US 2900040 A
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J. F. NOVAK LOUDSPEAKER SYSTEM Aug. '18, 1959 Filed Sept. 12, '1955 2 Sheets-Sheet l INVENTOR. (in New? Aug. 18, 1959 J. F. NOVAK LOUDSPEAKER SYSTEM 2 Sheets-Sheet 2 Filed Sept. 12, 1955 cfamw 3min United States Patent O LOUDSPEAKER SYSTEM I James F. Novak, Berwyn, 11]., assignor to The Muter Company, a corporation of Illinois One of the most difiicult and persistent problems in the high fidelity reproduction of sound involves the adequate propagation of speaker diaphragm impulses in the atmosphere in the low frequency ranges. Faithful reproduction of music, particularly, requires the proper emphasis be given to the sound waves throughout the audible frequency spectrum. The problem is somewhat aggravated by the tendency of listeners to prefer overemphasis of the bass component of musical renditions.
A variety of expedients have been employed to enhance the performance of loudspeaker units in the troublesome low frequency range, particularly in the region below 100 cycles per second. Acoustic horns have long been employed to provide more or less adequate coupling between the vibrating diaphragm of the loudspeaker and the atmosphere, the efiiciency of the system depending upon the size of the horn together with other factors. Use of the acoustic horn is not a satisfactory solution to the problem for the reason that the size of horn required for the efiicient propagation of the low frequency sounds may be excessive. Vented enclosures have also been employed as a compromise device to improve low frequency performance. This expedient, which has come to be known as the bass reflex system, provides auxiliary radiation from the rear surface of the speaker diaphragm at a port in the enclosure adjacent the direct outlet for the speaker. Such vvented enclosure systems effect improved performance in the low frequency range; however, the piston band efficiency of the loudspeaker is much lower than that of a horn loaded system.
The principal object of the invention is to provide a loudspeaker system which is capable of faithful reproduction of sound, including frequencies below 100 cycles per second, and which is of moderate size. Another object is to provide such a system which exhibits an improved piston band efficiency without the employment of the large acoustic horn which would itself be capable of attaining comparable efficiency. Otherwise stated, the invention provides a compact loudspeaker system including an abbreviated horn, the system being characterized in that the frequency cut-off characteristic of the system is substantially lower than that of the horn.
In the accompanying drawings,
Figs. 1 to 5 are somewhat schematic cross sectional views illustrating several embodiments of the loudspeaker system of the invention;
Fig. 6 is a cross sectional view taken at the line 66 of Fig. 5;
Fig. 7 is a somewhat schematic cross sectional view of a loudspeaker system embodying the invention and including a folded horn, and
Fig. 8 is a graphical representation of the response characteristics of certain of the elements and of the loudspeaker system described herein.
' In each of the several arrangements of the loudspeaker system herein shown and described, both sides of the loudspeaker diaphragm are utilized for the propagation of sound to the atmosphere. One side of 1 2,900,040 Ce Patented 7 Aug. 18, 1959 the diaphragm is horn loaded, the other side is exposed to a ported cavity having acoustical characteristics required for the reasonably efilcient coupling of the vibrating diaphragm with the atmosphere. The horn in each instance is designed for a cut-off frequency that is substantially higher than the intended frequency cutoff characteristic of the loudspeaker system. The desired lowered frequency cut-off characteristic of the system results when the ported cavity acoustic system is designed to resonate at the resonant frequency of the horn loaded diaphragm, within reasonable approximation.
In the arrangement of each of Figs. 1 to 5, a speaker unit designated generally by the numeral 1 is mounted by suitable means, not shown, within an enclosure 2 with the diaphragm 3 in position at an opening 4 in the front panel 5 for the propagation of sound from the diaphragm to the atmosphere. An acoustic horn 6 is mounted on the enclosure to receive sound waves from the loudspeaker at the opening in the enclosure. The horn in each instance may be regarded as an abbreviated horn since it is designed for a cut-off frequency substantially higher than the design cut-off frequency of the entire loudspeaker system. In other Words, the horn employed 7 in the loudspeaker system of the invention is shorter than a horn that would be capable of efiiciently propagating sound from the loudspeaker to the atmosphere in a conventional horn type speaker system designed for efficient performance in the low frequency range.
An acoustic cavity 7 is defined by the inner surfaces of enclosure 2 and the rear surface of diaphragm 3, this cavity being in all instances open to the atmosphere through ports 8 which may be single openings in the enclosure, may consist of several openings, or may be provided by spacing a speaker supporting panel 9 (Fig. 4) from the inner surfaces of the enclosure, or may be formed, as shown in Figs. 5 and 6, by providing suitable openings in the compliance annulus at the periphery of the diaphragm and inwardly of the diaphragm supporting panel 9c. Cavities 7 may open directly to the atmosphere at the ports, as in the structures illustrated in Figs. 1 and 3, or the location and arrangement of the ports may be such that the cavities open into the throat of the horn and are coupled with the atmosphere through the born as shown in the embodiments of Figs. 2, 4 and 5. If desired, additional acoustic inertance may be introduced into the ported cavity acoustical system, as by providing a somewhat elongated passageway or duct 10 (Figs. 3 and 4) at the port through which the sound waves must pass from cavities 7 to the atmosphere. In general, the effect of the interposition of such an acoustic inertance element in the ported cavity acoustical system is to lower the resonant frequency thereof with the practical advantage that the size of cavities 7 may be correspondingly reduced.
Preferably, ports 8 are located as close as possible to the horn. For this reason, the arrangement of Figs. 2, 4 and 5 may be considered to be preferred in that the ports are connected directly into the horn in each instance. This particular arrangement exhibits somewhat improved performance in the very low frequency range, due partly to the desirable close coupling of the port with the horn and partly to the mass loading of the cavity system by the horn.
In order to achieve the improved performance of the loudspeaker system in accordance with the invention, the ported cavity acoustical system must be designed to exhibit the necessary acoustical response characteristics in relation to the acoustical characteristics of the horn loaded diaphragm system. Specifically, the volume of cavities 7, area of port or ports 8, and dimensions of inertance ducts 10, if used, should be such that the to that of the horn loaded diaphragm, the latter being determined by specification requirements of the loudspeaker system.
In order to fully describe the desired acoustical design relationships, a specific example will be given. Assuming a loudspeaker system specification calling for a cut-off frequency of 70 cycles per second, a cavity volume of approximately one and one-half cubic feet is provided. The horn for this loudspeaker system may be one designed to exhibit a cut-off frequency about one and one-half octaves above the specification cut-ofi frequency of the system, about 180 cycles per second in the specific example being described. A speaker unit having a free air resonance at a frequency midway between the cutoff frequencies of the loudspeaker system and of the horn; i.e. 125 cycles per second, is selected. The horn is designed to have a throat area equal to the effective diaphragm area of the speaker and a mouth area such that the circumference of the horn at the mouth is equal to the wavelength at the horn design cut-off frequency, 180 cycles per second.
The total port area, taking into account passageway inertance, is such that the resonant frequency of the ported cavity acoustical system is approximately equal to the resonant frequency of the horn loaded diaphragm system. This will be somewhat lower than the resonant frequency of the speaker unit in free air for the reason that in the frequency region wherein the ratio of the circumference of the horn mouth to the wavelength is less than about 0.5, the horn exhibits the characteristics of a cylindrical tube, loading the diaphragm and resulting in a resonant frequency of the horn-diaphragm system below that of the diaphragm itself.
A loudspeaker system designed in accordance with the invention as thus described exhibits an improved efficiency due to the horn in the frequency region where in the ratio of the circumference of the mouth of the horn to wavelength lies between about 1 and 4. Auxiliary radiation from the cavity system diminishes as this ratio increases due to the shunting effect of acoustic capacitance of the cavity. In the frequency region wherein this ratio is greater than about 4, performance of the horn approaches that of an infinite horn and propagation of sound from the cavity system is substantially non-existent.
These performance characteristics are graphically illustrated in Fig. .8. With reference to the specific example described above, the broken (dash-dot) line curve 11 represents the frequency-response characteristic of the horn loaded speaker independent of the cavity system, broken (dash-dash) line curve 12 depicts the performance of the ported cavity system independent of the horn, and the solid line curve 13 shows the performance of the combination loudspeaker system. The improved performance of the loudspeaker system in the low frequency range is immediately apparent.
The structure of the loudspeaker system illustrated in Fig. 7 has the advantage of compactness inasmuch as it employs a folded horn; otherwise, the system is in every way similar to those described in connection with Figs. 1 to 5. Enclosure 2:! having opening 4a in what may be considered to be the front thereof although it faces rearwardly in the horn, is disposed desirably entirely within horn da. The speaker unit 1a is mounted within the enclosure, as by means of a supporting panel 9a. Cavity 7a is preferably vented through a passageway a which may be somewhat extended by flange 9b which extends rearwardly into the enclosure from the periphery of supporting panel 9a. As was pointed out more fully above, the effect of the interposition of passageway that between the cavity and the port 8a is to lower the resonant frequency of the cavity system with the desirable result that enclosure 2a may be somewhat smaller than would be the case if the passageway inertance were not employed, or the same size enclosure may be used with a port having a larger cross 2,900,040 r p r 5 4 sectional area than a port which employs no passageway inertance, thereby achieving a larger radiating area. In the structure of Fig. 7, sound waves are propagated into the throat of the folded horn at port 8a from both the front and rear surfaces of diaphragm 3a.
The principles and mode of operation of the loudspeaker system of the invention are fully disclosed by the foregoing description. It will also be understood from the particular examples herein specifically described that many structural variations may be employed in the implementation of such principles. It may be particularly mentioned that a multiple-diaphragm speaker unit may be employed, and such structures are intended to be included within the scope of the appended claims.
Invention is claimed as follows:
1. A loudspeaker system having a given frequency cutoff characteristic and including a speaker having a diaphragm and an enclosure for said speaker having an opening therein, and a horn having a frequency cut-off characteristic higher than said given frequency cut-off characteristic of said system, said horn extending outwardly from said opening, said speaker being mountedwith the diaphragm thereof at said opening to project sound through said opening and said horn and to define with said enclosure a cavity, the walls of said cavity having a port providing limited communication of said cavity with the atmosphere.
2. A loudspeaker system in accordance with claim 1 characterized in that the acoustic system comprising the cavity and port is resonant at approximately the same frequency as the acoustical system comprising the diaphragm and born.
3. A loudspeaker system in accordance with claim I wherein the cavity opens directly to the atmosphere at the port.
4. A loudspeaker system in accordance with claim 1 and including a duct forming a passageway from the cavity at the port.
5. A loudspeaker system in accordance with claim 1 wherein the cavity opens into the horn at the port and thence to the atmosphere.
6. A loudspeaker system in accordance with claim 5 and including a duct forming a passageway from the cavity at the port.
7. A loudspeaker system in accordance with claim 5 wherein the port comprises an opening at the periphery of the diaphragm.
8. A loudspeaker system having a given frequency cut-off characteristic and comprising an enclosure having a front panel having an opening therein, a supporting panel within said enclosure spaced from said front panel and having an opening therein in register with said opening in said front panel, a speaker having a diaphragm mounted with said diaphragm at said opening in said supporting panel to project sound therethrough and to define with said enclosure a cavity rearwardly of said supporting panel, said supporting panel defining an annular port at the periphery thereof providing limited communication of said cavity with the space between said front and supporting panels, and a horn having a frequency cutoff characteristic higher than said given frequency cut-off characteristic of said system, said horn being operatively connected with said opening in said front panel to acoustically couple both sides of said diaphragm with the atmosphere.
9. A loudspeaker system in accordance with claim 8 wherein the horn is of the folded type and the enclosure is disposed within said horn.
10. A loudspeaker system having a given frequency cut-off characteristic and including a speaker having a diaphragm and an enclosure for said speaker having an opening therein, and a horn having a frequency cut-off characteristic higher than said given frequency cut-off characteristic of said system, said horn extending outwardly from said opening, said speaker being mounted compliance annulus at the periphery thereof, said annulus having distributed openings therein providing limited communication of said cavity with the atmosphere through said opening in said enclosure and said horn.
11. A loudspeaker system having a given frequency cut-ofi characteristic and including a speaker having a diaphragm and an enclosure for said speaker having an opening therein, and a horn having a frequency cut-ofi characteristic higher than said given frequency cut-0E characteristic of said system, said horn extending outwardly from said opening, said speaker being mounted with the diaphragm thereof at said opening to project sound through said opening and said horn and to define with said enclosure a cavity, the Walls of said cavity having a port providing limited communication of said cavity with the atmosphere, said ported cavity having a resonant frequency approximately equal to the'resonant frequency of said horn loaded speaker.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Angevine et al.: Publication, Audio Engg, March 1948, page 25. (Copy in Library.)
Planar et al.: Publication, Audio Engg, page 29. (Copy in Library.