US2228886A - Electroacoustical apparatus - Google Patents

Description

Jan. 14, 1941. H. F. OLSON ELECTROACOUSTIGAL APPARATUS 8 Sheets-Sheet 1 Filed Oct. 51, 1938 (Ittorneg Jan. 14, 1941. L OLSON 2,228,886

ELECTROAOOUSTICAL APPARATUS Filed Oct. 51, 1938 8 Sheets-Sheet 2 Srwntor X (Ittorneg Jan. 14, 1941. F OL N 2,228,886

ELECTROACOUST I CAL APPARATUS Filed on. 51, 1958 8 Sheets-Sheet s H. F. OLSON ELECTROACOUSTICAL APPARATUS Jan. 14, 1941. 2,228,886

Filed Oct. 31, 1938 8 Sheets-Sheet Snnentor fiarwzw m Jan. 14, 1941. H. F. OLSON ELECTROACOUSTICAL APPARATUS Filed 001:. 31, 1958 Fig.3.

8 Sheets-Sheet 5 m4 2:: i! J :3 K25 57 T J RESPONSE Jan. 14, 1941.

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ELECTROACOUSTICAL APPARATUS Filed Oct. 51, 1938 8 Sheets-Sheet 6 way/m r5501 m/vr Jan. 14, 1941. H. F. OLSON 2,228,886

ELECTROACOUSTICAL APPARATUS Filed Oct. 51, 1958 s Sheets-Sheet 7 0N6 P/PE Lon/ PIPE con/57M, 404 6 F/PE amass SECTION P/PE WIT/l l/VERT/M/CE 0 w 90 I o o o 90 a" 90 00 0 /J0 attorney Jan. 14-, 1941. OLSQN 2,228,886

ELECTROACOUSTICAL APPARATUS Filed Oct. 51, 1938 8 Sheets-Sheet 8 Bnoentor zlm flam 40 acoustical resistance.

Patented Jan. 14, 1941 2,228,886 ELECTBOACOUSTICAL APPARATUS Harry F. Olson, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 31, 1938, Serial No. 237,960 15 Claims. (Cl. 181-05) This invention relates to electroacoustical apparatus, and more particularly to a sound translating device for translating acoustical waves into electrical variations, the invention having for its primary object the provision of an improved and highly directional sound translating device or microphone by means of which it will be possible to pick up sounds over very long distances.

More particularly, it is an object of my present invention to provide an improved directional microphone which is especially suitable for use in sound motion picture recording, television pickup, large stage productions in radio broadcasting,

sound reinforcing systems, etc., where it is desirable to keep the microphone out of the field of action and where it is, therefore, necessary to place the microphone at'a considerable distance from the sound source.

Another object of my present invention is to provide an improved directional microphone which has a directional characteristic independent of the frequency of the sound picked up thereby.

Still another object of my invention is to provide an improved directional microphone as aforesaid which has a relatively small angle of pick-up.

It is also an object of my invention to provide an improved ultra-directional microphone for the purpose set forth which is compact in construction, easily portable, and highly efficient in use.

According to my present invention, I provide a sound pick-up system which comprises a large number of pipes of different length connected 5 to a common junction which, in turn, is connected to one side of an element adapted to convert the sound waves which travel down the pipes into corresponding electrical variations, the said element being terminated on its other side by an The length of the line, that is, the distance between the longest and the shortest pipe, d termines the directional characteristic of the system. I

r The novel features that I consider character- 45 istic of my invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from to the following description of a few embodiments thereof, when read in connection with the accompanying drawings, in which Figures 1 to 21, inclusiv ;show various' modi- 'fications' oi microphonesformed according to my a 55' present invention,

Figure 22 is a diagrammatic layout of a complete microphone assembly according to my present invention,

Figures 23, 24, 25, 29 and 30 are vector diagrams showing the outputs of certain modifications of my invention,

Figures 26, 2'7, 28, 31 to 37, inclusive, and to 46, inclusive, show directional patterns of various I modifications of my invention herein described,

Figures 38 and 39 show response curves of two m of the modifications of my invention,

Figure 4'! shows a completely assembled microphone built in accordance with my present invention and mounted for operation, the microphone including a layout similar to that shown H in Figure 22, and

Figure 48 is a response curve showing the response of a microphone such as shown in Figures 22 and 47.

Referring more particularly to the drawings, 20 wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. 1, a microphone comprising a plurality of tubular elements I, 2', 3, 4, 5 and 6 of progressively varying length coupled to or having a common 25 junction at a casing 1 within which is vibratably mounted a ribbon element 8 for vibration in a magnetic field in well known manner. The tubular elements I to 6, inclusive, are of uniformly progressively varying length, so that their free, 30 or pick-up, ends lie along a straight line. The tubular elements I to 6 are preferably all parallel and are arranged to direct acoustical waves onto the front surface of the vibratile member, 35

or ribbon 8, the diameters of all the tubular elements l to 6 being preferably the same and small compared to the length of the ribbon 8. Coupled to the rear of the casing or junction 1 is a long tube or pipe 9 filled with tufts of felt ill, the pipe 9 with its felt tufts lo constituting an acoustical resistance which terminates the ribbon 8 and which has a value substantially equal to the surge resistance of the tubes l to 6 and large compared to the mass reactance of the ribbon 8. Prefer ably, the total cross-sectional area of the tubular elements I to 6, inclusive, is substantially equal to the cross-sectional area of the large pipe 9. This very largely minimizes reflection at the junction, or casing I, particularly when the acoustic impedance of'the ribbon is small comparedto the 50 acoustic resistance offered by the pipe9. If the impedance of the ribbon 8 is comparable .to the surge resistance, of the tubular elements I to 6, then the vector sum of theimpedance terminat-;

"lng the ribbon a and the'impedance oi the ribbon 8 itself should be equal to the surge impedance of the tubular elements I to 6.

If a plane sound wave traveling from left to right in a direction parallel to the axis of tubes I to 6 is considered, it will be noted that sound enters each of the small tubes I to 6 and travels down to the ribbon 8 and into the long, damped pipe 9. In going from the small tubes I to 6 into the large pipe 9, the sound waves must, of course, excite the ribbon 8. In the case under consideration, all the outputs of the tubes I to 6 are in phase and may be represented by the vector diagram shown in Fig. 23, wherein the numerals I to 6, inclusive, correspond to the tubes I to 6 of Fig. 1.

Suppose, now, that sound incident at an angle 0 with respect to the axis of the microphone is considered. Let the instantaneous pressure contributed by the tube I at the ribbon 8 be given by where p is the amplitude of the pressure,

A is the wave length,

t is the time, V

c is the velocity of the sound I is the distance from the opening of pipe I to the ribbon.

The pressure contributed by the tube 2 will be given by p =p sin %[ctl(dd cos 0 2 where dis the distance between the open ends of the adjacent pipes I and 2. Similarly, assuming that the length diflerence a between any pair of adjacent pipes I to 6 is the same, the pressure contributed by the tube 3 will be p sin [ctl(2d2d cos 0 (3) and so forth for each of the other pipes.

If it is assumed that the distance d between the ends of any pair of adjacent pipes I to 6 is of a wave length and that the angle 0 is 60, then the vectors will be as shown in Fig. 24. From this figure, it will be noted that the re sponse is considerably attenuated as compared to the case represented by the vector diagram of Fig. 23, in connection with which it was assumed that the angle 0 is equal to 0. Let it be assumed, now, that the distance d between the ends of any pair of adjacent tubes I to 6 is $4; wave length but that the angle 0 is In such case, the vectors will be as shown in Fig. 25, from which it will be noted that the response is zero. The directional characteristic of the microphone of Fig. 1 for the condition of d equals V wave length will be as shown in Fig. 26. There are, as will be noted, two small secondary lobes in the backward direction, but these are small and of little consequence in practice.

For the distance d between the ends of any pair of adjacent tubes I to 6 equal to 1; wave length, the directional characteristics will be as shown in Fig. 27, and for a distance d equals the wave length, the directional characteristic will be as shown in Fig. 28. The characteristic curves of Figs. 26, 27 and 28 show that it is possible to obtain practically any directional characteristic by choosing an appropriate ratio of the wave length. in Fig. 1, there may length of the pipe system t- In the microphone shown coupled to a 4" be some attenuation of sound in the small tubes I to 6 at the high frequencies. For example, in a system employing A", pipe, I found that the attenuation at 10,000 cycles is 2.5 decibels per foot of length, while at 1,000 cycles the attenuation is only .7 decibel per foot of length for the same size pipe. This loss can be counteracted by attaching small horns to the ends of the tubes I to 6, and the microphone of Fig. 2 shows the small horns II to I6, inclusive, formed on the free, or pickup, ends of the tubes I to 6, respectively. The characteristic of each of the small horns II to-IG can be made so that the horns will increase the pressure delivered to any of the small tubes I to 6 at the performance at low frequencies. This ineach of the small pipes due to attenuation. In an actual model which was constructed, a small exponential horn 1%" in length with mouth and throat diameters of and A", respectively, I pipe was used. Such a horn gave about 6 decibels gain at 10,000 cycles and compensated for the loss in the associated pipe.

In some cases due to losses in the tubes I to 6, to place the junction of the tubular elements I to 6 at the casing 1 within which the ribbon is housed. The

' microphone may, therefore, be made as shown in Fig. 3, where a section of pipe I1 is interposed between the casing Ti and a similar casing I8 which constitutes the iunction of the tubular elements I to 5. An arrangement of this sort does not impair the performance at all. The cross-sectional area made equal to the total cross-sectional area of the tubes I to 5 and the ribbon 8 is terminated, as before, in an acoustical resistance.

Fig. 4 shows a modification of the microphone shown in Fig. 1. In the modification of Fig. 4, each of the tubes 2, 3, 6 and 5 (but not the tube I) is provided with a double bend 2a, 3a, 4a and 5a, respectively, which introduces a delay. The bends 2a, 3a, 6a and 5a are progressively longer in correspondence with the lengths of the tubes in which they are formed, the longest tube 5 having the longest bend 5a, and each successively shorter tube having a correspondingly shorter bend, so that the tube 2 has the shortest bend 2a.

As in the case of Fig. 1, let the instantaneous pressure contributed by the tube I at the ribbon be given by Also, for the tube 3, the instantaneous pressure at the ribbon 8 is given by Assuming that there are six tubular elements in the system with a separation of V A and that the length of the bend is lie X, then, for 0 equals 0, the vector diagram will be as shown in Fig. 29. For the case where 0 equals 60, the vector diait is not convenient or desirable,

of the pipe I! is preferably.

the high frequencies without affecting I gram will be as shown in Fig. 30. It will be seen that theresuitant for the case where equals 60" is much smaller than in the case of Fig. 24,

where no delay was introduced in the tubular elements. The response for 0 equals 0 has also been reduced slightly. Thenet result of introducing delay in the tubular elements by reason of'the bends 2a to a, inclusive, is to sharpen the directional characteristic. For the sake of comparison. the-directional characteristics with and without delay are shown in Figs. 31 and 32, the one in Fig. 31 being without delay and the onein Fig.32 being with delay.

Fig. 5 shows a modification of my invention which operates on the difference in pressure of the resultants of the outputs of two tube systems or lines. One line comprises the tubular elements 2|, 22, 23. 24 and connected at a common junction 26 to which is also coupled a long pipe 21 filled with felt tufts 23, and the other line consists of a series of tubular elements 3| to 35, inclusive, connected at a common junction 36 to which is also coupled a tube 31 having the felt tufts 38 therein. The difference between the length of the two lines may be measured by the difference in the average length of the tubular elements 2| to 25, in one case, and average length of the tubular elements 3| to 35, in the other case; or, if the length increments of adjacent tubes in each of the lines is the same, then the difference may be measured by the difference in length of any corresponding tubular elements, for example, 25 and 35, or 22 and 32. The tube 31 in this modification is provided with a bend 31a which has a length equal to the difference in length of the two linesas above described. The ribbon 8 is located between the pipes 21 and 31 and is actuated by the difference in pressurein these pipes. The length of the bend 31a should preferably be as much less than /2 the wave length received by the system as good practice dictates. For this condition, the length of the bend 31a is small compared to the wave length, and the directional characteristics of the individual lines are multiplied by cos 0. This, then, causes considerable sharpening of the characteristics. For example, if the characteristic of an individual line is as shown in Fig. 33, then the characteristic multiplied by cos 0 will be as shown in Fig. 34.

Fig. 6 shows a modification which is similar to Fig. 5 but in which the individual tubes -2| to 25 of one line and the individual tubes 3| to of the other are run directly to the ribbon 8 on opposite sides thereof instead of to the junctions 26 and 36, as shown in Fig. 5, the tubes 3| to 34 being formed with progressively varying bends 3|a, 32a, 33a, and 34a therein, as shown, the bend 3|a being longest and the bend 34a being shortest.

Fig. '1 shows a modification which is a combination of those shown in Figs. 4 and 5. In this case, there is provided a microphone which includes the delays of Fig. 4 multiplied by cos 0.

For the sake of comparison, the characteristic curves of Figs. 35, 36 and 3'7 have been placed alongside each other, thatof Fig. 35 relating to a system with a plain line, that is, one having neither delay, nordelay and multiplied by cos 0, Fig. 36 relating to a systemhaving a line with delay alone, and Fig. 37 relating to a system which both has delay and is multiplied by cos 0.

Fig. 8 shows a modification of the system shown in Fig. 7, butissimilar to that of Fig. 6 in that the tubular members are allrun directly to the ribbon 3. .-l

Fig. 9 shows a modification of the system shown in Fig. 5. In Fig. 9, the tubular elements 2|3|, 22-32, 23-33, 2434, and 25-35, respectively, are made continuous, and the branches 3|, 32, 33, 34 and 35 are provided with appropriate bends 3|a, 32a, 33a, 34a and 35afor the purpose of delay. The branches 2| to 25, inclusive, are connected by the tubes 2|b to 2512, inclusive, to one side of the ribbon 8, and the branches 3| to 35, inclusive, are connected by the tubes 3|b to 35b, inclusive, to the opposite side of the ribbon 8. Each of the lines is terminated in 'an acoustic resistance 21-28, and 3l38, as previously described. This system, therefore, includes two lines which are designed to give an output in pressure which is a cosine characteristic. For example, the output of the pair of tubes 2|-3| is defined by where p is the difference in pressure in the two pipes of the pair. Similarly, the output of the pair of tubes 22-32 is and so on for the other pairs of tubes. Now, the outputs 1121,31, 1022,32, 1723,33, etc., can be added vectorially the same as in the case of the microphones previously described, and the result is the same as the modification of Fig. 5, but it is accomplished in a somewhat diiferent manner.

In some cases, it is desirable to limit the low frequency range of the microphone. Fig. 10 shows how this may be done. If the pipe 9 which terminates the back of the ribbon 8 is very long and filled with the damping material Hi, the impedance presented to'the ribbon 3 is practically a pure resistance over the entire length. If the pipe 9 is less than wave length, it acts as a capacitance, that is, the impedance becomes greater as the length decreases below wave length. Then the impedance presented to the ribbon becomes greater and the velocity of the ribbon decreases. The response with a 10 g pipe 9 which presents a pure resistance as compared to a short pipe, as above described, is shown in the response curve of Fig. 38. The system consisting of a short pipe 9 terminating the ribbon 8. as shown in Fig. 10, provides a means for obtaining a low frequency cut-off.

In pipe systems of the kind under consideration, there is an attenuation in the pipes which is proportional to the frequency and inversely proportional to the diameter of the pipes or tubes. Fig. 11 shows a system including the tubular elements 4| to 45, inclusive, of different diameters. The pipe has the smallest bore or diameter, with very little pressure at the high frequencies. However, at low frequencies it will deliver the same pressure as a short pipe of the same bore. This, then, provides a means for obtaining a line system which becomes shorter with increase in frequency and provides a means of obtaining a uniform directional characteristic. If the constants are adjusted so that the length is inversely proportional to the frequency, the directional characteristic will be independent of the frequency.

Fig, 12 shows another means i or shortening the effective length of the line. The free or pick up ends of the tubular elements 5| to are equipped with inertances5|a to 55a, respectively, by reducing the free ends of the. tubes, preferably progressively and in correspondence with the lengths of the tubes 5I to 55. The inertance is given by g pl 5 III where g p is the density of the air,

I l is the length of the small tube, and S is the cross-sectional area thereof.

The impedance is given by iwM. Then each pipe consists of a small inertance connected to the -end. The inertance becomes progressively smaller as one goes from the tube 5| to the tube 55. This means that the pressure becomes attenuated at the higher frequencies, the amount of attenuation becoming greater as the inertance is made larger. By a suitable choice of constants, the length of the system can be made to be inversely proportional to the frequency. Therefore, the directional characteristics will be inversely proportional to the frequency.

The system illustrated in Fig. 13 shows another method for shortening the effective length of the line with frequency. In this case, the tubes I to 5 are provided with expanded sections, or large volumes, 6|, 62, 53, 64 and 55, respectively, to provide acoustic capacitances, the volumes 5| so to 65 being progressively larger in going from the shortest tube l to the longest tube 5. In the equivalent electrical circuit, the capacitance of the volume under consideration appears as a shunt capacitance across the resistance. The acoustic capacitance is given by K pt: where V is the volume in cubic centimeters,

p is the density of the air, and c is the velocity-of the sound in centimeters per second.

The impedance of any pipe is given by 3 A where A is the area. Thus, it is possible to attenuate the pressure delivered by any pipe at the 0 higher frequencies. By a suitablechoice of constants, the length of the system can be made inversely proportional to the frequency.

The system shown in Fig. 14 provides means for attenuating the high frequency response of the to entire system. In this modification, an inertance is inserted between the Junction or casing I and the resistance I0 by constricting the pipe 9, as shown at 9a. As pointed out above, the impedance of an inertance increases with frequency.

0 The inertance 9a, placed as shown, will decrease the velocity of the ribbon at the higher frequencies. The relative responses, with and without an inertance, is shown by the response curve of Fig. 39.

05 Fig. 15 shows a simplified version of the line microphone. In this modification, the microphone consists of a pipe II whichreplaces the tubular elements I to 6 of Fig. 1 and which is provided with a number of small holes 13. A

system of this sort is, however, subject to the disadvantage that there is reflection from the closed end of the tube II. To minimize such reflection, the system may be built as shown in Fig. 16 wherein the pipe 1| is provided with a gradually expanding section by virtue of the inclined surface Ila which recedes from the openings 13 as it approaches the junction I from the pick up end of the tube I I. It is also possible to minimize reflection by providing the tube II with a damping material 12, as shown in Fig. 17, so that the 6 system has adamped pipe in both, directions relative to the casing I. However, in such a sys-. tem, there may be some interaction between the different holes 13 which tends to reduce the directional efliciency. To reduce the coupling beit tween the holes I3, the system of Fig. 18 may be provided wherein the pipe 'II is provided with apertured partitions I4 between adjacent holes 13. This constitutes an acoustic vfilter between the holes 13. 15

In all the systems heretofore described, ribbon elements have been used as the electro-acoustic transducer for transforming the electrical vibrations into the corresponding electrical variations. This is somewhat morefully shown in Fig. 19 20 wherein the magnet 8| provides the flux for the air gap in which the ribbon 8 is mounted for vibration between the pole pieces 82. The velocity of the ribbon is given by X: PA 20 ZI+Z2 where p is the pressure acting on the ribbon A is the area of the ribbon Z1 is the impedance of the ribbon (this can 30 be made a mass reactance for the audible range), and

Z2 is the resistance terminating the ribbon.

If the impedance Z: terminating the ribbon is a resistance and is large compared to the impedance of the ribbon, the velocity, for constant pressure, will be independent of the frequency. The voltage output of the ribbon is where B is the flux density and e is the length of the ribbon.

With a resistance controlled system, the output is independent of the frequency for constant 45 pressure.

Fig. 20 shows a microphone employing an electrodynamic system which includes the field structure 83 and a cone or the like 84 provided with a voice coil which .is located in the airgap between 50 the poles of the fleld structure 83 in well-known manner. The same reasoning as above may be used in the case of the system under consideration, since both are electrodynamic systems.

Fig. 21 shows another system for transforming 55 the acoustical vibrations into corresponding electrical vibrations. In this case, a condenser unit 85 is used to measure the pressure of the pipe.

'In general, the acoustical impedance of a condenser microphone is high and it will not upset so the impedance of the junction of the small tubes and the large pipe.

Fig. 22 shows the layout of a complete microphone assembly employing four individual lines IOI, I02, I 03, I04. These lines may be formed 55 according to any of the modifications of my invention heretofore described, and may be arranged to cover a range of from 50 to 10,000 cycles, for example, each line covering a predetermined range, whereby practically uniform directional 7o characteristics are obtained over the entire range. The low frequency line I04 may, in practice, be made about 10 in length, themid-low frequency line I03 may be made about 3' in'lhgth, the mid-highfrequency line I02 may be made about 7 one foot in length, and the high frequency line IOI may be about 4" in length. Suitable electric filters IOIa, I02a, I03a, Ila, may be associated with the several respective lines to separate the various outputs therefrom, and the response of each individual line with its associated filter systern may be as shown in Fig; 48. Such a system makes it possible to obtain practically uniform directional characteristics. Taking the mid-high frequency line I02, for example, it is found that at 1500 cycles,all the output comes from this line alone and the directional characteristic is as shown in Fig. 43. At 800 cycles, it will be-noted fromFig. 48 that the overlap of the mid-high frequency line I02 and the mid-low frequency line I03 occurs, and the'directional character'- istics of these two lines and of the combination are shown in Figs. 40, 41 and 42, Fig. 40 showing the characteristics of the mid-low frequency line I03, Fig. 41 showing the characteristics of the mid-high frequency line I02, and Fig. 42 showing the characteristics of the combination, all at 800 cycles. The combination is the same as that obtained at about 1,500 cycles. At 3,000 cycles, where the upper overlap occurs, the characteristics are substantially as shown in Figs. 44, 45 and 46, Fig. 44 showing the characteristics of the mid-high frequency line, Fig. 45 showing the characteristics of the high frequency line, and Fig. 46 showing the characteristics of the combination. These illustrations indicate that the directional characteristics of a system of this type are practically independent of the frequency.

In Fig. 4'7, there is shown a complete microphone assembly mounted for operation. The microphone includes the individual lines IM to I04, inclusive, mounted within a perforated casing I05 which is open at the pick-up end (the left-hand end as shown), and which is swivelly supported in the yoke I06 of a suitable stand I01. The stand I01 includes a pair of rotatable telescoping sections I08 and I09 which, together with the swivel mounting at the yoke I06, make the microphone universally adjustable so that it can be pointed in any direction.

From the foregoing description, it will be apparent that I have provided a novel microphone which is highly directional and the characteristics of which are practically independent of the frequency, and although I have shown a number of modifications according to which any of the individual lines may be built, I am aware that many other modifications thereof are possible. It will also be apparent to those skilled in the art that many changes may be made in the particular modifications shown and described within the scope of my invention. I therefore desire it to be understood that I do not wish to limit myself except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In electroacoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of open-ended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said memher.

2. In electroacoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openacoustical waves and transmit said waves along 1 their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, said tubular elements being of progressively varying length, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

4. In electroacoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, said tubular elements being of progressively varying length with their free ends lying along a straight line, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

5. In electroacoustical apparatus, the combination of a'vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one,

side of said member whereby to direct acoustical waves to said member, said tubular elements being of uniformly progressively varying length,

and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

6. In electroacoustical apparatus, the combination of a-vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, said tubular elements each having a diameter that is small compared to the length of said member, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

7. The invention set forth in claim 6 characterized in that said acoustical resistance comprises a relatively long pipe substantially filled with acoustic damping material, and characterized further in that the combined cross-sectional areas of said tubular members is substantially equal to the cross-sectional area of said pipe.

8. In electroacoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements of progressively varying lengths adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, and a relatively long pipe coupled to the opposite side of said member, said pipe being substantially filled with acoustic damping material whereby to coustitute a terminating resistance for said member, and said terminating resistance having a value which is large compared to the mass reactance of said member.

9. In electroacoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a pluralityoi' openended tubular elements ofprogressively varying length adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, certain of said tubular elements being provided with acoustic delays, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

10. The invention set forth in claim 9 characterized in that said delays are constituted by bends in said certain tubular members intermediate their lengths.

11. The invention set forth in claim 9 characterized in that said tubular elements have a common junction adjacent said acoustically actuable member, and characterized further in that said delays vary progressively in correspondence with the progressive variation in length of said tubular elements, said delays being proportional to the distance from the free ends of their respective tubular elements to said common junction. I

12. In electroacoustical apparatus, the combination of a vibratile member adopted to be actuated by acoustic waves, means associated with said member providing a pair of acoustical lines for directing acoustical waves to said member, each of said lines comprising a plurality of openended tubular elements of progressively varying and the other of said lines leading to the opposite surface thereof, and means providing acoustical resistances coupled to each of saidlines and terminating said lines.

' 13. The invention set forth in claim 12 characterized in that the average length of the tubular lements of one or said lines is shorter than the average length of the tubular elements of the other line, and said one line having an acoustic delay therein.

14. The inventionset forth in claim 12 characterized by the addition of a pair of pipes, one of said pipes being joined to the tubular elements of one of said lines and the other of said pipes being joined to the tubular elements of the other of said lines, and characterized further in that the average length of the tubular elements of one of said lines is shorter than the average length of the tubular elements of the other of said lines, the pipe joined to the shorter of said lines having a bend therein between its junction with its associated tubular elements and said acoustically actuable member, and said bend being equal in length to the diiference between the respective average lengths of said tubular elements.

15. The invention set forth in claim 12 characterized in that the average length of the tubular elements in one of said lines is shorter than the average length of the tubular elements of the other line, and at least certain of the tubular elements of one of said lines having bends therein to provide acoustic delays, said bends being progressively longer in correspondence with the lengths of the respective tubular elements in which they are formed.

HARRY F. OLSON.

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