US2604182A

US2604182A - Loud-speaker with a tapered horn coupled to the speaker diaphragm

Info

Publication number: US2604182A
Application number: US32607A
Authority: US
Inventors: Massa Frank
Original assignee: Individual
Current assignee: Individual
Priority date: 1948-06-12
Filing date: 1948-06-12
Publication date: 1952-07-22
Anticipated expiration: 1969-07-22

Description

F. MASSA LOUDSPEAKER WITH A TAPERED HORN COUPLED July 22, 1952 TO THE SPEAKER DIAPHRAGM Filed June 12, 1948 rflv FIG. 2

. 3nventor Fag-5.7

5N CYCLES FREQUENCY Fla-1.8

Patented July 22, 1952 UNITED STATE s PATENT" oFFlcs LOUD-SPEAKER WITH A TAPERED HORN COUPLED TO THE SPEAKER DIAPHRAGM Frank Massa, Cleveland Heights, Ohio Application June 12,1948, Serial No. 32,607

9 Claims;

My invention is concerned with loud speakers and, more particularly, with loud speakers of very-small size such as aregenerally employed in the so-called personal types-of battery operated portable radio receivers." Due to the space limitations in thesesmall sizeradio sets, the dimensions of the loud speaker must necessarily bevery small. Loud speakers employing diaphragmshaving a vibrating surface corresponding to the area of 'a 2" diameter disc, more orless, are typical-of theloud speakers generally employed for this application: Because of thesmall sizeof the vibrating diaphragm, the acoustic efiiciency and the corresponding power output from these loud speakers-is-very low and since the small portable radios which employ this type of loud speaker are operated with batteries,

a considerable drain onthe battery power isnecessitated if areasonable acoustic power out put is to be established. Another "disadvantage of the tiny loud speaker is that the resonant frequency'of the-vibrating diaphragm is generally in the neighborhoodof 300'cycles or higher and,- as'a result, thereis a complete absence of any low-frequency reproduction from the loud speaker, resulting in the well-known ftinny-"* quality of reproduction that-istypical of this type'radio set.

The general object of my invention is to imgprove both the efficiency of reproduction of smallsize loud speakers and also -ito increase their low-" frequency response;

Another object of thisinvention istoprovide' a low-cost loud speaker design which permits the economical use of my new loud speaker system in a small-size portable radio set-.

Still another object of this invention-is'to pro--- vide a design such that the space'requirement for my improved'loud speaker will be kept-at a minimum.

A further object of my inventionis to reduce the resonant frequency of the vibrating system of the loud speaker by means externalto thedia phragm structure, thereby resulting in increased low-frequency range of reproduction without sacrifice in the high-frequency performance.

One method for increasing the low-frequency response of a loud speaker would'beto reduce the natural resonance frequency of the vibrating diaphragm by the addition'of a weight tothemovingstructure, but if this means were em"- ployed, there would bea lowering ofthe mid'" and high-frequency sensitivity of the loud speak er because'ofthe added weight'that would have to be set into vibration by the driving mechanism,

thus resultingdn 'still further "r educed- "eiiji'cien'c'y ofthe loud "speaker over frequency range." f.

In one specificembodl I provide an incrasf surface'of the diaphragm" quencies, so that the r "so the"diaphragm"is material lat same time the effective inassbith diaphragmat' the higher frequencies}, unchanged; I achieye v tli ing an air chamber-i111 diaphragm operatesfwhlfe trueexponenti'al hornat but which acts-as an acous the lower frequencies.

In one form of my invention midget-size loud speakrfifl coup shaped air chamber whi'c "(its actance at the low frequencie erv mfiifi s substantially w at the higher frequencies toine'reasef utput efficiency'at the higher frequencies; 5

In another form' of my invention] the air radio Cabinet. thereby requiring minimum spa e;

inside theiarge opening of the air 'chamber and coupled I to the small throat," thereby 'efi'ecting a considerable saving in spacef'requirerxientl" In still another form of my invention; I show" an economicaldesign of the" air chamber strue-' ture which can be "fabricated bymo'lding.,.two

symmetrical halves and then cementing them to gether.

The novel "features that'I consider characterls tic of my invention are'set forth" with partic u lanty in the appended claims. itself,- however; both as to its organization and method of operation; as well'asadvantages there- 1 of,- will best be'understoodfrom the following 1 description of several 'embodiments'thereof, when readin connection with theacc'ompanying draw ings. in which:

Fig.1 is an end view of a portable radioincorporating one' form" of my invention."

Fig; 2 is a *sec'tlomtaken along. the li ne 2-2 I Y hicl ij he vibrating. sea ater-w ems-he r queaau,

the resonant frequency of theleud speakergdi'aphragm and also acts as an" exponential "horn.

The invention, V

Fig. 6 is asection taken along the line 6 of Fig. 5.

Fig. 7 is a section taken along the line 'i7 of Fig. 6.

Fig. 8 is a graph showing the measured improvement in the performance characteristics of one type of loud speaker built in accordance with one of the teachings of my invention.

Referring more particularly to the figures in which the same reference character represents the same part throughout:

Figs. 1, 2, and 3 show three views of one embodiment of my invention; The reference character 1 represents the outer case of a portable type radio receiver. Instead of employing a conventional arrangement for the loud speaker in which the sound is radiated directly from the diaphragm through a grill placed on one wall of the radio cabinet, I provide a tapered air chamber which is designed to act as an exponential horn at the higher frequencies, resulting in improved efiicien-cy for the loud speaker 2 at these higher frequencies. The air column is also designed so that at the low frequencies, it acts as a sufficiently large mass reactance load on the loud speaker diaphragm 3 to cause a substantial reduction in the resonance frequency of the Vibrating diaphragm. In order to achieve this dual function for the air'chamber, I found it necessary to make the developed length of the air column less than one-quarter wavelengthof the lowered frequency of resonance of the diaphragm and air column combined, and I also found it desirable to give the air column an approximate exponential taper so-that it behaves effectively as an infinite exponential horn at the higher frequencies. This results in the elimination of the mass reactance air load on the diaphragm at these higher frequencies and also results in an increase in the acoustic efficiency of the loudspeaker at these higher frequencies because of the increased acoustic radiation resistance which is imposed on the vibrating diaphragm by the exponential horn attachment.

In the form of my invention shown in Figs. 1, 2, and 3, the tapered air chamber is formed by several'adjacent walls of the radio cabinet and the fabricated portions 4, 5, 5, l, and 8. The strip 1 shown in section in Fig. 2 and in partial section in Fig. 3, is joined to a pair of opposite outer walls of the radio housing I to form the relatively large funnel-like mouth opening of the air column which discharges the sound, in this particular instance, through the sound transparent grill 9 which covers one end of the radio cabinet, as indicated. The flat portion 6 flares in a plane at right angles to the direction of fiare of the portion 7, as is indicated in the plan view of Fig. 3. The portion 6 of the horn structure, in combination with a pair of side walls, one of which, 8, is shown in Fig. 2, forms a second portion of the tapered air chamber of the loud speaker system. The final portions 5 and 4 complete the tapered air chamber which is continuously expanding from its small throat opening shown adjacent to the diaphragm 3 to its large mouth opening shown adjacent to the sound transparent grill 9. An additional flat portion l0 shown in section in Fig. 2 provides a flat platform around the small throat opening of the air column to permit the loud speaker 2 to be mounted upon it without air leakage between the diaphragm 3 and the throat opening of the air chamber. Angular blocks II are placed in the corners of the right-angle bends, as indicated in Fig. 2, in order to minimize the discontinuity in the rate of increase of the cross-sectional area of the air chamber at the particular bends. The reference character 12 illustrates a carrying handle for the radio set which may be mounted, as illustrated. Although the design shown in Fig. 2 employs some of the walls of the radio cabinet as part of the air column structure, it would obviously be possible to fabricate the air chamber as a separate complete unit, if desired, and then mount it in the same relative position, as indicated in Fig. 2. Such a construction, however, would add thickness to the radio cabinet and it would also increase the cost of production. The space which remains within the cabinet I outside of the horn structure may be used for housing the radio set proper and batteries in any manner desired. The radio chassis and batteries are not shown in these figures since no claim is made to any particular arrangement of these parts.

For the ordinary loud speaker generally employed in small-size portable radio sets, the resonant frequency of the small diaphragm is generally in the vicinity of or greater than 300 cycles. As a result of this high resonant frequency, which results inherently from the small diaphragm size, the sound reproduction is lacking in low-irequency response, with the result that the quality of reproduction is very thin. Also, because of the small radiating area of the diaphragm, which in many cases is less than that of a piston having a 2diameter, the efficiency of reproduction is extremely low, which means that in addition to the lack of low-frequency tone reproduction, the volume of sound reproduction is also weak, especially for those sets which are designed for economical use of batteries. By employing a tapered air column designed according to the teachings of this invention such that the developed length of the air column is less than one-quarter wavelength at the lowest frequency of reproduction, I am able to effectively load the loud speaker diaphragm 3 with a relatively large mass of air such that the resonant frequency of the diaphragm is very substantially reduced. In one embodiment of my invention, for example, in which the air chamber as shown in Fig.2 was adjusted so that its developed length was approximately 8", the area at the small end of the chamber was onehalf sq. in., and the area at the large end of the tapered column was 8 sq. in., I was able to reduce the resonant frequency of the diaphragm, whose effective vibrating diameter was in the neighborhood of 1% inches, by about cycles, thereby adding a little more than one-half octave additional low-frequency range to the loud speaker reproduction. By designing the air column to have an approximately exponentially increasing cross-section, the air chamber becomes an efllcient exponential horn for the loud speaker at the higher frequencies and thereby serves to materially increase the efliciency of reproduction. These combined advantages of the speciallydesignedair chamber are clearly evident from the experimental response curves shown in Fig. 8. The dotted curve in Fig. 8 shows the meas- 193d "5 9 15 ha a te is o he; ouds ak r h n. un e gnv na yo po i e a r l opening on one wall of a small radio cabinet. The solid curve in Fig. 8 showsthe measured response characteristics of the same loud speaker arranged as illustrated in Fig. 2 and coupled to the specific-air chamber above described. The great improvement in low-frequency reproduction as well as the indicated increase in the efflciency at the higher frequencies actually produced a most spectacular improvement 'ir; the quality of reproduction from the small portable radio. The reproduction was very considerably louder and muchrnore pleasing in tone balance because .of the e t i d w-f equ ncy, range.

The important principle which achievesthe added low-frequency response. without sacrifice in high-frequency performance 7. is that. in my design of the air chamber, I. add mass to. the loud speaker diaphragm only atthe low frequencies, thereby reducing the diaphragm resonance. At the higher frequencies, the air chamber becomes effectively a long exponential horn and the load on the diaphragm becomes essentially a resistance and the mass reactance disappears. In this way, the mass load is eliminated at the higher frequencies and the acoustic efficiency is materially increased.

Figs. 5, 6, and 7 illustrate another embodiment of my invention. Fig. 5 is a view similar to Fig. 1 except that the grill cloth 9 and the carrying handle I2 have been removed. Fig. 6 illustrates the essential difference between this second embodiment of my invention over the previous embodiment shown in Figs. 1, 2, and 3. The portion I 3 in Fig. 6 runs from wall to wall in the radio cabinet I the same as was the case with portion 1 in Fig. 2. The portion 6 in Fig. 6 is the same as the portion 6 shown in Figs. 2 and 3. The side pieces 8 shown in Figs. 5, 6, and 7 combine with the portion 6 to form a tapered horn section in the same manner as

part

6 and 8 were employed in the construction shown in Figs. 2 and 3. Parts 4 and I serve to complete the sealing of the small end of the air column so that the loud speaker 2 may have its diaphragm 3 coupled to the air chamber exactly in the same manner as was described for the corresponding parts in Figs. 2 and 3. The essential difference between the structure in Figs. 5, 6, and 7 as compared with the structure in Figs. 1, 2, and 3 is that the tapered air chamber in Fig. 6 is made re-entrant and, in this way, provides a different space factor for accommodating the radio set components than results from the construction of Fig. 2. Although the construction illustrated in Figs. 5, 6, and '7 utilizes some of the walls of the radio cabinet for part of its structure, it is likewise possible to fabricate the re-entrant horn as a separat unit and then install it within the radio cabinet. One method for economically fabricating the re-entrant horn is illustrated in the perspective view of Fig. 4. In this design, the bell-shaped portion l4 surrounds a second portion l5 which has side walls I6 and an opening ll such that when a symmetrical mating half, having the same shape as Fig. 4, is joined to the cross-sectioned face, a complete re-entrant horn similar to that illustrated in Figs. 5, 6, and 7 will be produced. The type of design shown in Fig. 4 can be very economically produced in molded plastic or molded paper pulp. The horn shown in Fig. 2 could likewise be molded as a separate complete unit, if so desired, and then assembled within the carrying case I.

Although I havechosen certain speciflcnems bodiments of. my. invention forv illustrating the basic features of myinvention, it. will beobvious tonthoseskilled in the art that numerous departuresmay be. made in. the specific details forv executing the required. functions .and I, therefore, desire that my invention shall not be limited except-insofar asis madenecessary by the prior. art. and by. the.- spirit of the, appended claims.

I'claim asmy invention:

1. In combination in. a sound reproducing system, a loud speaker including .ayibratile diaphragm whose ,resonantfrequency lies in the approximate range .200; cycles persecond to 400 cycles. per second, means for substantially reducing. the resonant frequencyof said diaphragm said means. comprising a tapered horn coupled to said diaphragm, the cross-sectional area of said tapered horn increasing along the horn axis such that the area of the open end of said horn is greater than the area of the horn in the vicinity of the diaphragm, the length of said tapered horn being greater than the diameter of the diaphragm and less than A wavelength at the reduced resonant frequency of the diaphragm.

2. The invention set forth in claim 1 further characterized in that the effective vibrating area of the diaphragm is less than four square inches.

3. The invention set forth in claim 1, further characterized in that said horn has a cross-sectional area which increases along its length approximately in accordance with an exponential law whereby said horn behaves essentially as a long exponential horn at the higher frequencies of sound reproduction, thereby serving to increase the acoustic emciency of said vibratile diaphragm in said higher frequency region.

4. In combination in a small size radio set of the portable type, aloud speaker having a fundamental resonant frequency within the approximate range 200 cycles per second to 400 cycles per second, means for substantially reducing the resonant frequency of said diaphragm said means Y comprising a tapered horn coupled to said diaphragm, the cross-sectional area of said tapered horn increasing along the horn axis such that the area of the open end of said horn is greater than the area of the horn in the vicinity of the diaphragm, the length of said tapered horn being greater than the diameter of the diaphragm and less than wavelength at the reduced resonant frequency of the diaphragm.

5. The invention set forth in claim 4, further characterized in that said horn is constructed with some of the walls of the radio set serving also as walls for said air column.

6. The invention set forth in claim 4 further characterized in that said horn has a cross-sectional area which increases along its length approximately in accordance with an exponential law whereby said horn behaves essentially as a long exponential horn at the higher frequencies of sound reproduction thereby serving to increase the acoustic eillciency of said loud speaker.

'7. The invention set forth in claim 4 further characterized in that said horn has a cross-sectional area which increases along its length approximately in accordance with an exponential law whereby said horn behaves essentially as a long exponential horn at the higher frequencies of sound reproduction thereby serving to increase the acoustic efliciency of said loud speaker, and further characterized in that the large opening 7' of said horn terminates at one-endof the radio set through which end the sound is radiated.

8.. The invention set forth in claim 4 further characterized in that-said horn has a cross-sectionalarea which increases along its length approximately in accordance with an exponential law Wherebysaid horn behaves essentially as a long exponential horn at the higher frequencies of sound reproduction thereby serving to increase the acoustic efiiciency of said loud speaker, and further characterized in that said horn is folded back on itself such that the'larger portion of the horn surrounds the smaller end portion.

9. The invention set forth in claim'4 further characterized in that said horn has a cross-sectional areaa which increases along its length approximately in accordance with an exponential law whereby said horn behaves essentially as a long exponential horn at thehigher frequencies of sound reproduction thereby serving to increase REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,730,425 Harrison i Oct. 8, 1929 1,878,018 Stephens Sept. 20, 1932 1,984,542 Olson Dec. 18, 1934 2,035,108 Weil Mar. 24, 1936 2,338,262 Salmon Jan. 4, 1944 Knowles May 10, 1949