker with said other loudspeakers and whereby the overall loudspeaker system operates at optimum efficiency.
an autotransformer included in said LC crossover network and connected between an amplifier and said at least one less efficient loudspeaker for boosting the audio frequency input to said at least one less efficient loudspeaker, whereby the output of said at least one less efficient loudspeaker is increased to enable use of said at least one less efficient loudspeaker with said other loudspeakers and whereby the overall loudspeaker system operates at optimum efficiency.
2. In a loudspeaker system having at least n loudspeakers, wherein n is greater than one, and an LC crossover network dividing the audio frequency output of an amplifier into n frequency bands, each said frequency band driving a separate one of said loudspeakers, there being n-1 crossover frequencies, the improvement in said LC crossover network, comprising:
at least one peaking circuit means included in said LC crossover network and resonant near at least one crossover frequency for enhancing the amplitude response of said loud-speaker system, whereby said loudspeaker system has a smooth amplitude response characteristic in the region of said at least one crossover frequency.
at least one peaking circuit means included in said LC crossover network and resonant near at least one crossover frequency for enhancing the amplitude response of said loud-speaker system, whereby said loudspeaker system has a smooth amplitude response characteristic in the region of said at least one crossover frequency.
3. The loudspeaker system of claim 2 wherein there are three loudspeakers, including low, midrange, and high frequency loudspeakers, and two crossover frequencies, and said LC cross-over network has peaking circuit means resonant near each said crossover frequency to enhance the amplitude response of said loudspeaker system in the region of each said crossover frequency.
4. The loudspeaker system of claim 3 wherein said cross over frequencies are approximately 400 and 6,000 Hz. and said peaking circuit means are resonant at approximately 350, 450, 5,500 and 6,500 Hz.
5. The loudspeaker system of claim 2 wherein said at least one peaking circuit means includes an adjustable impedance, whereby said at least one peaking circuit means is adjustable for obtaining the lowest peak-trough ratio for said amplitude response characteristic of said loudspeaker system.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA373,128A CA1111775A (en) | 1977-06-02 | 1981-03-16 | Loudspeaker system with improved lc crossover network |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/802,734 US4138594A (en) | 1977-06-02 | 1977-06-02 | Small dimension low frequency folded exponential horn loudspeaker with unitary sound path and loudspeaker system including same |
| US802,734 | 1977-06-02 | ||
| CA304,598A CA1098450A (en) | 1977-06-02 | 1978-06-01 | Small dimension low frequency folded exponential horn loudspeaker with unitary sound path and loudspeaker system including same |
| CA373,128A CA1111775A (en) | 1977-06-02 | 1981-03-16 | Loudspeaker system with improved lc crossover network |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1111775A true CA1111775A (en) | 1981-11-03 |
Family
ID=27165686
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA373,128A Expired CA1111775A (en) | 1977-06-02 | 1981-03-16 | Loudspeaker system with improved lc crossover network |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1111775A (en) |
-
1981
- 1981-03-16 CA CA373,128A patent/CA1111775A/en not_active Expired
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