US2031500A - Sound reproducing system - Google Patents

Description

Feb. 18, 1936. B. oLNl-:Y 2,031,500

SOUND .REPRODUCING SYSTEM Filed Sept. 1'7, 1934 2 Sheets-Sheet l ATTOR N EY Feb. 18, 1936. B. OLNEY SOUND REPRODUCING SYSTEM 2 sheets-sheet 2 Filed Spt. 17, 1954 mwN mami

INVENTOR BENJAWHN OLNEY. BY Wm ATTORNEY Patented Feb. 18,1936

`UNITED STATES PATENT OFFICE l manson soUNn nEPnooUcmG SYSTEM New York Application September 17, 1934, Serial No. 744.382

1i) Claims.

'Ihis invention relates to sound reproducing systems.

When a loud speaker of the open radiator type such as a cone type loud speaker is mounted in a cabinet, the sound reproduction thereof is always marred by so-called cavity resonance which manifests itself in a disagreeable boominess and an apparent lowering of the pitch of speaking voices. This is due to the fact that a loud speaker cabinet, although open at a side or an end, otherwise encloses a column of air at the rear of the loud speakerand therefore acts like any wellknown resonator in which the resonance is due to the reaction between the mass andthe stiffness of a partially enclosed column of air. The resonant frequency and the intensity of resonance of such a cabinet varies with its position relative to the walls of the room and surrounding objects, the resonant frequency Vbeing lowered and the strength of the resonance being augmented when the back of the cabinet is -placed close against a wall. vided with an open radiator type of loud speaker may be minimized by careful cabinet design, in

so far as applicant is aware, such resonance has never been completely eliminated. Y It is recognized that a. cone type loud speaker mounted in a baille of limited size is very inefficient in handling power at low frequencies due to the fact that the loud speaker has an insufficient acoustic load at certain of the lower frequencies and consequently, at these lower frequencies, the loud speaker may chatter as a resuit of its moving coil striking the pole pieces of the speaker on relatively large surges of current in the moving coil such as occur when loud passages of music or the like are reproduced.

Commercial open radiator or cone types of loud speaker have a poor response at the lower frequencies, notably frequencies below one hundred cycles, so thatbass voices and the music from bass instruments are only weakly reproduced thereby at such frequencies.

In accordance with the main feature of the present invention, means are provided in a sound reproducing system which substantially eliminates the air cavity resonance effect of a loud speaker cabinet .forming a part of such system.

An additional feature of the invention relates to means in a sound reproducingsystem for suppressing or absorbing substantially. all of the sound waves of higher frequencies transmitted -from one surface of an exposed radiator such-as a cone type loud speaker incorporated'in said system. while discharging into free air, certain un- While cavity resonance inV a cabinet proabsorbed low frequencies transmitted `from said surface.

A further feature of the invention relates to a loud speaker system provided with a conduit or passageway lined with sound absorbing material and having substantially the form of a labyrinth into which the sound Waves radiated from one surface of a sound radiator of the loud speaker,

are discharged.

Still a further feature of the invention relates to a loud speaker system provided with a conduit suitably lined with sound absorbing materialand of such length as to resonate at those lower frequencies at which the loud speaker is an inefficient reproducer so that' the low frequency response of the loud speaker is. extended.

These and other features of the invention will appear from the detailed description and claims when taken with the drawings in which Fig.- 1 is a fragmentary plan view of a cabinet containing the sound reproducing arrangement of the present invention; Fig. 2 is a vertical section section thereof taken substantially on the line 2'-2 of Fig. 3; Fig. 3 is a vertical sectional view taken substantially on the line 3-3 of Fig. 2; Fig. 4 is a chart showing the differences in response of a given sound reproducing system with and Without the arrangement disclosed in the foregoing figures; Fig. 5 is a fragmentary plan view of a cabinet containing a modified type of sound reproducing system; Fig. 6 is a vertical section view thereof taken on the line 6 6 of Fig."7; andFig. 'l is a sectional view taken substantially on the line'l-l of Fig. 6. While the invention may have various embodiments, one form thereof is illustrated in Figs. 1, 2 `and 3 in which the sound reproducing system is housed in a cabinet made of wood or like materiPl. This cabinet includes a top 5, sides 6 and l, and a front or baille 8, vthe rear and bottom of thisv cabinet being conveniently left open. A loud speaker of the open radiator type such as a dynamic cone type loud speaker El is mounted in the cabinet with the frame ill (Fig.` 3) of its diaphragm Il closely engaging the margin of an opening i2 in the baille 8 so that the front surface of the diaphragm transmits reproduced sound waves through this opening. It will be understood that. in the construction just described, which follows current practice, the rear surface of the diaphragm transmits sound waves'into the space within the cabinet. The top, sides and front Such a sound reproducing system is also deficient in thatit affords poor reproduction ofthe low frequencies notably those below. one hundred cycles as indicated at D in the response curve A of Fig. 4. This poor reproduction at such ylovv frequencies arises from the fact that it is imprac- -tical to provide a commercial cabinet having .ai

suiiiciently large baille area to prevent the circulation of airfrom the front surface ofA the diaphragm to the rear surface'thereof and vice versa. This poor'reproduction at the mentioned low frequencies also arises from the fact that prior loud speakers having an open Atype of radiator, could not be provided with a sufiicient acoustic load to prevent the moving coil of the diaphragm from striking the pole pieces of the actuating unit of the loud speaker on relatively large surges of current in the moving coil, such as occur in the reproduction of loud passages of music and the like. If the moving coil were permitted to strike the pole' pieces in the manner just described, itwould cause a rattling or chattering sound which would be highly unpleasant to the listener. In -order to prevent such rattling of the loud speaker, ithas been the common pracitice-to provide such a rigid mounting for the `moving coil thereof that this coil could notstrike the pole pieces of the actuating unit. However, such a rigid' mounting prevented the diaphragm vfrommoving freely in response to the currents'of lower frequency and consequently vthe sound reproducing system had a. poor low frequency response.

In accordance with the present invention an arrangement is provided which' eliminates the disadvantages of such former sound reproducing systems, While this arrangement maytake various forms', as shown in Figs. 1, 2 and 3,it com'- prises a housing made in two parts designated I 3. I3 which with the bracket III" forming a .part of the frame of the'lo'ud speaker completely encloses.

the rear surface of the diaphragm I I so that none of the sound waves 'from the rear surface of this diaphragm are transmitted into the space within the cabinet. It will be noted that .the interior of /the housing comprising the parts I3, I3, is lined with felt Il or other suitable sound absorbing material. The parts I2 and I3 respectively communicate with the conduit I1 through theslots or openings I5 land I6 which are symmetrically placed with respect to therear surface of the diaphragm II Substantially all of the inner sur- Y faces of this conduit are lined with a sound absorbing material I8 such as rock wool or other -material of like characteristics. Since it is desir- 'able to have the conduit I1 of relatively great length and since it is highly desirable in commercial cabinets that they be of relatively small sue. the conduit is herein shown as being folded back and forth upon itself, in other words, it com'- prises a circuitous passageway. The conduit I1 discharges into free air through a slot t9 at the front surface of the diaphragm to .the rear surface thereof and vice versa.

It should bementioned `,that the conduit is preferably made of uniform rectangular cross section since by this arrangement a maximum area of sound absorbing material can be brought in contact with sound waves in any given cabinet.

the air in the conduit may not become suflicientr 1y great to. interfere with the motion of the loud speaker diaphragm at low frequencies. This cross sectional area is also made large because of the fact that in a pipe of large proportions,

the overtones are thrown somewhat out'of tune with the' pipe and are not readily excited therein. The cross section of the con`duit and consequently, the shape of its orii'lce is in a form of an elongated sl`ot which provides poor efciency for the radiation of low frequency sounds. 'This reduces the radiation which occurs at 'the fundamental resonance of the conduit. As a result of' the aeoustically remote positioning of the orifice I9 with respect to the front surface of the diaphragm, the loss of lowfrequenciesdue to circulation between the two surfaces of the diaphragm is largely prevented. While there/can be reasonable variations in the cross sectional area of the conduit and its length, it has been found empirically that a conduit having -a cross-section of practice by the amount of room available in the cabinet: with some sacrifice in results, but the ,ideal length however, is suchthat the fundamental resonanceof the pipe occurs just below the frequency at whichthe loud speaker cabinet be- .cmes increasingly ineffective as a baille as diagrammatically indicated by the portion D of the response curve A of Fig. 4. By 'proper proportioning of the'dimensions of the conduit, this reson- `ance may be caused to extend, in a smooth manner, the response of the loud speaker at the lower frequencies where ordinarily the efilciency would be poor due to limitations in bale size. 'I'he sound absorbing lining of theA conduit together with the' poor radiation afforded by the slot shaped orifice I9 prevent the fundamental resonance from being excessively strong and also serve to broadenA this resonance considerably. As aresult of adding the present invention to the cab- `inet previously referred to, the response of the sound reproducing system is transformed from its condition as indicated in the broken line response curve' of Fig. 4 tothe condition indicated by the response curve B illustrated in full lines in this figure. From this chart it will. be seen that the low frequency response of the system has been greatly extended in rangeand the resonant or peaked frequencies have beeny largely reduced so that the system affords/a uniform o'r substantially straight line response over an extended frequency range. v

It has been mentioned that the openings IB and I6 in the housingare symmetrically placed with respect to the rear surface of the diaphragm.

This is done so that the diaphragm may yniet be driven out of line due to an unbalanced acoustic load.

It should also be mentioned that experiments thus far conducted indicate that the acoustical impedance presented to the rear of the diaphragm by the conduit I1, at very low audible frequencies has the characteristics of a mass reactance which is large as compared with that of said diaphragm including the acoustic load on its freely transmitting side, so that it is possible to greatly reduce the rigidity of the center mount ing of the diaphragm Il without its moving coil lstriking the pole pieces on relatively large surges of current. This reduced rigidity of the mounting permits the diaphragm to respond more freely at the mentioned lower frequencies. This acoustic impedance at one low audible frequency lined with felt 32.

has the characteristics of a pure resistance while at audible frequencies higher than the last-mentioned frequency, this impedance has the characteristics of a compliance reactance associated with a resistance, the magnitude of the resistance component with respect to the compliance component becoming greater as the frequency increases and becoming predominant at the higher audible frequencies. These characteristics of the mentioned acoustic impedance together with the resonance of the conduit l1 at the lower frequencies explain the greatly improved lower frequency response of the present sound reproducing system.

In certain sound reproducing systems where it is desirable to provide aloud speaker having a diaphragm of `greater diameter than that illustrated in Fig.v 1, it may not be convenient to arrange the conduit in the manner shown in Fig. 3. Under such conditions, a modified form of the invention as shown in Figs. 5, 6 and 7 may be employed. As before the system is housed in a cabinet having a top 2l, sides 26 and 21,' and baffle of well-known construction which cabinet can conveniently be open at the rear and the bottom. In this arrangement the loud speaker which includes the diaphragm or radiator 28, is suitably mounted behind their-ont` or baille of the cabinet with the front surfaee'jgf the diaphragm transmitting sound waves generated thereby through the diaphragm 28 is enclosed'in a housing generally designated 3i which-housing is suitably This housing has openings 33 and 33 respectively communicating with two separate conduits 34 and 35. Each of these conduits is lined with a sound absorbing material 38 such as rock wool or the like and is folded back and forth upon itself as a circuitous passageway in. order to afford relatively great length' for the conduit while still maintaining it in compact form. These conduits respectively open into free air through rectangular slots 36 and 31 in the bottom o f the cabinet. This arrangement of these mentioned slots effects the discharge of the sound waves from the rear surface of the diaphragm as acoustically remote as'possible from the front surfaceof`the diaphragm as is consistent with a given cabinet size. these conduits preferably hasits dimensions comparable with those of the conduit of Figs. 1, 2 and 3.

Listening tests of the above systems show an entire lack of the customary boominess associated with cabinet loud speakers and it has been found that the naturalness of the reproduction has been greatly increased.l

The rear surface of Each 'of In addition to the improvement in reproduction described above, there are other advantages to be had from the use of the system, of the present invention. The loud speaker cabinet may be placed tightly against the wall without changing its operating characteristics or it may be used, for example, on a stage where the acoustics at the back of the loud speaker may be poor and which would give inferior results with the loud speaker radiating freely from both sides of its diaphragm. 0n account of the loud speaker radiating from one side only of its diaphragm over the greater part of the frequency range, the standing wave pattern in the room due to the interference of sound waves is greatly simplified. Furthermore, since sound is not being discharged into the back of the cabinet, the likelihood of acoustic coupling due to the mechanical vibration of the vacuum tubes and condenser plates in the radio receiver chassis which is commonly mounted within the cabinet, is greatly reduced. For a like reason there is much less trouble from rattling of the cabinet and the radio receiver chassis parts, due to acoustic driving.

What I claim is:

1. The method of improving the sound reproduction of an open diaphragm type of loud speaker which comprises transmitting to the atmosphere substantially unmodified sound Waves from one surface of said diaphragm, absorbing in a folded passageway by `multiple reflection from the surface of material having a high degree of acoustic absorption, the major portion of 'the range of audio frequencies of sound waves transmitted from the other surface of said diaphragm, and simultaneously resonating a certain remaining portion of the frequency range of sound waves transmitted from said last-mentioned diaphragm surface'.

2. The method of improving the sound reproduction of an open diaphragm type of loud speaker which comprises transmitting substantially unmodified sound waves to the atmosphere from one surface of said diaphragm, absorbing substantially al1 frequencies above 100 cycles `transmitted from the other surface of said diaphragm, resonating certa-in of the unabsorbed frequencies radiated from the last-mentioned surface of the diaphragm, discharging the resonated frequencies at a point remote from the first-mentioned surface of said diaphragm. and utilizing the resonated frequencies to augment the response of the loud speaker at certain frequencies.

3. In vcombination with a loud speaker having an exposed radiating diaphragm and actuating means therefor, a housing enclosing one surface only of said diaphragm, the other surface of said diaphragm being in free and unrestricted communication with the atmosphere, and a sound damping conduit communicating with the interior of said housing and extending therefrom to free air at a, point remote from the other surface of said diaphragm, said conduit comprising naterial having a high degree of acoustic absorpion.

4. In combination with a loud speaker having an exposed radiating diaphragm and actuating means therefor, a, housing enclosing one surface only of said diaphragm the other surface of said diaphragm being in free and unrestricted communication with the atmosphere and a folded sound absorbing conduit of substantial length opening in one wall thereof, a loud speaker having an exposed radiating diaphragm one surface of which is arranged to transmit sound unrestrictedly through said opening to the atmosphere and actuating means for said diaphragm, alhousing enclosing the other surface of said diaphragm and a sound damping structure within the cabinet defining a circuitous sound passageway extending from the interior of said housing to free air at a pointflocated as accustically remote as possible from Athe first mentioned surface of the diaphragm consistent with the size of the cabinet.

6. In combination with a loud speaker having an open type of radiating diaphragm and actuating means therefor, a housing enclosing one surface only of said diaphragm the other surface of said diaphragm being in free and unrestricted communication with the atmosphere, and a circuitous sound damping conduit communicating with the interior of said housing and extending therefrom to free air, the cross sectional area of said conduit being at least equal to one-half of the projected area of said diaphragm.

7. In combination with a loud speaker having an open radiating diaphragm and actuating means therefor, a housing enclosing one surface only of said diaphragm. the other surface of said diaphragm being in 'free and unrestricted communication with the atmosphere, and a circuity ous sound damping conduit lined with sound absorbing material communicating with the interi-or of said housing and extending therefrom to free air.

8. In combination with aloud speaker having an open radiating diaphragm and actuating means therefore, a housing enclosing one surface only of said diaphragm, the other surface of said Adiaphragm being in free and unrestricted communication with the `Iatmosphere, and a pair of sound damping condits opening into said housing symmetrically with respect to said surface of said diaphragm and communicating with freeair,

said conduits having their inner surfaces treatedl point outside of said cabinetsaidlconduit in-y cluding sound damping material and having a length greater than the greatest dimension of said cabinet whereby sound radiated from the other surface ofy said' radiator is not discharged within the enclosure of said cabinet. g

10. In-a sound reproducing system, a cabinet having anopening in one wal thereof, a. loud speaker mounted in said cabinet and including a radiator the front surface of which radiates sound only through said opening in free and unrestricted communication with the atm'osphere, and a folded conduit for resonating and conducting certain of the frequencies of the sound radiated from therear surface of said radiator to apoint acoustically remote from saidffront surface.

11. In avsound reproducing system, a cabinet having an opening in one wall thereof, a loud speaker mounted in said cabinet and including a radiator of the type which radiates sounds from both surfaces thereof. the front surface of said radiator radiating -unmodified sound only through said opening to the atmosphere, and a folded conduit for conducting sound radiated from the rear surface of said radiator toa point acoustically remote from said surface, said conduit being lined with acoustically absorbent material.4

12. In a sound reproducing system, a cabinet having an opening in one wall thereof, a loud speaker mounted 4in said cabinet and including .a diaphragm of the type which radiates sounds from both of its surfaces, the front surface of said diaphragm radiating unmodified sound to `the atmosphere only through said opening, and aconduit of uniform cross section throughout its length for conducting sound radiated from the rear surface of the diaphragm to a point acoustically remote from the front surface thereof, said conduit being lined with acoustically absorbing material.

13. In a sound reproducing system, a cabinet having an opening in one wall` thereof, aloud speaker mounted in said cabinet and including a diaphragm of. the type which radiates sounds from both of its surfaces, one of said surfaces radiating unmodified sound to the atmosphere only throigh said opening, and a conduit of uniform cross section throughout its length for couducting sound radiated from the other surface of the diaphragm to a point acoustically remote from said opening, said conduit having a cross section of which the ratio of length -to width thereof is greater than 1 to 3.7.

14,. Ina sound reproducing system, a cabinet having an opening in one wall thereof, a loud speaker mounted in said cabinet and including a diaphragm of the type which radiates sounds of effective conduit length in inches to cross sectional area in square inches less than 1 to 3L5.

15. In a sound reproducing system, a cabinet having an opening in one wall thereof, a loud speaker mounted in said cabinetv and including,

said diaphragm through said barile opening, ac-

tuating means for said diaphragm, and means including a conduit treated internally with sound vabsorbing material and leading from the other surface of said diaphragm to free air at a point remote from said first-mentioned surface of said diaphragm, said conduit having a relatively s'mall cross-sectional area as compared with the area of said baille.

17. In combination, a baille having an opening therein, a loud speaker including an exposed radiating diaphragm having one of its surfacesl adjacent said openingfor transmitting substanradiate unresonated sound to the atmosphere from said surface only through said opening, and means including a folded conduit having its inner surface treated with sound absorbing material and leading from the other surface of said diaphragm to free air at a point acoustically `remote from said first-mentioned surface.

19. In combination, a baille havingan opening therein, a loud speaker including an exposed radiating diaphragm having one of its surfaces `adjacent said opening transmitting unresonated sound therethrough to the atmosphere, actuating means for said loud speaker, a housing enclosing one surface only of said diaphragm and a. folded sound absorbing conduit of substantial length leading from the interior of said housing 'to an opening discharging into substantial free air, the general plane of the margin of said discharge opening extending in a direction substantially at right angles to the principal plane 15 of said baffle.

BENJAMIN OLNEY,

Images