US2688373A - Sound translating apparatus - Google Patents

Description

Sept. 7, 1954 H. F. OLSON 2,688,373 SOUND TRANSLTING APPARATUS Filed May l 1951 VA Ms CAS Cm: 1Q/f Q/a A K ,5 Pl

Xl una /ao 20o 40o @0c ffvi/yc'y 1;'72 E r MS, CAs| MS2 CAs'z-esz l 1 .5 l

fifa/[Nay ENTOR ULSDN ATTORNEY Patented Sept. 7, 1954 SOUND TR-AN SLATIN G APPARATUS Harry F. Olson, Princeton, N. J., assgnor to Radio Corporation of America, a corporation of Dela- Ware Application May 1, 1951, Serial No. 223,865

11 Claims. l

The present invention relates to sound translating apparatus, and more particularly to an improved loudspeaker and method for converting electrical energy into acoustical energy.

An important consideration in providing high quality sound reproduction is that the sound translating apparatus have uniform acoustical power output over a wide range of lsound frequencies. Although many excellent systems have been proposed in the past, the sound reproducing quality of relatively small apparatus in use today,

particularly apparatus of the kind used in the home or in places where space is relatively limited, is not too good. In many home instruments, in order to obtain adequate low frequency response, a large open back cabinet is used. Other acoustical systems employ relatively large, closed cabinets, having a base reflex opening for accentuating low .frequency acoustical power output. In others, a plurality of loudspeakers are mounted in a single cabinet to provide suitable coverage over a wide frequency range.

The use of a closed back loudspeaker cabinet is, however, generally accepted as superior to that of the open back cabinet for sound reproduction,

`since the later type of cabinet exhibits a serious resonance which accentuates the acoustical power output at the particular resonant frequency. In addition, the low frequency acoustical power output of the open back cabinet below the resonant frequency of the loudspeaker is highly attenuated.

It is a primary object of the present invention to provide an improved sound translating apparatus the dimensions of which are relatively small and the acoustical power output of which extends over a wide range of useful sound frequencies.

It is also an object of the present invention to provide an improved sound translating device of relatively small dimensions the acoustical power output frequency characteristic of which is comparable to that of larger instruments having excellent sound reproducing qualities.

Another object of the present invention is to improve the sound reproducing qualities of a loudspeaker mounted in a relatively small, closed back cabinet.

Still another object of the present invention is to provide a method of operating a loudspeaker mounted in a closed cabinet which will enhance the sound reproducing qualities thereof.

A further object of the present invention is to provide a method of operating a loudspeaker mounted in a closed cabinet which will extend the acoustical power output frequency range thereof.

It is another object of the present invention to provide a sound reproducing apparatus having a closed back cabinet of relatively small dimensions with excellent fidelity over a wide frequency range, and which is also highly efficient in use.

In accordance with the present invention, the sound reproducing apparatus comprises a cabinet structure arranged to provide two acoustically closed chambers. An opening is provided in a wall of the cabinet, between one of the chambers and the exterior of the cabinet. A loudspeaker diaphragm is disposed over this opening and forms a closure therefor. A sound transmitting passage is provided between the chambers and a second r loudspeaker diaphragm is mounted in a manner to form a closure therefor. The two loudspeakers are connected to the output stage of a conventional Vaudio frequency amplifier. The loudspeaker mounted between the chambers is adapted particularly to reproduce most efficiently low audio frequency sound waves whereby mechanical coupling between the diaphragms through the air medium common to both diaphragms serves to assist in driving the other diaphragm in the low frequency range thereby enhancing the sound reproducing quality of the apparatus by extending the overall acoustical power output frequency range.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be understood better from the following detailed description when read in connection with the accompanying drawing, in which Figure l is a side view, partly broken away, of a single loudspeaker mounted in a closed back cabinet,

Figure 2 is a wiring diagram of an electrical circuit corresponding to the acoustical network of the vibrating system of the apparatus shown in Figure 1,

Figure 3 shows two curves which indicate the acoustical power output frequency characteristics of a loudspeaker mounted in closed back cabinets of diierent volumes,

Figure 4 is a side View, partly broken away, of a sound reproducing apparatus in accordance with one embodiment of the present invention,

Figure 5 is a wiring diagram of an electrical network corresponding to the acoustical network of the vibrating system of the apparatus shown in Figure 4:

Figure 6 shows two curves, one of which indicates the electrical impedance frequency characteristic of one of the loudspeakers shown in Fig- 3 uie 4, and the other of which shows the electrical impedance frequency characteristic of the other loudspeaker,

Figure 7 shows two curves representing a comparison of the acoustical power output frequency characteristics of the apparatus shown in Figures 1 and 4, both cabinets having the same volume,

Figure 8 is a sectional view of a second embodiment of sound reproducing apparatus in accordance with the present invention, and

Figure 9 is a-wiring diagram of an electrical network depicting the electrical impedance of the two voice coils of the systems shown in Figures 4 and 8.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Figure l, for comparison purposes, a closed back cabinet l in which a loudspeaker 3 is mounted with the diaphragm thereof disposed behind an opening 'I provided in a wall 9 of the cabinet so as to form a closure for the opening. The acoustical network for this type of enclosed loudspeaker cabinet may be represented by yan electrical circuit diagram, as shown in Figure 2 of the drawing, wherein:

p1=the driving pressure of the loudspeaker, which may be calculated from the equation:

fm1 Pi- A1 (l) where and where Ms=inertance of the cone diaphragm, the voice coil and the air load on the cone diaphragm of the loudspeaker;

CAs=acoustical capacitance of the cone diaphragm suspension system;

CAQ-:acoustical capacitance of the cabinet, as calculated from the equation:

where V=volume of the cabinet; p.=d.ns'1ty of air; and cvelocity of s ound.

`The acoustical volume current in the system is given by the equation:

j(l: l p1 TA +7wMs-C'As-.wCAC

vwhere yew/1 1' cp=21rf f :frequency i The acoustical power output of the system is given by the equation:

P 7 AX 12 (4) Referring to the Equations 3 and 4- above, it will be seen that the acoustical power output lin the low frequency range is influenced by the cabinet volume. As the cabinet volume is decreased, the low frequency cut-off in acoustical power output is increased in frequency.

If, for example, a loudspeaker having a twelve inch diameter diaphragm with a resonant frequency of 70 cycles per second is mounted in a closed back cabinet l, in the manner shown in Figure 1, the acoustical power output frequency characteristic of the apparatus using cabinets having volumes of 5 and 2 cubic feet, respectively, may be represented by curves as shown in Figure 3. In this family of curves, curve A represents the acoustical `power ouput frequency characteristic for the twelve inch loudspeaker mounted in a ve cubic foot cabinet, and curve B represents the acoustical power output frequency characteristic for that same loudspeaker mounted in a two cubic foot cabinet. A comparison of these two curves shows that, in the case of the ve cubic foot cabinet, the resonant frequency of vthe loudspeaker is approximately cycles per second, whereas, in the case of the two cubic foot cabinet, the resonant frequency is approximately cycles per second. A comparison of these two curves shows that, the smaller the cabinet cavity the higher the resonant frequency of `the loudspeaker. In either of these cases, the resonant frequency is too high and the acoustical power output falls off rapidly below the resonant frequency thereby reducing Vthe output in the low frequency range, a condition unsuitable for excellent sound reproduction.

On the other hand, if a loudspeaker having a lower resonant frequency were used, for example, one having a 50 cycle per second resonant frequency alone, the resonant frequencies of that loudspeaker mounted in cabinets of 5 and 2 cubic feet volumes, would be respectively, 86 and 120 cycles per second. However, experience has shown that a twelve inch loudspeaker with a 50 cycle resonant frequency is not only diiicult to constructy but it is also dimcult to. maintain uniform acoustical power output in the mid-frequency range due to break up of the `flexiblesuspension system for the diaphragm.

The current trend being toward the use of smaller cabinets. for sound translating instruments, the present invention is particularly concerned with the improvement in low frequency acoustical power output of conventional. types of loudspeakers mounted in closed back cabinets of relatively small volume. For average use, a tolerable cabinet would have a volume of approximatelyV two cubic feet.

In accordance with one embodiment of the present invention, improved sound reproducing quality of relatively small apparatus is obtained by employing an acoustically closed cabinet or walled enclosure lfhaving an opening l'l through awall i9 thereof. A partition 2 Il having an opening 2.3 therein is mounted with-in the cabinet so that it divides the interior into two chambers 25 21.

A sound reproducer comprisingz a loudspeaker 29 isA mounted with the diaphragm 3L! thereof covering the wall opening lil-'so that it forms with the cabinet wal-l a closure forY that opening. A second loudspeaker 33. is mounted with the diaphragm 35 thereof covering the partition opening 23 so thatit forms with the-'partitiona closure for f that opening. Thus, the loudspeaker diaphragms together with the enclosure walls and the partition provide two acoustically closed chambers.

In one sound reproducing apparatus, made for experimental use, a cabinet having a volume of two cubic feet is provided. A loudspeaker 29 having a twelve'inch diameter cone diaphragm 3| and a resonant frequency of 70 cycles per second is mounted behind the opening I1 connecting the chamber 25 with the exterior of the cabinet. A second loudspeaker 33 having a ve inch cone diaphragm is mounted with its diaphragm 35 covering the partition opening 23. The partition ZI is arranged in the cabinet I5 so that an acoustically closed chamber 25 of one-half cubic foot volume is provided behind the larger diaphragm. The other chamber 21 has a volume of one and one-half cubic feet. The respective electro-mechanical driving means for the loudspeakers 29, 33 are connected electrically in series with a common source 'of energy provided by a conventional audio frequency amplier (not shown). If desired', the electro-mechanical driving means may be connected electrically in parallel with the source of energy. The acoustical network for this arrangement is shown by an electrical circuit diagram, such as that illustrated in Figure 5 of the drawing, wherein:

p1=driving pressure of the large loudspeaker, as

determined from the equation,

where B1=ux density in the air gap in the large loudspeaker;

l1=length of the conductor in the voice coil of the large loudspeaker;

i1=current in the voice coil of the large loudspeaker; and

A1=area of the cone diaphragm of the large loudspeaker;

p2=driving pressure of the small loudspeaker, as

determined from the equation:

where B2=fiux density in the air gap in the small loudspeaker;

l2=length of the conductor in the voice coil of the small loudspeaker;

i2=current in the voice coil of the small loudspeaker; and

A2=area of the cone diaphragm of the small loudspeaker;

m=acoustical radiation resistance presented to the cone diaphragm of the large loudspeaker;

Ms1=inertance of the cone diaphragm, the voice coil and the air load on the diaphragm of the large loudspeaker;

Ms2=inertance of the cone diaphragm, the voice coil and the air load on the diaphgram of the small loudspeaker;

CAs1=acoustical capacitance of the diaphragm suspension of the large loudspeaker;

CAs2=acoustical capacitance of the diaphragm suspension of the small loudspeaker;

Casa=acoustical capacitance of the volume of air in the small chamber common to both loudspeakers, as determined from the equation V3 Cass-p c2 Where The volume current in branch I of the electrical circuit is given by the equation:

Where D 'ZAiZAz -I- ZAiZAs -iZAzZAa v(The acoustical impedances ZM, ZM and Zas refer to the acoutical impedances of the branches I, 2 and 3 respectively of the network of Figure 5.)

The acoustical impedance of branch I of Figure 5 is given by the equation:

J', wC'Asi branch 2 of Figure The acoustical impedance of 5 is given by the equation:

wC'Asa (11) The acoustical impedance of branch 3 of Figure 5 is given by the equation: I

J' J' A3 MM2 wCAsz wC'Asi (12) The volume current in branch 3 is given by the equation:

Since the two loudspeakers are connected in series, i1=i2, and

P1 BiZiAz -==K P2 Bzlzl where K: constant.

The acoustical impedance at p1 is given by the equation:

D Zai ZAK) 15) ZA 2 l-ZA a The acoustical impedance at p2 is given by the equation:

D Z :m

2 zA2+zA1+KcA2 (16) The electrical circuit of the Voice coils of the large and small loudspeaker connected in series is shown in Figure 9. The electrical impedance and the quantities in this circuit will now be dened. In the electrical circuit, the electrica-l impedacef oft the; two.. loudspeakers: connected--K in seriesuisgiverrby the` equation; 1

ZEMz--motional electrical impedance of the voice coil of the small loudspeaker. The motional electrical impedance of the voice coil ofthe small loudspeaker isy given by theequation:

eine (19) The;t current. .through y the voicey ycoils..is given-.by

tnaequation:

where e=voltage applied.to-the=.voice coils;

The acoustical power output is the power output deliveredzbytthe diaphragm of the large loudspeaker, and isgivenby. the equation:

PwncV (2r) Employing llquationsi)` .toA 2l inclusive it is possible to determine-theacoustical power output of theisystem.. The'above analysis shows. thatit is possible to eirtendtheflowl frequencyk rangefby the use of a second loudspeaker.

Several modelsoflthissystem have been built. As a typicalexample; inf one model, the diameter of thee cone; diaphragm of the largeradiating loudspeaker was 12 inches and the,dame.ter. of the cone diaphragmof4V the, small driving loudspeaker wasinchesl. The massIof the voice coil of the small loudspeaker was about twice that of thelarge.- loudspeaker.. Theiux densityv inA the air gap was 12,000 gausses as comparedto 10,000 for the large loudspeaker. The resonant frequency of the smallfloudspeaker was 50 cycles per second and the largeloudspeaker 80 cycles per second. rf'he total cabinet volume-was 2f cubic feet. The. volume behind the small loudspeaker Was ll/g cubic feet: Thevolurne b'eiweerrthev two loudspeakers was less than 1/2 cubic foot: E1n'- ploying this system, an examination of the acoustical circuit shown in; Figure 5 and Equations 9 to inclusive, discloses, that the load on the back ofthe large loudspeaker diaphragm is Very small in' the region" from' 50 to 300` cycles' perfsecond. Thus, the acoustical power output conditions; as far as the large loudspeaker is, concerned, are practically the same. asif. the large loudspeaker were mounted in a.. cabinet having a very large volume. The electricalz impedanceA frequency characteristic of the largey speaker valone is shown by' curve A in Figure 6i |The electrical impedance frequency characteristic of the smalll loudspeaker mounted in al cabinet oi one and one half cubic 8f. feet.. is shown. by curve Br liligure.v 6. Iny the regioniabove 30.0,cyclesfper. second,vthe.acoustifcal impedance-o thechamber of one half, cubic foot volume is; smallcompared to the acousticall im pedancevof; the large-diaphragm and, therefore, does-notinuence the acousticalpower output. It. will beyobserved fromv the curves shown` in Eiguree` that above. 30,0 cyclesl per secondtheelectr-ical. impedance, of, the. small loudspeaker is very smallrcomparedto theelectricalimpedance ofthe largefloudspeaker. As av result, since the two loudspeakers are; connected in seriesv practicallyr allatlfieinput` electrical power sfed to the large loudspeaker in the region above 300 cycles. Under the above-.conditionsthelarge loudspeaker operates alone and; iny the,Y conventional manner above :300;cycles per second.

The acoustical power output frequency. char,- acteristic; for, thecombination or closely coupled loudspeakers. mounted in a cabinet of two cubic feet volume: is shown by,A curve A in Figure '7. For.- comparison, purposes, the acousticaly power outputmharacteristicfor the 12 inch loudspeaker mountedzalonein\a12cubic foot cabinet, as shown byA curve-.Bin Figure 3, is slperimposed on curve Aeinrigure 7. It will be seen fromacomparison of these two.curves that there is considerable extension of the low frequency acoustical power output. This is` explained` by the introduction of the smaller loudspeakerI and: coupling it mechanically with the large loudspeaker through the enclosed volume of airinlthe small chamber 25. Thus, in the region below 300 cycles, the small loudspeaker, which is adapted to reproduce most eiiciently sound waves of relatively low audio frequencies in the desired range of frequencies, functions as. a. supplementary driving element for the large loudspeaker, which is adaptedto reproduce. eiciently. sound waves over that, entire range. Eflectively, the sound pressure developed by. the small. loudspeaker. reduces the acoustical impedance presented to the back of the large loudspeaker, over thatwhich would exist if'the large loudspeaker operated directly into, the entire cabinet.. Inthe example referredito above, over the frequency range, from` 50 to 300 cycles per second, the acousticall impedance presented to the back of the large loudspeaker is practically the same as that of a.. very large cabinet.

From the foregoing description, it will be apparent that the present invention provides animproved sound reproducing system for completely closed loudspeaker cabinets of relatively small dimensions..

It will be obvious to persons skilledinA the. art that: various modicationsand changes in the sound reproducing apparatus shown and describedV herein ,are-possiblewithin the spirit of the present invention. For example, as. shown in Figure 8;the partitionzl may be disposed diagonally of the interior of the cabinet. On the other hand,y two separate, acoustically closed cabinets may be provided; with means for transmitting sound" from one or'the loudspeakers mounted in onecabinet tothe chamberprovidedlby the other cabinet. Other changes oflike nature will, or course, readily suggest themselves. Therefore, it is-desiredithatthe particularforms of the present invention showny anddescribed herein shall' be considered as illustrative andnot aslimiting.

is claimed is:

l. Sound reproducing apparatus comprising awallecl4 enclosure; meansdividing the interior of said enclosure into separate acoustically closed chambers, an exterior wall of said enclosure having an opening therein connecting one of said chambers with the exterior of said enclosure, said dividing means including an opening therein deiining a sound passage between said chambers, sound reproducing means including a diaphragm mounted over each of said openings and forming a closure therefor, and electro-mechanical driving means connected to each of said diaphragms.

2. The invention as dened in claim l wherein said sound reproducing means comprises a plurality of closely coupled loudspeakers, one of said loudspeakers being adapted to reproduce efiiciently sound waves over the entire range of desired audio frequencies and the other of said loudspeakers being adapted to reproduce most eniciently sound waves of relatively low audio frequency in said range.

3. The invention as defined in claim 1 wherein the volume of the chamber common to both said diaphragms is substantially smaller than the volume f the other of said chambers.

4. The invention as defined in claim 1 wherein said electro-mechanical driving means for each of said diaphragms is connected to a. common source of electrical energy.

5. The invention as dened in claim 1 wherein one of said diaphragms is larger in area than the other of said diaphragms.

6. Sound reproducing apparatus comprising a walled enclosure, means including a partition having an opening, a loudspeaker diaphragm covering said opening, said last named means dividing the interior of said enclosure into two acoustically closed chambers, one wall of said enclosure having an opening therein connecting one of said chambers with the exterior of said enclosure, and a second loudspeaker diaphragm mounted over said opening.

7. Sound reproducing apparatus comprising walled enclosures defining first and second acoustically closed chambers, said first and second chambers having a common wall, the rst one of said chambers having an opening in an exterior wall thereof, a loudspeaker including a diaphragm mounted over said opening and forming a closure therefor, an opening in said common wall, a second loudspeaker including a diaphragm mounted in said opening in said common wall and forming a closure therefor.

8. Sound reproducing apparatus as defined in claim 7 wherein said enclosures are dimensioned to provide acoustically closed chambers of different sizes.

9. Sound reproducing apparatus as defined in claim 8 wherein the enclosure having an opening in the exterior wall thereof is dimensioned to provide the smaller one of said chambers.

10. Sound reproducing apparatus as dened in claim 9 wherein one of said loudspeakers is adapted to reproduce most efciently low audio frequency sound waves, said one of said loudspeakers having its said diaphragm mounted over the opening in the enclosure having the smaller one of said chambers.

11. Sound reproducing apparatus as defined in claim 10 wherein said loudspeakers are connected electrically in series.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,969,704 DAlton Aug. 7, 1934 1,988,250 Olson Jan. 15, 1935 2,114,680 Goldsmith Apr. 19, 1938 2,143,175 Waite Jan. 10, 1939 2,217,279 Karns Oct. 8, 1940 2,539,327 Reid et al Jan. 23, 1951 FOREIGN PATENTS Number Country Date 585.963 France Mar. 12. 1925

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