US2643728A - High fidelity loud-speaker cabinet - Google Patents

Description

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Patented June 30, 1953 UNITED STATES PATENT1 OFFICE HIGH FIDELITY LOUD-SPEAKER CABINET William R. Anthony, San Juan, P. R.

Application August 15, 1951, Serial No. 241,943

4 Claims. 1

This invention relates to acoustical devices, and more particularly to a high-ndelity acoustical enclosure for a loud speaker.

A main object of the invention is to provide a novel and improved acoustical cabinet devi-ce for high fidelity reproduction of the sound vibrations from a loud speaker, said device being simple in construction, being relatively compact in size, and being neat in appearance.

A further object of the invention is to provide an improved high-fidelity loud speaker acoustical cabinet Which provides accurate reproduction of the sound vibrations from a loud speaker over a range much Wider than that obtained from loud speaker enclosures of comparable size and cost of the prior art.

-A still further object of the invention is to provide an improved high-iidelity loud speaker cabinet which is inexpensive to manufacture, which is of pleasing proportions, and which accurately reproduces the high and W sound frequencies, as well as the middle range of sound frequencies of the loud speaker employed therewith.

A still further object of the invention is to provide an improved acoustical cabinet for a loud speaker, which is capable of high-delity, widerange reproduction of sound vibrations, including the very low frequencies.

A still further object of the invention is to provide an improved loud speaker assembly of smaller dimensions than has heretofore been possible for equal acoustical results and eiliciency.

A still further object of the invention is to provide an improved loud speaker assembly of simpler construction and which is less expensive to manufacture than previous loud speaker as.- semblies of equivalent performance, yet which is of pleasing proportions aesthetically While maintaining `superior acoustical results and efficiency.

A still further object of the invention is to provide an improved loud speaker arranged so that its positioning and location in a room has no elect or limitation on its eiiiciency of operation, and which is further arranged so that directional effects of the sound reproduced thereby are minimized.

A still further object of the invention is tor provide an improved loud speaker as above set forth, Which may employ a plurality of driving units in accordance with the volume of sound desired, whereby the loud speaker may be employed in auditoriums, theaters, and other loca- 2 tions of public assembly where a large volume of sound is required.

Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings, wherein:

Figure 1 is a front elevational view illustrating one form of the present invention.

Figure 2 is a vertical, transverse cross-sectional view taken on line 2--2 of Figure 1.

Figure 3 is a horizontal cross-sectional View taken on line 3--3 of Figure l.

Figure 4 is a horizontal cross-sectional view similar to Figure 3, illustrating a modied form of the present invention.

Figure 5 is a front elevational View illustrating another modified-'form of the present invention.

Figure 6 is a vertical, transverse cross-sectional view taken on line 6-6 of Figure 5.

Figure 7 is a front elevational view illustrating still another modified form of the present invention.

Figure y8 is a` perspective theoretical sketch illustrating the relationship between the three planes of sound reproduced by the improved loud speaker of the present invention.

Figurev 9 is a 'horizontal cross-sectional view illustrating a still further modied form of the present invention, the loud speaker enclosure being arranged for placement in `the `corner of a room.

Figure l0 is a diagrammatic sketch illustrating the various acoustic elements oi the loud speaker of the present invention.

Figure 11 is a horizontal cross-sectional view similar to Figure 3, illustrating a further modified form of the invention wherein sound deadening material is employed to prevent back reiiections or standing sound waves.

Practical limitations have in the past restricted most wide-range sound reproducers to vf-requencieswhich did not include the very low frequencies. .Extremely large theater-type horns and very complicated folded horns (see, for example, U. S. Patent No. 2,373,692 to Paul W. Klipsch) -have been the best available means for reproducing the loW frequencies. However, both of these methods have serious physical, constructional and acoustical limitations. l

It has been generally recognized that very low frequencies must be included in the overall widerange reproduction of sound for satisfactory high-fidelity results. Together with this require- :menty is the requirement that the loud-speaker unit must be suitable for use where space is comparatively limited, such as in the home. Also, simplicity of construction is reflected in lower cost of manufacture.

The loud speaker assembly of the present invention is designed to fulll all high-fidelity sound reproduction requirements, yet is of compact size, is simple in construction, and is of aesthetically pleasing proportions. The overall efciency of the assembly is high and is not affected by variations in audio power. Therefore, it is suitable for both residential use and for circumstances where higher power is required, such as in theaters, and the like.

Previous designs have employed folded horns. The folded horn acts like a band pass filter in that it has a low-frequency cut-off, determined primarily by the dimensions at the mouth of the horn, and a high-frequency cut off, determined primarily by the number of folds. In the attempt to conserve space, previous designs employed multiple folds and hence lost the ability to reproduce high frequencies, because said frequencies cannot follow an excessively circuitous path and are dissipated by multiple reflections in the folds.

A straight horn, however, acts like a high pass filter, with only the low cut-off frequency, determined primarily by the mouth dimensions. A straight horn, however, would not be practical to use for very low frequencies because of size limitations. Therefore, in the present invention the straight horn is used only for the lower middle and higher frequencies.

Referring now to the drawings, wherein like characters of reference designate similar parts in the various views, Figures 1, 2 and 3 show a cabinet with the opening 2| in the front and the opening 22 in the back thereof. Mounted from the back over opening 22, against baffle 34 in chamber 26, is a suitable electroacoustical driver 23, such as a dynamic loud speaker, which includes at least one cone or diaphragm 24 and which should preferably be capable of reproducing a wide range of acoustic signals, especially including the very low frequencies. Compartment 26 for the acoustical driver 23 is formed by baffles 35, the solid top 3|, the solid bottom 32, and the speaker mounting baille 34.

A short, straight-axis horn 25, which preferably, but not necessarily, expands exponentially in a horizontal plane at its mouth portion, is located in the front section of the cabinet and loads the front of the diaphragm 24. By terminating at the opening 2|, said horn serves to establish communication between the diaphragm 24 and the opening 2|, as well as the atmosphere.

Chamber 26 may be partially closed, if desired, but the back of cone 24 of the acoustical driver 23 is preferably left open to free space, because the self-resonant frequency of said cone will then be lower than the resonance frequency in an enclosure. If desired, the back of compartment 26 may be covered with protective mesh material or perforated sheet material to provide protection against mechanical damage.

An acoustical resonator 21, externally excited by diaphragm 24 of the acoustical driver 23, is dened by completely enclosing a volume of air by means of side pieces 28, back pieces 29, and front pieces 30, together with top 3|, bottom 32, and horn 25.

A long air column 33, externally excited at its midpoint by diaphragm 24 cf the acoustical driver 23, is defined by baies 36 together with Vmerely as heat loss.

,4 every other piece making up the cabinet structure.

Shown at 31, 31 are deectors having a combined function, in that their opposing, inwardly facing surfaces form part of horn 25, extending said horn well into proximity to the diaphragm 24, thereby assuring the maximum possible acoustic coupling and resultant efficient reproduction of the higher frequencies by the horn; the surfaces of deflectors 31 facing the rim of diaphragm 24 serve to help deect very low frequencies produced by diaphragm 24 into and out of the air column 33.

The horn 25 is employed to transmit middle and high audible frequencies with great efficiency, the resonator 21 is employed to transmit low audible frequencies with high efficiency, and the long air column 33 is employed to transmit very low audible frequencies with high efficiency. All three of these acoustic elements are coupled together and to the front side of the diaphragm 24- in the portion of the enclosure directly in front of baffle 34. All three acoustic elements then establish communication with the atmosphere through opening 2|. Each of the three acoustic elements is designed to operate with highest efficiency over a relatively small frequency range. This results in a substantially constant acoustic loading of the driver diaphragm 24 over the entire very low to high frequency range, the efficiency therefore remaining relatively uniform at all volume levels.

The assembly thus described may be regarded as a three-way loud speaker because of the above described incorporation of three distinct acoustic elements into one loud speaker. Each of these elements operates independently of the other because of a novel inherent acoustic phasing effect, resulting in an acoustical crossover network. The crossover frequencies in this acoustical crossover network are adjustable in the dimensional design of the assembly, but are normally at approximately 60 cycles per second and 200 cycles per second, which values will be assumed in further discussion.

The present invention operates as a sound wave guide in its ability to transmit sound from the three distinct acoustic elements 2'5, 21 and 33. A separation or acoustic cross-over effect is due to a acoustical phase difference between most of the sound waves transmitted by the respective acoustic elements, occurring at the single opening 2|.

Heretofore, insuflicient front air loading of the acoustic driver cone has demanded additional loading and damping from the rear of the cone, resulting in a dissipation of acoustic power The need for high loading has been generally accepted as a means for limiting otherwise excessive cone movement, especially when reproduction of audible frequencies at 0r near the natural resonant frequency of the cone is required.

In the present invention practically all air loading is accomplished on the front of the driver cone and is in turn coupled to the atmosphere, resulting in eicient acoustical work being accomplished by the air loading. The front air loading is so complete that cone movement is reduced to a minimum throughout the audio range, thus reducing non-linear distortions for given amounts of useful power output. The overall acoustic efliciency of a loud speaker system, exclusive of acoustic driver design factors, is

proportional to the effectiveness of the atmosphere air load upon the driver cone.

It is generally agreed that systems employing multiple acoustical crossovers produce less harmonic distortion than single-crossover systems. The harmonic distortion in the system -of the present invention is low because the frequency range of at least two of the three acoustic elements in this system is preferably held to less than two octavos.

The exponential horn isa natural acoustical high pass iilter, and the low frequencyat which it begins to pass sound is its cut-off frequency. The cut-off frequency of the exponential horn employed in the system of the present invention is made as low as possible compatible with size limitations. horn of this nature is capable of excellent very high frequency distribution. However, at frequencies higher than approximately 1000 cycles per second the importance of external air loading is secondary to the capability of the acoustic driver cone or diaphragm, or multiple cones or diaphragms, to generate and properly distribute the higher frequencies.

In the present invention, at the cut-off frequency of the exponential horn 25, another acoustic element comes into operation, namely,` the resonator 21. The resonator 21 also has a cutoff frequency, just below its natural resonant frequency. Therefore, at the cut-off frequency of the resonator 21, another acoustic element, namely, the long air column 33, comes into oper ation to provide eiiicient very low frequency response, thus adding the full and desirable bottorn range of audible frequencies.

rlhe response characteristics of a resonator are normally not quite as smooth as those of an exponential horn, but because a straight exponential horn would have to be of enormous size to cover the same frequency range, resonator 21' proves to be a much more practical acoustical element. In the present invention the strategic use of sections of sound deadening material, as shown, for example, at 56, 5|, 52 and 53 in Figure ll, results in a much smoother response from resonator 21 than has heretofore been possible in an ordinary resonator. As shown in Figure 11, the sound deadening material is placed at the interior corners of the resonator, to prevent disturbing reflections at said corners.

The long air column 33 serves as an acoustic impedance matching device between the acoustic driver cone or diaphragm 24 and the atmosphere, in the frequency range which includes the natural resonant frequency of said cone or diaphragm. If said acoustic driver has multiple cones and/or diaphragms, the frequency range of said air column covers the natural resonant frequency of the cone or diaphragm used to reproduce the very low frequencies. The length of said air column is adjusted so that its acoustic capacitive eiect on said cone or vdiaphragm effectively damps the otherwise excessive cone or diaphragm excursions. The very low frequency cut-olf depends only on the natural resonant frequency of said acoustic driver cone or diaphragm and the proper matching length of said air column. Therefore, for best very low frequency response, said acoustic driver cone or diaphragm should have as low a natural resonant frequency as is practical.

Sound waves in the frequency range of the exponential horn 25 reach the atmosphere through opening 2l in a manner approaching ver-tical polarization, or in vertical planes of sound perpendicular to the front of the loud speaker, as shown, for example, at VL and VR in the Itheoretical diagram of Figure 8. Sound waves in the frequency range of the acoustic resonator 21 reach the atmosphere through opening 2| in a manner also approaching vertical polarization, but in a vertical plane parallel to the front of the cabinet, or a pressure plane, as shown at P in the diagram of Figure 8. Sound waves in the frequency range of the long air column 33 reach the atmosphere through opening 2| in a manner approaching horizontal polarization, or in horizontal planes of sound perpendicular to the front of the loud speaker, as shown, for example, at HA rand HB in the theoretical diagram of Figure 8.

Deflectors $51 serve both horn 25 and air column 33, and indirectly affect resonator 21. Not only do said deflectors complete the physical form of horn 25, but do so primarily in the aforesaid ver-tical planes, therefore delivering to said horn sound waves primarily vertically polarized, as required. The shape of said deectors is such that they deflect very low frequency sounds into the air column 33 in horizontal planes symmetrically located above and below the horizontal center line of acoustic driver 23. The return paths of very low frequency sounds are in two horizontal planes, one plane being in space sa near the top piece 3l, and the other plane being in space 38h near the bottom 32. Thus, upon returning to opening 2i, two horizontal planes of very low frequency sounds are coupled to the atmosphere in phase with the very low frequency sounds reaching opening 2l by direct radiation from the upper and lower sides of the acoustic driver cone or diaphragm 24.

rThe indirect effect of denectors 31 on resonator 21 is tliat the sise and shape of said deflectors control the efficiency and quantity of acoustic output in the frequency ranges of both air column i3 and horn 25, thus allowing the efliciencies of said air column and horn to be adjusted to match the best attainable efficiency of said resonator, the final result being to obtain substanfor 'a given type of acoustic driver.

As shown in Figure 4, the lengths of the resonators may be effectively increased, if so desired, by employing speaker baiiie elements 54, 54 which extend laterally a substantial distance beyond the side baiiie elements 35, 35, and by employing inwardly extending wing elements 55, at the ends of the walls S3, opposing and parallel to the baiile elements 54.

As shown in Figures 5 and 6, a conventional tweeter unit 55 may be mounted at the top of the cabinet to provide additional high frequency emphasis, said tweeter unit being located immediately over the central portion of the top wall 3l, as shown, and being housed in a suitable enclosure 51, which mayform part of the main enclosure..

` Figure '1 illustrates a modiiied form of the present invention suitable for use in an auditorium, theater, or other location where a large volume of sound is required. In this form of the invention, a double enclosure is employed, containing two acoustic drivers 23, 23, arranged in vertically spaced relation and cooperating with respective resonators 21 and 33, and respective horns 25, similar to those shown in the single enclosure of Figures l to 3. As shown in Figure '1, the deflectors 31 define the rear portions of the sides of the respective horns 25, 25, and extend adjacent the cones 24, 24 in the same manner as in Figures 1 to 3. Similarly, since the horizontal cross-section of the enclosure, taken through the axes of the drivers 23, 23, is the same as in Figure 3, respective low and very low frequency resonators are provided for each of said drivers, as in the single enclosure of Figures 1 to 3. A conventional tweeter unit 56 may be mounted at the top of the cabinet to provide additional high frequency emphasis, as shown.

The loud speaker assembly illustrated in horiaontal cross-section in Figure 9 is generally similar acoustically to the assembly of Figures 1 to 3, except that it is arranged to fit into a corner of a room. r`he enclosure of Figure 9 comprises a cabinet having side walls 28', 28 at right angles to each other and converging rearwardly toward the rear opening 22 of the cabinet. The acoustic driver unit 23 is mounted in the rear of the cabinet against baffle 34, as shown. The compartment 255 for driver unit 23 is formed by baille 3 and the rear portions of side walls 28', 28. A straight-axis horn 25 is located at the front section of the cabinet, said horn having the inner deflectors 31, 31 terminating adjacent the cone 24 of the driver, said horn establishing communication between the cone 24 and the atmosphere in front of the loud speaker assembly. Designated 35', 36 are internal wall elements which are arranged to define respective air columns 21 extending from the inner sides of the deflectors 3l', 31 to the sides of the forward portion of horn 25', said air columns 21 having low frequency natural resonance, and the relatively long air column 33 dei-ined above and below the deflectors 31, 31, having very low frequency natural resonance, as in the enclosure illustrated in Figures l to 3. As in the previously described embodiments oi' the invention, the air columns 21 extend from cone 24, along the deflectors 31, and around the barile elements 35 to the regions rearwardly adjacent the forward portions of the horn 25. The air column 33 extends from the region rearwardly adjacent one side of the forward portion of horn 25', around the baffle element 36 at said one side, above and below the deflectors 31, 31, and around the baille element 36 at the other side of the horn to the region rearwardly adjacent the forward portion of the horn at said other side.

While certain specific embodiments of an improved acoustical cabinet device for high fidelity reproduction of the sound vibrations from an acoustical driver unit have been disclosed in the foregoing description it will be understood that various modifications within the spirit of the invention may occur to those skilled in the art. Therefore it is intended that no limitations be placed on the invention except as defined by the scope of the appended claims.

What is claimed is:

1. A loud speaker cabinet comprising a main housing having a top wall, a bottom wall, respective side walls connecting said top and bottom walls, wall means extending between said top and bottom walls connecting the rear portions of said side walls, and a front wall extending between said top and bottom walls and connected to the forward portions of said side walls, said means including a chamber arranged to receive an acoustic driver unit, said chamber having a forward wall formed with an, opening, said front wall including a pair of rearwardly convergent vertical wall elements extending between said top and bottom` walls, said vertical wall elements having rear end portions which are reduced in height and which extend into but are spaced from the side edgesy of said opening, and respective inner vertical baiiles connected to and extending from the intermediate portions of said convergent wall elements into the opposite side portions of the main housing between the respective side walls thereof and said chamber, said baies extending between said top and bottom walls and being spaced at their free vertical edges from the inner vertical surfaces of the housing to define respective folded air conduits of relatively low resonant frequency communicating with said opening, the vertical height of said reduced rear end portions being substantially less than the vertical height of the opening, whereby said folded air conduits communicate with each other adjacent said opening and denne a resultant air chamber having a still lower resonant frequency.

2. In combination with the structure of claim l, a cone speaker mounted in said chamber with the cone of the speaker facing forwardly through said opening, said reduced rear end portions being spaced inwardly from the opposite side edges of said cone, whereby the outer side portions of the cone are acoustically coupled to said folded air conduits and a remaining outer portion of the cone is acoustically coupled to said resultant air conduit.

3. The structure of claim 1, and wherein said housing is substantially rectangular' in plan.

4. The structure of claim 1, and wherein said side walls are substantially at right angles to each other, whereby the housing may be received in the corner of a room.

WILLIAM R. ANTHONY.

References Cited in the file of this patent UNITED STATES PATENTS Number

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