US1984542A - Acoustic device - Google Patents

Description

Dec. 18, 1934. OLSON 1,984,542

ACOUSTIC DEVICE Filed March 31, 1932 2 Sheets-Sheet 1 RESPONSE IN DECIBELS FREQUENCY IN CYCLES PER SEOOND INVENTOR HARRY F OLSON BY NQQM ATTORNEY Dec. 18, 1934. H QLSON 1,984,542

ACOUSTIC DEVICE Filed March 31, 1932 2 Sheets-Sheet 2 mil/( mm nmlmnwwr INVENTOR HARRY F. OLSON I wT/M ATTORNEY Patented Dec. 18, 1934 UNITED STATES PATENT OFFICE Radio Corp ration of Delaware Application March 31,

8 Claims.

This invention relates to acoustic apparatus.

More specifically it relates to an improved type of acoustic apparatus which is particularly adapted toreproduce sound and radiate the sound waves into a theatre or auditorium in such amanner A parts of the apparatus so as tosecure the desired uniformity of frequency response characteristics.

Other and more specific objects of the invention will become apparent upon reading the following specification and appended claims in connection with the accompanying drawings which illustrate an approved type of acoustic apparatus embodying the invention.

In the accompanying drawings Fig. 1 is a diagrammatic illustration of a simple type of acoustic device.

Fig. 2 is a circuitwhich is the electrical equivalent of the acoustic device shown in Fig. 1.

Fig. 3 is a diagrammatic illustration of an acoustic device according to the present inven- Fig. 4 is a circuit which is theelectrical equivalent of the acoustic device shown in Fig. 3.

Fig. 5 is a chart showing the response characteristics of apparatus em dying the invention.

Fig. 6 is a cross sectional view showing certain details of acoustic apparatus embodying the invention. v

Fig. 7 is an isometric view of the apparatus illustrated in part in Fig. 6, and diagrammatically in Figs. 3 and 5, and

l ig. 8 is a perspective view of amodified form of the invention.

An essential distinguishing characteristic between difierent kinds of loudspeakers lies in the method of coupling between the loudspeaker diaphragm and the medium into which the soundis radiated. I .have' found that in the prior art 'lou there B a loss of coupling between the diaphragm and the medium (the air) which materially aifects the reproduction of'sound of the loudspeakers. The efiect of the loss of cou- Amerlca, a corporation of 1932, Serial No. 602,158

pling diifers according to the method of coupling which is used in any specific loudspeaker but in all cases the loss of coupling is objectionable. In general, the lossof coupling between the diaphragm and the air occurs at the lower frequencies. It is a purpose of my invention to decrease the loss of coupling between the diaphragm and the air at all frequencies within the audible range, and to provide acoustic apparatus in which vthe loss of coupling is substantially constant over 10 at least the greater portion of the audible frequency range.

Among the common methods employed to increase low frequency radiationfrom diaphragms are the use of large diaphragms, groups of diaphragms, various shapes of baiiles, and various shapes of horns. Fig. 1 illustrates diagrammatically one of the latter methods for increasing low frequency radiation.

The loudspeaker shown in Fig. 1 consists of a diaphragm 1. an electrodynamic driving unit 2 for vibrating the diaphragm, and a horn or tube 3. The throat of the horn or tube 3 is positioned adjacent the edge of the diaphragm so that all the sound waves from the front of the diaphragm pass through the horn or tube. Its-function is to reduce the coupling losses at the lower frequencies between the diaphragm and the air that is set in motion by the diaphragm. It also has a baille effect by, means of which it prevents interference between sound waves from the two sides of the diaphragm and the consequent cancellation or distortion of the sound waves, especially at the lower frequencies.

It has been shown that a diaphragm vibrating with constant velocity and coupled to a tube of infinite length, generates the same acoustic power for all frequencies. This is due to the fact that the mass of the diaphragm is negligible compared 40 to the acoustic impedance of the infinite tube. Nowassumingthatthediaphragm1ofFig.1is a dynamic cone of mass m coupled to a tube 3 of acoustic impedance R; if the mass m is chosen so that the acoustic reactance of the cone is negligible compared to the acoustic impedance R for the'desired frequency range, a system is obtained that dissipates the same power in the acoustic resistance for any frequency within the range. According to the present invention the mass of the diaphragm is so chosen with respect to the acoustic impedance of the horn or directional baflle associated with the diaphragm, that the acoustic v is negligible compared with the acoustic impedance of the horn,--over at least the greater portion of the audible frequency range.

An infinite tube of exponentially increasing cross section can now be substituted for the infinite tube of constant cross section. It has been shown by Webster (Journal of National Academy of Sciences 1919, pages 275-282) that the acoustic resistance at the small end of this tube will be a constant for all frequencies above the cut off frequency. The cut off frequency is determined by the rate of flare. It may be located below the lowest frequency to be reproduced. If the tube of exponentially increasing cross section is now cut at some point along its length so as to provide a finite tube with its open end terminating in air, the action will be altered depending upon the cross section of the resulting mouth. If this cross section is sufliciently large, a very slight reflection will occur in the transition from the mouth to the medium (air) and the impedance presented to the cone by the tube will be practi cally constant above the cut off frequency. The system, as before, will dissipate the same power into the tube for the frequency range chosen. Consequently, neglecting slight reflection at the mouth, it will dissipate constant power into the medium for this range. Systems consisting of a finite flaring tube of exponentially increasing crosssection coupled to a cone diaphragm, are known as directional baflie types of loudspeakers. Another purpose of the present invention is to provide a finite flaring tube or directional baffle having a mouth sufiiciently large in cross section so that only a very slight reflection occurs at the transition from the mouth of the tube or directional baflle to the air.

In the dAlembertian wave equation for the axial motion in an exponential horn (Webster Journal of the National Academy of Sciences, 1919, pages 275-282) it is assumed that the phase is the same over a plane normal to the axis of the horn. This condition is practically satisfied provided the cross section is not greater than a wave length. It has been found experimentally that, for any particular frequency within the transmission band, additional length of the horn beyond a certain point (the radius of ultimate impedance) does not affect the performance of the horn. That is, the working portion of the horn decreases with increase of frequency, therefore, in a horn in which the axis has a straight line, the

condition of the same phase over a plane normal to the axis is automatically satisfied. To maintain the same phase over a plane normal to the axis in a folded or curled up horn is, exceedingly difllcult. The condition is practically satisfied provided the diameter at any bend is less than the wave length of the highest frequency reproduced. This places a limitation upon the amount of folding or curling that may be accomplished without impairing the horn action. If these conditions are not satisfied destructive interference will result. In addition, certain portions of the horn will act as reflectors at the higher frequencies. These conditions ultimately result in a non-uniform response characteristic. It is therefore, a further purpose of the present invention to provide the apparatus with a horn or directional baflie having a straight line axis.

The low frequency cut off of a finite exponential horn is determined by the rate of flare and the mouth opening. When the cut off has been set the mouth opening and rate of flare are fixed. The remaining factor, the throat area, therefore determines the length of the horn, or, if the length of the horn is fixedby the limits of the over-all dimensions of the acoustic apparatus, the throat area is determined by the length of the horn. For example, if the acoustic apparatus is to be used in a talking moving picture installation and is to be placed behind the motion picture screen, the space in which the loudspeaker can be located is limited and therefore the horn must be comparatively short. When the horn is comparatively short the throat of the horn becomes comparatively large due to the fixed cross section of the mouth and the exponential character of the flare. The diaphragm must then be associated with this relatively large throat in such a manner that the acoustic reactance of the diaphragm bears the proper relation to the acoustic impedance of the horn.

Fig. 3 illustrates diagrammatically acoustic apparatus embodying the principles ofthe present invention. The apparatus consists essentially of a large throat horn 5, a vibratile system 6 including a diaphragm and a driving coil, a box 7 substantially enclosing the rear of the diaphragm, and an air chamber 8 between the cone diaphragm and the horn.

The horn 5 is of the exponential type. The equation expressing the area at any distance X along the axis is given by The solution of equation may be written in the form s=so E m= 211, 1': frequency The pressure at any point in the horn is given by The volume velocity at any point in the horn is given by 56 (V) V=- -S[ae"(A cos bx-i-b sin bxH- be(-A sin Bx+ 8 cos Bxfl where =density of air.

These equations give the expression for the pressure and volume velocity for any point in the horn. Equations for the pressure and volume velocity at the throat in terms of the pressure and volume velocity at the mouth, may be obtained as follows:

The impedance at the throat is given by 'givenby- At the mouth the impedance is given by From the tour equations (Equations (11) to (V)) containing A, B, P1. P2 and Va, A and B may be eliminated to obtain vthe ratio of P1 to V1, i. e., the impedance 31 at the throat of the horn. This impedance in terms of Z, is given by the expression (vr as I! lm(b1)-l-- sa (1, 4

where l=length oi the horn 'I'hereactivecom entma bee essedby pen y m The acoustic capacitive reactance presented to P 10 Wm) T '2' a? 3 I The impedance Z: for the mouth of the horn is (1x) sl m- 4x1 Y The impedance at the small end of thehom can be calculated by substituting the values of 5: above in equation (VI).

The vibratile unit of the system consists of a paper cone 9 fitted with an aluminum wire voice coil 10. The air chamber 8 couples the area of the cone 9 to the throat of the horn 5. The-back oi the cone is enclosed by a box 7 having a ielt back 11.

The inertance oi the cone and voice coil is The impedance presented behind the'cone must also be considered. Crandall (Theory of Vibrating Systems and Sound, p. 110) has derived an expression for the impedance presented to a piston in an infinite plane. Experiments conducted upon cones in fiat bailies indicate that the size of the cone employed in this unit behaves as a piston to approximately 3,000 cycles. The further stipulation that the side of the box containing the cone shall be an infinite plane is fulfilled for the range in which the impedance referred to is of appreciable magnitude. The impedance presented to the back oi the cone consists oi two (XI) X,

parts; the resistive and reactive components. The resistive component is given by.

The reactive component is given by pc x (QKR') (Km) The air chamber behind the cone is enclosed by a box which has a felt back. The purpose of the felt is to absorb any sound striking it and thus prevent standing wave systems which would cause abrupt changes with frequency in the impedance presented behind the cone. At the higher irequencies the absorption of the felt is practically unity and the sound wave flows from the cone into the ielt. At low frequencies, however, the absorption is very small and, as a consequence, a capacitive reactance is presented to the cone. Assuming the most unfavorable condition, in

which the absorption is zero, the capacitance of the box is then given by' I (XIV) op -E,

. where V=volume of the box c=velocity of sound =density of air.

the cone in series with the horn load is This equation holds until the dimensions of the box are comparable to a quarter wave length. Above this irequency the absorption due to the felt is practically. unity andalso the impedance due to the box may be neglected.

The purpose of the air chamber is to act as a transformer between the area of the cone and the smaller area of the throat of the horn. In accomplishing this, a capacitance results which is indicated by C1 Figure 3. The capacitance of the air'chamber is given by c'llgemgglouigtic capacitive reactance oi the air (XVII) .xw -f;;

The equivalent electric circuit of the acoustic system illustrated by Fig. 1, is shown in Fig. 2. It consists of a series circuit including a reactance (the mass of the vibratile system divided by the square or the area of the diaphragm) and an impedance R (the acoustic impedance of the tube). In accordance with the present invention the value of reactances and impedances.

Referring, for convenience, the electric circuit, 4, the acoustic impedance at the point F will be indicated by the expression- The mechanical impedance of the acoustic system at the point F is (XIX) Z=r+ix=A(R +iX where A=the area of the cone. In the case of a moving coil loudspeaker the motional impedance is given by the expression.

where B=flux density and I=the length of wire in the voice coil The principal object in obtaining the motional impedance is to predict the efficiency which, in

tum; indicates theperformance of the loud-' speaker. The efliciency is given by the expression Rm XXI r! Rm Rd where Rm=motional resistance Ra =clamped resistance of the voice coil. The total acoustic current through the system at the point F, Figure 4 is given by (xxn) v1 over the frequency range from approximately 100 cycles per second to over 4,000 cycles per second.

Fig. 6 is a cross sectional view showing certain details of the vibratile system and the elements associating the vibratile system with the directional bailie. The vibratile system consists of a corrugated cone diaphragm 15 and a voice coil 16. The voice coil is supported on the usual cylindrical member secured to, or forming a part of the diaphragm. The voice coil is located in the magnetic field formed between the pole pieces 17 and 18. Leads such as 19, supply current to the voice coil. The peripheral edge of the diaphragm is secured by supporting means such as the leather ring 20, to a supporting member which, in the present instance, consists of an annular ridge 21 on the member 22. Any other suitable supporting means may be used however.

The member 22 which is preferably of metal, provides the connecting link between the diaphragm and the throat of the horn or directional bame 23. It is known as an adapter or as a transformer according to how its function is viewed. Heretofore it has been customary to shape the throat of the horn so that it is circular in cross section and of a diameter conforming The acoustic power output of the loudspeaker is given by the dissipation in Z1. The total impedance of $1 shunted by Cl is The dissipation in inwill be the real part of (XXIV) Power=zprv r The actual values of firrVr can be inserted in equation (XXIV) and the total acoustic output of the loudspeaker determined. For a constant value of F applied to the system it is desired to make'the acoustic output as large and as uniform with frequency as possible. The values of the individual component parts have been derived in equations (1) to (XXIII). The magnitudes of these parts should be substituted in equation (XXIV) and the optimum value for each part determined so that equation (XXIV) will be a maximum.

Some of the dimensions of the loudspeaker are determined before hand. For example, the maximum length of the loudspeaker is usually a definite quantity. With the length determined the rate of flare is determinedby the lowest frequency that it is feasible to reproduce. This, in

turn, determines the mouth and throat openings. Thus it will be seen that some of the parameters in expression (XXIV) are fixed by limitations as to space and the frequency range to be covered. It then becomes a question of adjusting the remaining parameters in the manner pointed out hereinbeforato yield the best results. The best results are obtained when equation (XXIV) is the largest possible value obtainable over the frequency range to be reproduced.

Fig. 5 is a chart showing the frequency characteristic of an acoustic device embodying the principles of the invention. The chart contains a r-J r s) 1 w'C R with the diameter of the peripheral edge of the diaphragm. The present adapter is shaped so a that the throat of the horn or directional baflie may be square in cross section, and slightly smaller in its dimensions than the diameter of the peripheral edge of the diaphragm. The adapter is provided with a portion 24 which extends within the cone diaphragm and conforms with the shape of the diaphragm. The portion 24 is spaced from the diaphragm by approximately one eighth of an inch. This permits the desired vided with a felt lining or a rear wall of felt 2'7 for the purpose of absorbing sound waves from the rear of the diaphragm.

It has been found that the response and dynamic characteristics of a six inch cone are es--v pecially suitable for the directional baiiie type of loudspeaker. As will be seen from the foregoing equations and the equivalent electrical circuit Fig. 4, it is important that the mass of the cone be small in order to maintain a uniform dissipation in $1. This is accomplished by preferably employing an aluminum wire voice coil and an extremely light rigid paper cone. Non-uniform frequency response at the higher frequencies which is commonly encountered when the light paper of high stiffness is employed, is prevented by suitable corrugation of the cone.

The size of the throat of the directional bailie, that will present a tolerable acoustic impedance to the cone and at the same time not impair the high frequency response due to absorption along the walls or cause destructive interference in the air chamber, is 4" x 4". The mouth of the baiiie,

if made 43" x 58" will insure good radiation characteristics at low frequencies. The axial length of. a baiiie which will place the cut off due to flare in the neighborhood of 100 cycles, is fifty inches.

The size of the enclosure controls the capacitive reactance Xb at the back of the diaphragm. If the enclosure is made in the form of a box 12" x 12" x 10 deep, the reduction in current in the equivalent circuit Fig. 4 is not appreciable above 100 cycles.

The reactance x1e due to the air chamber 25 is controlled so as to eliminate destructive interference up to the highest frequency reproduced. It has been found that a separation of approximately one eighth of an inch between the diaphragm and the adapter, allows the diaphragm to vibrate with the desired amplitude of movement necessary for full power output at the low frequencies, and provides a reactance within allowable limits.

Fig. 7 is an isometric view of the apparatus described. It illustrates a loudspeaker of the directional baflie type, in the form in which it is adapted to be used intalking moving picture installations.

Fig. 8 is an isometrieview of a modified form of apparatus in which a pair of directional baiiie loudspeakers are connected together on a plane baffle for the radiation of sound waves in a predetermined manner required by certain theatre constructions. The doublet arrangement includes two separate vibratile units and a directional baflie 31 for each unit. Each unit is provided with an enclosure in the same manner as in the apparatus illustrated in Fig. '7. I

While I have illustrated and described an approved form of acoustic apparatus embodying my invention, it is to be understood that I do not desire to be limited to the exact construction shown, as various modifications can be made thereto without departing from the spirit of the invention. It is to be understood that I contemplate various changes and modifications and that I desire to be limited only by the scope of the appended claims.

What I claim is: 1. An acoustic device comprising a corrugated conical diaphragm of relatively large diameter,

an aluminum driving coil for vibrating said diaphragm, a box-like enclosure surrounding the rear of said diaphragm, a felt back in said enclosure, a horn in front of said diaphragm, said horn having a throat opening of dimensions comparable with those of the cone and having its acoustic impedance of such. order of magnitude that the acoustic. reactance of the diaphragm is negligible compared therewith, a member at the throat of said horn extending into said conical diaphragm,

and an air chamber between said member and said conical diaphragm.

2. Acoustic apparatus comprising a conical diaphragm approximately six inches in diameter,

said diaphragm, said directional baflie having a throat opening approximately four inches square and a mouth opening approximately forty-three inches by fifty-eight inches, and an adapter connected between the throat of said directional baffie and said diaphragm.

3. Acoustic apparatus comprising a vibratile diaphragm and a directional baille associated therewith, said directional baille having a mouth opening and a flare such that the apparatus has a cut oil? in the neighborhood of one hundred cycles per second, said diaphragm having so small a mass that its acoustic impedance is negligible compared with the acoustic impedance of said directional baflle.

4. Acoustic apparatus comprising a conical diaphragm, a directional bame associated therewith, said directional baflie having a substantially square throat opening comparable in size to the size of said diaphragm, and an adapter between said diaphragm and said directional bailie, said adapter having an opening therein corresponding to said throat opening, said adapter having a portion extending into said conical diaphragm.

5. Acoustic apparatus comprising a conical diaphragm, a directional bafile associated therewith, and driving means connected with said diaphragm, said driving means and diaphragm being so light that the acoustic reactance of said diaphragm is negligible compared with the acoustic impedance of said directional baiiie.

6. As an article of manufacture, a member adapted to be inserted between the vibratile conical diaphragm and a directional baflie, comprising a body having a portion constituting the sole support for the peripheral edge'of the diaphragm, a portion conforming with the shape of said diaphragm and adapted to extend within the diaphragm, and a square opening of substantially the same size as the throat opening of said directional baiiie extending through said body whereby sound waves from said diaphragm are conveyed to said directional baiiie.

7. Acoustic apparatus comprising a substantially plane baflle member, a pair of openings in, said baflle member immediately adjacent each other, a pair of vibratile diaphragms positioned adjacent each other, and a pair of directional baflies arranged so that their throats are immediately adjacent said diaphragms and so that their mouth openings terminate at the openings in said plane baflle member.

8. The method of reproducing sound at substantially uniform intensity over the greater p013 tion of the audible frequency range by means of a vibratile diaphragm and a directional baliie associated therewith, which consists in providing the directional baflie with a relatively large mouth opening and a flare which will produce a cut of! in the neighborhood of the frequency of one hundred cycles per second, and in matching the acoustic reactance of the diaphragm with the acoustic impedance of the directional baliie, in such a manner that the same power is dissipated in the acoustic resistance of the directional battle at all frequencies within the desired range.

HARRY F. OLSON.

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