US2552309A - Acoustic diaphragm and baffle - Google Patents

Description

w. 1.. CARLSON ET AL ACOUSTIC DIAPHRAGM AND BAFFLE Filed Oct. l0, 1947 May s, 1951 May 8, 1951 w. CARLSON ET AL ACOUSTIC DIAPHR/acm` AND BAFFLE 3 Sheets-Sheet 2 Filed Oct. l0, 1947 May 8, 1951 W. CARLSON ET AL ACOUSTIC DIAPHRAGM AND BAFFLE Filed Oct. 10, 1947 DB HELAT/VE JOU/V0 RESPONSE 3 Shets-Sheet 5 FOR CI//PVE E FIL TER WIT/'l FIBER PACK/NG /VD 46005776 FILTER O/STA/VCE 7'0 MIC. =4FT.

:2.5 @HMS 24 0.5.

| l l l Ja Y fao 2a@ fw@ Z000 TABL HAD/0 JPEA/fE/-P RESPONSE (FREQUENCY) TABLE HAD/a :PEA/ffl? @f5/amfe- W/TH A/vo #wr/mar 6A wry (FREQUENCY) Inventors Patented May 8, 1951 UNITED STATES PATENT OFFICE ACOUSTEC DIAPHRAGM AND BAFFLE Wendell L. Carlson and Loy E. Barton, Princeton,

N. 3;, assignors toRadio Corporation of America, a corporation of Delaware Application ctcber 10, 1947, Serial No. 779,176

8 Claims.- l.

This invention relates to radio receiving apparatus, and more particularly to an arrangement for improving the acoustic characteristics ofthe sound output fromV receivers of small size.

The most common objection to the small, table model'receiver is its poor low-frequency response. Poor response is due largely to the small barile for the speaker. which allows the back and iront waves from the speaker to cancel each other. The principal object of this invention is to overcome this detect without increasing the size or interfering with the portability of the receiver.

Those skilled in the art have long recognized the problem of obtaining low-frequency sound output from dynamic speakers using the very lirnited balile area available in small cabinets. It has been proposed, and tried, to obtain a synthetic bass by' accentuating the'harmonics ofthe missing bass notes to give the ear Ythe impression of bass response. Attempts have also been' made to boost existing low frequenciesl by means of' selective regenerative feedback in the electrical'v These arrangements have' not proven' pension, having a much lower than normal natural resonance to allow for the stiffness added by the confined air in the small rear enclosure to the compliance of the diaphragm suspension. It has generally been considered impractical, if not impossible, to reproduce frequencies lower than the natural resonance oi the speaker, that is, below the point where the speaker diaphragm is mass controlled, in the stiiness controlledre-4 gion. This is true regardless of the size of the baille, contrary to the general supposition that low frequency reproduction is limited only bythe baille area used.

In accordance with the invention, the lowfrequency response of sound reproducing'apparatus using an ordinary dynamic speaker'is 'ex-v tended byenclosing the space to the rear ofthe speaker except for a restricted opening in the` basel and removably placing the opening overav corresponding opening in a table top or cabinet' havingW an acoustick device, the'back Wave'being In the latter case, it' is fedthroughV the-openings into said device; They invention is based upon threeassumptions:y (l) That, inthe home, small table modelreceivers-'are usually located more or less permanentlyI onapiece of iurnituresuch as an end table ;"(2) thatl a special tableor the like could be providedespe# cially for thispurpcseand 'so constructed in cornbinationwith the' receiver as to increase the efffective bailie area or otherwisev to increasethere'` model `with ordinary performance.

Another object of the invention is to provide" a small, portable radidreceiver having unusual perform ancer when'used iny onelocation in combination with auxiliary apparatus; which may readily'be disassociated from said auxiliary-'apparatus and transported to other locations for operation as a complete, small receiverwithatl least usual performance.

Another object ofY the invention is 'to-provide means forobtaining from an ordinary small; table model radio receiver an extended range of lowlfrequencyA sound output comparable' to that ob tained with a relatively large` receiver 'and'withv out using a special speaker-of lower resonant frequency than usual.

A further object of the' invention is to provide an' inexpensive low-pass acoustic lter forV a radio receiver, preferably using some of vtl1e"e1ki'st ing apparatusas 'parts of the filter, for'reducing high audio-frequency disturbances.

Astill further' object of the inventionis 'to providev a supporting table stand or' cabinet for' a table model radioA receiver, constructed vto im'" prove `the idelityV of the sound output as well as to increase the radio-frequencyinput to the"a`n`' tenna built within 'the receiver; particularly for operating within shielded' buildings," when the" receiver is used with the' table;

Other objects of the' invention and details of structure Will` be clear from the following descripu tion, taken in connection with the accompanyin'gy drawings, in which:

Fig; 1 is a View in perspective of a radio receiver and table comprising a preferred 'embodiment of' theinvention, the dotted lines showing the norrnalr operating positicnof the receiver, and

portions of the table legsbeing cut away'i Fig. 2 is'an elevationalview partly in cross'- section', o1" th'e apparatus shown' in Fig.t l', with" the receiver in operating' 'position on the table?" s a 2,552,309" o e 4 Fig. :i is a simplified electrical circuit diagram of the apparatus shown in Fig. 1;

Fig. Li is an enlarged view, in perspective, of the chassis of the radio receiver shown in Fig. 2;

Fig. is a side elevational view, partly in section, of a species of the invention;

Fig. 6 is a front elevational view of another species of the invention; and

Figs. '7 and 8 are actual characteristic curves of performance taken of the apparatus of Figs. 1 and 2.

Referring to Fig. 1, a radio receiver I, made in accordance with the. invention, is shown on top of a table 3, also embodying the invention, the normal operating position of the radio receiver being shown in dotted lines I. The table is provided with a hinged lid 5, which maybe closed when the radio receiver is not used in combination therewith, to present a substantially smooth table surface. For improving signal reception, a coil of wire 'I is embedded in the surface of the hinged lid and is connected, through the hinges 9, to antenna and ground, respectively, or in some cases to a dipole antenna. When the radio receiver is placed over the opening I I, for reasons hereinafter described, the loop antenna of the receiver, shown at I3 in Figs. 2 and 3, is adjacent the coil 'I for increasing the signal pickup. The latter feature is particularly desirable in shielded locations where a loop antenna loses its effectiveness and where an outside antenna is necessary. While only one turn 'I is shown, and while this would operate materially to increase signal pickup, actually several turns were used to obtain sufficient inductance to resonate the antenna circuit below the broadcast band.

For the purpose of obtaining an increased reproduction of low audio-frequency sound waves,

, and materially extending the low-frequency end of the range, a loudspeaker I3, of the moving coil, dynamic type, shown in Fig. 2, is provided, which has a natural resonance, as measured in a large baffle, substantially below the useful audio frequency range to be reproduced. The loudspeaker includes a direct acting diaphragm I5, and a highly compliant suspension system for the diaphragm whereby the diaphragm is controlled by mass reaction. The loudspeaker is mounted in a closed and acoustically sealed compartment I5 made up of a sheet metal housing Il and a rear enclosure Wall I9, preferably of plywood. The compartment is sealed around the rear surface region of the diaphragm I6, except for an opening 2l in the bottom thereof which is arranged to communicate I with a corresponding opening II near the top of the table when the radio receiver is positioned in a recess 6 above a sub-top 26, as shown. A felt gasket 25 provides an acoustic seal between the rear compartment I5 of the speaker and a closed cavity 30 below the table top formed by the top 23, bottom 29,- and side walls 2l. In effect, there are two compartments coupled acoustically through openings II and 2|.

The felt gasket 25 may be cemented to the chassis upon the base around opening 2l, as shown in Fig. 4, or it may be cemented to the sub-top` member 25. In the former case, the

compartment I5 is sealed when the receiver is mounted upon any fiat surface. In the latter case, there is leakage around the lower edge of the opening 2l through the Ventilating openings lll (Fig. 4) in the cabinet back 43 when the receiver is operated upon an ordinary flat surface. The leakage may be increased,V if desired, by spacing the receiver Vbase above the surface, as by means of a few books or by feet, in either of the two cases, depending upon results desired. The low-frequency response changes in the former case as the base opening is raised above a flat supporting surface, changing from a sealed, small back enclosure to a partially open rear enclosure. The felt seal may be entirely omitted, as in Figs. 1 and 2, it having been found that beneficial results still are obtained by spacing the openings 2I and I I a small distance apart.

For the purpose of minimizing resonant peaks and eliminating anti-resonant dips, shown below 1000 cycles in dotted lines in Fig. 8, the table cavity is filled with sound damping material 3l, such as fibre glass, loosely packed. Other ways of damping known to the art may be used if desirable, for example that shown in Carlisle et al. Patent 1,837,755, disclosing coupled compartments but involving a different acoustic function. The cavity has little or no effect on the response above 1000 cycles.

In accordance with the invention, the cubical contents of theV rear speaker enclosure or comdesired relation, the latter being much greater than the former. In an actual working model made in accordance with the invention, the cubical content-of the region to the rear of the speaker diaphragm, that is, the enclosed air, was about l0 cubic inches, whereas the cubical content of the cavity was around 900 cubic inches. The latter volume maybe increased for speakers having lower speaker resonant frequency. The speaker resonance in this case was cycles (Fig. 8), as measured in operation with a very large baiiie. The throat opening at 2I and II bears a certain relation to the cubical contents of the communicating chambers and the diaphragm area. The table cavity inside dimensions are 16" x 16" x 31/2, and the table was 23 inches high. In the model mentioned, the area of the throat or housing opening was about the same as the diaphragm area and measured Yabout 2.25 x 5". Experiments were made With-the throat opening smaller, and it was found that the effect was to lower the low-frequency resonance point, but there was some loss in amplitude of response. The speaker housing dimensions are, height 5, width 5, and depth 21A". The cabinet dimensions are, height 51A, width lOl/4, and depth 31/2.

Referring to Fig. 4, which shows the invention in greater detail, the metal chassis or plate 32 to which the speaker is secured at points 28, in front of the speaker diaphragm, is provided with a plurality of small holes 33 of the order of 1/8 in diameter, spaced about 1/2 from each other for the purpose of providing an acoustic filter in combination with a grille 2 (Figs. 1 and 2), preferably also made of metal and having holes of similar size and spacing. The diameter of the holes and the thickness of the metal determine the acoustic inertance.

4bursts, as shown to the'right in Fig. 8. The clot-V ted line curve C shows the effect of the output response without the filter and the solid line curve'D shows the effect of the lter.

The spaced per-` forate plates provide a low-pass filter for effec- The lter has little effect upon the low-frequency response .1f the holes are about the size showngorlarger,

The perforate plate 32 serves also during production to protect the conical paper diaphragm from injury, such as often occurs in cases where the diaphragm is unprotected on the chassis during production, and in use the plate 32 may take the place of a grille cloth.

It will be observed that certain of the radio components are mounted upon the chassis within the speaker compartment and others are mounted on the outside thereof. The tubes 3l and other heat radiating elements are mounted on the outside of the chassis. The tuning oondenser 39 and other components are also mounted on the outside, but for other reasons. Certain of the connections to the tubes are made on the inside of the chassis, within the compartment, as shown at 4i, for example.

The loop antenna i3 is preferably cemented to an insulating composition board dit which forms the rear enclosure wall for the cabinet of the receiver i. rEhe loop and mounting board d3 are preferably spaced from the rear of the chassis to reduce losses. It will be noted that the rear wall of the chassis is made of plywood or other insulating material to reduce losses into the loop circuit.

It will be observed that the closure board 43 is out away at the corners, at and d8, to provide openings for the purpose of circulating air to cool the heat radiating elements o-n the chassis within the spaces between the chassis end walls and the cabinet. When the small receiver is mounted on an ordinary surface, for example, as shown by the receiver l in Fig. 1, the air enters the openings at cl at the bottom of the cabinet and leaves through openings 4.3 at the top of the cabinet. When, however, the receiver is placed in normal operating position upon the table, as in Fig. 1 (dotted lines l) and Fig. 2, the lower openings at 4T! are closed by the proximity to lid 5. Tubular membersS have been provided for the entrance of cooling air in place of the covered openings at 4l. 'I'he tubular members '.S extend all the way from the top to the bottom surface of the closed cavity in the table, while preserving the acoustic seal within the cavity.

Referring to Fig. 5, an earlier species of the invention, enhanced results were obtained with an increased baiile eiect between the iront and rear surfaces of the diaphragm, by providing in a cabinet housing 5i an enclosed rear compartment 53 for the speaker, similar to that shown in Fig. 2, arranged to communicate with an opening 55 in a table top 57 through a corresponding opening 55S in the bottom of the speaker compartment. As in the other case, the heat radiating elements, such as the ampliner tubes 6l, were mounted outside of the speaker compartment for the purpose of air cooling.

An improvement over the arrangement of Fig. 5 can be obtained by the use of a horn, as shown in the modiiioation of Fig. 6. In this arrangement, a table or cabinet 63 of appropriate ornamental design is provided with a horn 65 extending vertically downwardly from the top S1 to the base of the table or cabinet, as shown in dotted lines. A small radio receiver I, with an enclosed speaker, as shown in Figs. 1 and 2, is mounted over the throat of the horn in the top of the cabinet. This arrangement results in a substantial extension of the low-frequency end of the operating range. The horn is not critical in design, although an exponential type is to be preferred. While this arrangement is satthe small receiver with the bottom opening 2l' closed, as used on any ordinary table or surface. The speaker used for this measurement had a natural resonance of cycles, as determined with a very large baie. Curve B shows the subi stantial improvement in the extended low-frequency end of the range when used with appli. cants invention. In view of the increased lowfrequency response, it was found desirable to improve certain of the circuit components, such.

as the output transformer to the speaker, which was redesigned to handle the increased low frequencies.

Referring to the curves in Fig. 8, curve C in dotted lines shows, in the region below 1000 cycles, the undesired peaks and dips occurring in the cavity without the sound damping material. The full line curve D shows a slight reduction in the nrst peak and a substantial elimination of the first bad dip in curve C. These curves were made with a speaker having 1Z0-cycle natural r,eso

nance as measured in a very large baiile, for example, the wall oi a room. The 10D-cycle speaker used in the case of Fig. 7 was damaged; and it was necessary to obtain another speaker` It had' of the same type to complete the tests. a different natural period.

Curve E, the low-frequency portion only being shown, was taken with the same speaker as in the oase of curves C and D, but operating in Va single, large, sealed rear cavity or compartment. In this case, the speaker was mounted in a hole out in the top wall 23 of the table cavity, with,

the opening Il closed. The compartment i5 was not involved. A comparison with curve D shows that the low-frequency resonance is about This shows the benecial re-` 25 cycles higher. sults from the two coupled compartments made in accordance with the invention, as distinguished from a single, large, rear compartment.

From the above, it is apparent that, as a result of the invention, substantially improved results are obtainable with a small, table model radio receiver using a standard design of speaker when combined with a table or other support which is made to extend the low-frequency endy of the range. At the saine time, the small receiver may readily be carried to other locations and operated in the normal manner as a small radio receiver with the usual acoustic characteristio output. able, as in the case of A. C./D. C. battery operation.

In accordance with the invention, a substantial improvement can also be realized when using a small receiver with a sealed rear enclosure and extra-low frequency speaker, as in the above mentioned Olson et al. patent. In one aspect, the invention is an improvement over said Olson et al. patent. For example, if the speaker, preferably of the folded edge type, disclosed in said Olson et al. patent, has a low natural resonance of, say, 40 cycles, and is mounted in the sealed enclosure, the stiiness introduced by the small enclosure raises the resonant point to around 70 The receiver may be fully port-' cycles. Y When such an enclosure, preferably larger than that in the above figures, is provided with the above disclosed base opening and other features shown in Figs. 2 and 4, and mounted upon the table embodying the invention, in communication with the large cavity, as in Fig. 2, there is much less stiffness introduced into the diaphragm suspension system and the net result is a lower resonance, say around 50 to 60 cycles. In such a case, the operation of the small set, with a special low-frequency response speaker, operated upon a plane surface with sealed rear enclosure, is better than normal as to low-frequency response, but when mounted on the table and coupled with the cavity according to the invention, the performance is exceptional. While the main purpose of the invention is to improve the operation of an ordinary small set, with an ordinary speaker, the invention is not limited thereto in that substantial improvement results in applying the invention to a small set with 'a speaker of special acoustic design.

What we claim is:

1. In a sound reproducing device, the combi nation with a loud speaker having an acoustic diaphragm of the direct acting type of a housing for said diaphragm, said housing having an acoustically closed compartment therein for enclosing the rear surface of said diaphragm and having an opening in one surface thereof for the passage of sound waves originating from said diaphragm rear surface, and a support for said housing, said support including a vwalled enclosure defining an acoustically closed cavity cf substantially larger cubical content than said cornpartment, one wall of said enclosure having an opening therein in registry with said housing opening.

2. The invention as set forth in claim 1 wherein damping material is included in said acoustically closed cavity for substantially reducing resonance peaks and dips in the frequency response characteristic of the low-frequency end of the range.

3. The invention as set forth in claim 1 where- J in said housing includes a walled enclosure for the front surface of said diaphragm, one wall of said enclosure having a plurality of spaced` small holes therein to provide a low pass filter effect.

4. The invention as set forth in claim 1 Wherein said speaker has a natural resonance, as measured in a large baffle, substantially. below the useful audio-frequency range to be reproducedv by said device.

5. The invention as set forth in claim 1 wherehousing independently of said walled enclosure opening for providing circulation of air'in said housing.' l f t 7'. `In combination, an acoustic apparatus comi# prising a table having a top provided with" an opening, a relatively portable sound producing device comprising an acoustic diaphragm of the direct acting type, a housing for said device defining around the region rearwardly of said diaphragm a compartment acoustically closed eX- cept for a base opening, and means providing a closed acoustic cavity of relatively larger cubical content than said compartment, said means being carried by said tableand coupled withfsaid compartment through said openings for conducting sound Waves from the4 back of said diaphragm through said openings to enhance the reproduction of low frequency sound waves. 1

8. In a sound reproducing device, the combi-` nation with a loud speaker of the moving coill type having a direct acting diaphragm and aV of substantially larger cubical content than said first-mentioned compartment, said cavity being acoustically sealed except for an opening disposed in a wall of said enclosuresaid last-men-j tioned opening being in registry with said rstmentioned opening whereby the effective enclosure for the 'rear of said diaphragm is substantially increased. WENDELL L. CARLSON.

LOY E. BARTON.

REFERENCES CITED UNITED STATES PATENTS Number Name Y Date 1,443,688 Hurd Jan. 30, 1923 1,693,470 Thomas Nov. 27, 1928 1,677,509A Hallberg July 17, 1928 1,705,778 Munroe Mar. 19, 1929 1,715,831 Hahnemann June 4, 1929 2,031,500 Olney Feb. 18, 1936 2,065,751 Scheldorf Dec. 29, 1936 2,067,582 Sperling Jan. 12, 1937 2,072,035 Hayenga et al Feb. 23, 1937 2,129,184 Roy Sept. 6, 1938 2,160,283 Roberts May 30, 1939 2,160,284 Roberts May 30, 1939 2,224,919 Olson ,Dec. 17, 1940 2,277,525 Mercurius Mar. 24, 1942 2,315,896 Dumas Apr. 6, 1943 2,337,213 Topping, Jr. Dec. 21, 1943

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