US2034014A

US2034014A - High-fidelity sound reproducing apparatus and method of improving the fidelity thereof

Info

Publication number: US2034014A
Application number: US14600A
Authority: US
Inventors: Harold A Wheeler; Vernon E Whitman; Nelson P Case; William O Swinyard
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1935-04-04
Filing date: 1935-04-04
Publication date: 1936-03-17
Anticipated expiration: 1953-03-17
Also published as: DE691238C; FR804389A; NL51541C

Description

March 17, 1936.

H. A. WHEELER ET AL 2,034,014 HIGH FIDELITY SOUND REPRODUCING APPARATUS AND METHOD OF IMPROVING THE FIDELITY THEREOF Filed April 4, 1955 3 Sheets-Sheet 1 III INVENTORS. HAROLD A. WHEELER VERNON E. WHITMAN 1 V ATTORNEY March 17, 1936. A WHEELER ET AL 2,034,014

HIGH FIDELITY SOUND REPRODUCING APPARATUS AND METHOD OF IMPROVING THE FIDELITY THEREOF Filed April 4, 1935 5 Sheets-Sheet 2 INVENTORS HAROLD A. WHEELER VERNON f. W/l/T/VA/V NELSON P. (1455 ATTORNEY.

March 17, 1936. A, WHEELER E L 2,034,314

HIGH FIDELITY SOUND REPRODUCING APPARATUS AND METHOD OF IMPROVING THE FIDELITY THEREOF Filed April 4, 1955 3 Sheets-Sheet 5 SOUND RE PRODUCING 05 VIC E frequency range.

Patented Mar. 17, 1936 PATENT OFFICE HIGH-FIDELITY SOUND REPRODUCING AP- PARATUS AND METHOD THE FIDELITY THEREOF Harold A. Wheeler, Great Neck, Vernon E. Whitman, Flushing, and Nelson P. Case and William 0. Swlnyard, Bayside, N. Y., assignors to Hazeltlne Corporation, a corporation of Delaware Application April 4,1935, Serial No. 14,600

17 Claims. Our invention relates to sound-reproducing ,apparatus and, whileit is of general application,

it is particularly suitable for use in connection with radio broadcast receivers.

In the satisfactory reproduction of sound, as from a radio broadcast receiver, two distinct problems arise. One is that of the fidelity of reproduction, that is, the uniform and distortionless reproduction of sound throughout the audible Among the important factors present to a greater or lesser'extent in the soundreproducing devices of the prior art tending to impairthe fidelity of sound reproduction, might be mentioned the non-uniform frequency-re-' sponse, characteristic of the primary sound-reproducing element; the resonant or non-uniform frequency characteristic of the housing or cabrnet in which such adevice is usually mounted; and interference patterns set up by waves'gerrerated by different portions of the sound-reproducing device.

A second important problem is that of the ef-- fectiveness of the device and the closely related problem of the distribution of the sound waves in their projection from the device to the audience,

usually involving the distribution over a room or auditorium. Among the important factors dis advantageously affecting any arrangements of the prior art in this respect may be mentioned the lack of proper direction of the sound waves from the device into the room; *the reflectionofthe sound waves from the walls, floor and ceiling,

giving rise to interference patterns; and the nonuniform angular, and improper vertical distribution of the sound waves as they are projectedinto 'the room, resulting in dead spots, that is, areas of very low sound intensity, accompanied by other areas of abnormally high intensity.

' stantlally reduced and which will reproduce device having an approximately uniform fre- It is an object of our invention to provide a sound-reproducing device in which the short-= comings of the arrangements of the prior art, discussed above, are largely eliminated or subsound waves with a maximum fidelity an'd effectiveness.

More specifically, it is an object of ourinvention to provide a high-fidelity sound-reproducing sated for externally to the sound-reproducing device, per se, preferably in the electrical network to which the device is connected.

or the casing cavity. v In order still further to improve the fidelity of vided a high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, such as a corner of a room. Within the casing is mounted a large-diaphragm type of sound-reproducing device disposed to discharge, preferably vertically, into the sound cavity at a point displaced from the vertex. sound cavity includes means for directing or defleeting the sound waves generated by the device outwardly from the vertex with a substantially uniform angular distribution and a proper vertical distribution, as determined by the physical relationship of the sound-reproducing device to an audience.

More specifically, in accordance with our invention there-is provided a high-fidelity soundsubstantially to the corner of a room so that the walls and floor or ceiling constitute effectively a continuation of the walls of the sound cavity. A partition separates the cavity from the rest of'the casing and a large-diaphragm type sound-reproducing device is disposed in an opening in the partition to discharge vertically into the cavity. A conical deflector is positioned between the sound-reproducing device and the opening of the sound cavity and is effective to direct outwardly from the vertex. the sound waves generated by the device. One or more additional segmental coni- Thev cal deflectors and a plurality of vertical fins radi- "ating from a point of the interior of the cavity substantially on the axis of the sound-reproducin'g device effect a uniform angular distribution and a proper vertical distribution of the sound waves generated by the device into the room-0r auditorium in which the apparatus is placed. The remaining portion of the casing comprises a rigid, substantially anti-resonant structure lined with a sound-absorbent material. A large mass of sound-absorbent material is disposedln the end of the casing opposite the sound cavity in the form of a pyramid having as its base the end of the casing and having its vertex in the corner including the vertex of the sound reproduction ofapparatus embodying our invention the electrical transmissionnetwork supplying energy to the sound-reproducing device is compensated for resultant or residual lack of unir formity in the frequency-response characteristic of the apparatus by associating therewith. an

auxiliary network. Such a 'network may comprise oneor more series resonant circuits connected in parallel and having natural frequencies orresponding tothe peaks of the frequency-respouse characteristic of the device and one or more parallel resonant circuits having natural frequencies corresponding to the dips of the frequency-response characteristic.

For a better understanding of our invention,

together withother'and further objectsthereof,

reference is had to the following description, taken in conection with the accompanying drawings, and its scope will be pointed out in the appended claims.-

Referring now to the drawings, Fig. 1 is a view in perspective of "a sound-reproducing apparatus" constructed in accordance with ourinvention; Fig. 2 is a longitudinal section of the apparatus ofFig. 1 taken along the lines 2-.-2 of 'Fig. 4; Figs, 3 and 4 are cross-sectionalviews of the ap paratus of Fig. 1 taken along thelines 3-8 and. 4-4, respectively, of Fig. 2; Fig; Sirepresents certain operating characteristics of the apparatus of Figs. 1-4 inclusive,- and certain corrections applied thereto in accordance with our invention; while Fig. 6 is a'schematic circuit diagram of a complete radio broadcast receiver, to which the sopndereproducing-apparatus of our inven tion may suitably be-applied, including a network for compensating for any residual lack of uniformity in the frequency-response characteristic of our improved sound-reproducing apparatus.

Referring now more particularly to Figs. 1-4,

inclusive,- of the "drawings, there .is shown a sound-reproducing apparatus embodying our. inv vention comprising a casingor housing l terminating at its lower end in a'sound cavity I I which is separated from the upper portion. of the easing by a partition I2. An opening Ila is pro-' vided in the partition in which or over which is mounted a sound-reproducing device or element I 3,0! the large-diaphragm type. By the term large-diaphragm type sound-reproducing de me is meant a device having a relatively large vibrating diaphragm, usually conical, which, because of its size, produces directly sound waves of sumclent intensity for usual purposes. Such a device is to-be distinguished from the small-dia- 1 phragm type of sound-reproducing device which is useful only in connection with a sound-amplifying device, such as a horn. The sound cavity II is'open on two sides while its two rearsides terminate in the comer Il a, shaped to conform corner of the casing I0, as indicated at, Illa.

to the vertex of a trihedral angle,-such asthe corner of;a room, the intersections of the walls of such a 'room being-indicated schematically bythe lines I4 of Fig. 1. with such' an arrangement the walls and floor or ceiling constitute effectively an extension of the sound cavity -II and secure the maximum effectiveness .of the sound-reproducing device I3. I

In order properly to direct and uniformly to distribute sound waves 'generatedby the device I3, there is disposed in the sound cavity. II a conical deflector I the axis of which lies in the vertical through the vertex of the sound cavity II and the base of which is'substantially 'coextensive with the base of the sound cavity II. To

this end, and also for purposes described hereinafter,- it may be desirable to-cut of! the front angle of the conical deflector I 5 is not critical but is determined, among other considerations, by the height of the partition I:

above the base of the should be such as todirect the sound waves gen-- sound waves.

cavity I I and, preferably,

erated by the device I3v substantially to the head of a listener located in the center of the room in which the apparatus is to be utilized. This vertical angle is preferably of the order of 130, although it may bevaried within limits of about 120 from this value. The deflector I5 serves principally to reflectthe downwardly projected sound waves outwardly into the room.

prove, the angular distribution of the sound waves, there are also provided a plurality of vertical fins, in this instance the two flns I1 and I8,

extending between the conical deflector I5 and the partition I2 and radiatingoutwardly from r substantially the center of the sound cavity ,and/or center of the sound-reproducing device IS.

The fins I1 and I8 may terminate in a fin 19 extending to the rear comer of the sound cavity I I.

The conical deflectors I5 and I6 and the verticalflns I I, I 8 and I9 must be of suflicient rigidity to prevent substantial mechanical vibration in the audio-frequency range, and to'thise'nd they may comprise sheets of moderately heavy gauge metal, welded, soldered, or otherwise fabricated into a unitary structure.

The upper portion of the casing lfl above the partition I2 constitutes av closed sound-absorb ing chamber to prevent'interference'of the sound waves generated at the rear of the diaphragm of the device I3 with the main sound waves which it is desired to utilize. This portion of the casing is also preferablyconstructed of relatively heavy.

rigid, anti-resonant walls and may include brac- I ing structure to assist in securing this characteristic. This portion of the casing is substantially completely lined with a sound-absorbing sheet material 20 which may be of any suitable type, as, for example, a fibrous sheetmaterial commercially available as Acousti-Celotex.' In addition,

a screen or other sonnd-pervious sheet-or strip 2I divides off from the upper portion of the 09.5-- ing a pyramidal cavity the baseof which constitutes the top of theca'sing III and the vertex of which lies at the rear corner of the casing; that is, in a vertical through the vertex of the sound cavity I I.. This chamber 22 is filled with a sound-' absorbent material such as rock wool or slag wool,

and, if desired, a fabric covering may be provided for the screen 2| to retain dust, dlrt and small particles of the sound-absorbent material.

It is believed that the operation of our improved sound-reproducing apparatus will be clear from the foregoing description. In brief, the sound waves generated by the device I3 and projected downwardly into the sound cavity II are deflected or reflected from the conical deflector I5 outwardly into the room. The conical configuration of the deflector I5 tends to produce substantially uniform angular distribution of the As stated above, the inclination of the deflector I5 should preferably be suchas to deflect the sound waves upwardly at such anangle as to reach the head of a listener in substantially the center of a-room in which the apparatus is to be utilized; This result maybeseto provide a more equal or uniform angular distribution of the sound waves, the deflector I5 being effective also to increase the uniformity of the vertical distribution of the sound waves. as they leave the sound cavity I I. It has been found that the fins l1 and I8 and deflector l5 are particularly effective over a range of audio frequency of from 600 to l200cycles.

in a particular case.

The portion of the casing 10 above the sound cavity I! is effective to absorb sound waves generated by the rear of the diaphragm of the device l3, and this absorption should be effected without mechanical vibration of any of the casing structure. Such mechanical vibration is sub stantially avoided, as described above, by utilizing relatively rigid walls, with or without bracing, and by lining the walls with a sound-absorbent material. The resonant properties of the casing avoided by this means should be distinguished; however, from the resonant properties of the enclosed body of air. Resonance of the body of air is minimized by the pyramidal mass of sound-absorbent material 22, which renders the portion of the casing above the baiiie I! of non-uniform depth throughout its cross-sectional area, and by the beveled portion liia of the easing. This beveling of the front corner of the casing substantially eliminates any parallel surfaces in this portion of the casing and thereby substantially eliminates the resonance effects or standing waves; facilitates high-angle reflection and aids in the'better utilization of the corner space of the room.

'The dimensions, both relative and absolute, of the above-described apparatus will be determined largely by therequirements of each particular type of installation, although the following relationships represent those found to be satisfactory The included angle between the side walls 01 the sound cavity is, of

course, that determined by the trihedral angle constituting the corner-of the room in which the apparatus is to be used, and is usually 90. The vertical opening of the sound cavlty- H is preferably of the order of magnitude of the diameter of the diaphragm of the device l3. The total height of the housing It is not critical but is preferably atleast 5 or 6 times the diameter of the diaphragm and should provide sufilcient volume to build up the low notes generated by the device. The diaphragm of the-device I3 is preferably parallel to the floor, although it maybe inclined slightlythereto, the inclination of the. deflectors l5 and i6 being correspondingly modified.

While the casing l0, and'particularly the soun cavity H thereof, has been illustrated as conforming substantially to a righttrihedral angle such as the corner of a room, it will be apparent 1 'that it may conform to any polyhedral angle, or, in general, to any conical shape, in case it is to" verted with the sound cavity immediately adjacent the ceiling and the same principles of oper ation will govern the performance of the apparatus.

The frequency-response characteristic of the sound-reproducing apparatus described above,

as determined by a microphone situated in such a way with respect to the apparatus as to simulate normal operating conditions, is shown by curve A of Fig. 5. From this curve it is seen that the fidelity of reproduction is reasonably uniform over the audible frequency range of from 70 to 6000 cycles. In this same figure, curve B represents a correction curve which, if applied to the apparatus whose characteristic is represented by the curve A, would produce absolutely uniform frequency response. The curve B is, obviously, the inverse of the curve A. Curve C, in this same figure, represents a correction curve obtainable by a frequency-selective network, to

. be described more particularly hereinafter, ap-

plied to the electrical transmission system to which the sound-reproducing device is connected. It is seen that this represents a very close approximation to the ideal correction curve B. The resultant of curves A and C is shown in curve D, which represents the frequency-response characteristic of a complete system embodying our invention. The average of the curve I) has been moved upwardly from'the other curves to avoid confusion and since it is relative, and not abso-' lute, values of the response which are important. It is seen that the curve D does not depart from its average value more than about i3 decibels.

An electrical transmission system by means of which the characteristics of Fig. 5 may be obtainedis shown in Fig. 6 of the drawings, which is a schematic diagram of a complete radio broadcast receiver modified in accordance with our invention. This system includes an input circuit 30 for connection between an antenna and ground, a radio-frequency amplifier 31, an oscillator-modulator 32, an intermediate-frequency amplifier 33, and a detector 34, all coupled in-cascade. The output of the detector 34 is fed to an audio-frequency amplifier including the vacuum tubes 35 and 36 connected in any conventional manner, the circuits of which form 'no part of our present invention. The audio-frequency amplifier is, in turn, coupled to a pushpull power amplifier including vacuum tubes 31 and 38 and associated circuit elements, also connected in a conventional manner, which, in turn, are connected to sound-reproducing device 39, which is of the type illustrated in, and described in connection with Figs. 1-4 inclusive.

In order to modify the frequency-response characteristics of the above system to. produce the correction represented by the curve C of'Fig. 5, a frequency-selective network 40, represented by the dotted outline, is included in the cathode circuit of the vacuum tube 35. This network comprises a parallel resonant circuit consisting of an inductance 4| and a condenser 42, shunted by a resistor 43, connected in series with a second parallel resonant circuit consisting of an inductance 44 and a condenser 45. Connected in parallel with the two serially connected parallel resonant circuits just described, through a blocking condenser 46, are the pairs of series resonant circuits comprising inductances 41,49, 5i and 53 and condensers 48, 50, 52 and 54, respectively. Further, there is connected in parallel with the voice coil or the sound-reproducing device 39 a frequency-selective network 55, represented in dotted outline, comprising series resonant in-. ductance 56 and'condenser 51.

It will be understood that the impedance constants of -the several elements of the above-dc: scribed frequency-selective networks will be dependent entirely upon the particular frequency- I apparatus, per se.

response characteristic of the sound-reproducing v 13y way of example only, as illustrative of a frequency-selective network for producing the'correction curve C of Fig. 5, the frequency-selective networks may have the following values:

Inductance 41 .12 henries Inductance 44 75 henries Inductance 47 1'7 henries Inductance 49 5.8 henries Inductance 51 5 henries Inductance 5 3 1.5 henrles Inductance '56 88 microhenries' Condenser 42 .027 microfarads Condenser .15 microfa'rads Condenser 46 .01 microfarads Condenser 48 .5 microfarads Condenser '50 ".07 microfarads Condenser 52 .007 microfarads Condenser 54' .006 microfarads Condenser 57 3 microfarads Resistor 43 3000 ohms.

, As indicated in the drawings, the parallel resonant circuit Al, 42 produces the dip in curve C at'2800 cycles; the parallel resonant circuit 4 5, 45 produces the dip at 150 cycles; the series resonant circuit 56, 51 in the circuit of the soundreproducing device produces a desired dip at 10,000 cycles. 0n the other hand, the series resonant circuit 41, 48 produces the peakat 55 I cycles; the circuit 49, produces the peak at 250 cycles; the circuit 5|, 52 produces the peak at 850 cycles; the circuit 53, 50 produces the peak at 1700 cycles; and the output circuit including the reactor and condenser 51 produces the The circuit constants for procuring the correction curve C may be determined empirically or computed in any of several well known manners. Fromthe foregoing'description it is apparent that, in accordance with our invention,

- there is provided a sound-reproducing apparatus having a nearly uniform frequency-response characteristic, together with a method of, and

apparatus for, compensating for any residual non-uniformity in the frequency-response characteristic within any desired limits, whereby the deviation of the frequency-response curve from its average value may be maintained within any assigned error, by proper design of the frequencyselective network.

.' tions as fall within the true While we have described what we at present consider the preferred embodiment of our invention, it will be obvious to those skilled in the I of a trihedral angle, as a corner of a room, a large-diaphragm type sound-reproducing device disposedto radiate sound waves into said cavity at a point displacedfrom said vertex, and means for directing the sound waves radiated by said 2. A high-fidelity sound-reproducing apparatus comprising a'casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a cornerof a room, a large-diaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity, and means for directing outwardly and distributing uniformly the sound waves radiated by said device.

- 3. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a largediaphragm type sound-reproducing device disposed to radiate sound wavesapproximately vertically into said cavity, and aconical deflector positioned below said device and effective to direct and distribute sound waves radiated by said device outwardly from said vertex.

4. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, asa corner of a. room, a largediaphragm type sound-reproducingdevice disposed to radiate sound waves approximately vertically into said cavity, and rigid conical deflecting means positioned below said device and efiective to direct and distribute sound waves radiated by said device outwardly from said vertex.

5. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a largediaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity, and a rigid conical deflector having its axis lying in a vertical intersecting said vertex and substantially coextensive with said cavity, said deflector being efiective to direct and distribute outwardly from said vertex the sound waves radiated by said device.

6. A high-fidelity sound-reproducing apparatus comprising a casing including a'sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a largediaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity, and a rigid conical de-y fiector having a vertical 'angle of the order of and disposed with its axis lying'in a vertical intersecting said vertex, said deflector extending substantially to the periphery of the bottom of said cavity and being efiective to direct and distribute outwardly from said vertex the sound waves radiated by said device.

7. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shapedto conform substantially to the vertex of a trihedral angle, as a. corner of a room, a largediaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity,- and a rigid conical defiector having a'vertical angle of from 110 to and disposed with its axis lying in a vertical' intersecting said vertex, said deflectorbeing efl'ective to direct and distribute outwardly from said. vertex the sound waves radiated by said device.

8. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner 01' a room, a largeinto said cavity," the vertical depth of said cavity diaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity, a conical deflector positioned below said device and effective to direct outwardly from said vertex sound waves radiated by said device, and additional means for effecting uniform angular distribution of said deflected waves.

9. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavityshaped to conform substantially to the vertex of a trihedral angle, as a comer of a room, a large-diaphragm type sound-reproducing device disposed to radiate sound waves approximately vertically into said cavity, a conical deflector positioned be-' low said device and effective to direct outwardly from said vertex sound waves radiated by said device, and a plurality of fins radiating from the interior of said cavity effecting-a uniform angular distribution of said-deflected waves.

10. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a largediaphragm type sound-reproducing device dis-.

mental deflector and said fins'being effective to secure uniform angular distribution of said deflected waves.

y 11. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a large- Y diaphragm type sound-reproducing device discluding a segmental conical deflector parallel to said first-named deflector and a plurality of vertical fins, said segmental deflector and said fins radiating substantially from the axis of said diaphragm to the periphery of said cavity.

12. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a horisontal partition defining the upper boundary .of said cavity, a large-diaphragm type sound-reproducing device disposed in an opening in said partition and radiating sound waves vertically into said cavity, and means for directing outwardly and distributing uniformly the sound waves radiated by said device.

- 13. A high-fidelity sound-reproducing apparatus comprising a casing including a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a horizontal partition defining the upper boundary of said cavity, a large-diaphragm type sound-reproducing device disposed in an opening in said 7 partition and radiating sound waves vertically being of the order of the diameter of said diaphragm, and means'for directing outwardly and distributing uniformly the sound waves radiated by said device.

14. A high-fidelity sound-reproducing apparatus comprising a casing terminating in a sound cavity shaped to conform substantially to the vertex'of a trihedral angle, as a corner of a room, a partition separating said cavity from the rest of said casing, a large-diaphragm type sound-reproducing device disposed in an opening in said partition to radiate sound waves substantially, vertically into said cavity, means for directing the sound waves radiated by said device outwardly from said vertex, and means for rendering the casing above said partition substantially non resonant.

'15. A h gh-fidelity soimd-reproducing apparatus comprising a casing terminating in a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as 'a corner of a room, a partition separating said cavity from the rest of. said casing, a large-diaphragm type soundreproducing device disposed in an opening in said partition to radiate sound waves substantially vertically into said cavity, means for directing the sound waves generated by said device outwardlyfrom said vertex, and means for rendering the casing above said partition substantially non-resonant, comprising a lining of sound-ab- .sorbent material and a mass of sound-absorbent material at the opposite end of said casing from said cavity and formed with a surface inclined to said partition.

16. A high-fidelity sound-reproducing apparatus comprising a casing terminating in a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room,

a partition separating said cavity from the rest of said casing, a large-diaphragm type sound-reproducing device disposed in an opening in said partition to radiate sound waves substantially vertically into said cavity, means for directing the sound waves radiated by said device outwardly from said vertex, and means for rendering the casing above said partition substantially anti-resonant, comprising a lining of sound-absorbent material and a mass of sound-absorbent material disposed to form a pyramid having as its base the end of said casing opposite said cavity and having its vertex at the corner of said casing including the vertex of said cavity.

17. A high-fidelity sound-reproducing appara.. tuscornprising a casing constructed with sufiicient rigidity as to be substantially anti-resonant and terminating in a sound cavity shaped to conform substantially to the vertex of a trihedral angle, as a corner of a room, a horizontal partition separating said cavity from the rest of said casing, a lining of sound-absorbent material for the lastmentioned portion of said casing, a sound-pervious screen dividing a pyramidal chamber out of said casing having as its base the end of said casing opposite from said cavity, a mass of soundabsorbent material filling said chamber, a large diaphragm type sound-reproducing device disposed in an opening in said partition and radiating sound waves substantially vertically into said cavity, and means for directing outwardly from said vertex sound waves radiated by said device.

. HAROLD A.-WI-IEEI.-ER. VERNON E. WHITMAN. NELSON P. CASE. WILHAM O. SWINYARD.