US2775309A - Sound translating devices - Google Patents

Description

Dec. 25, 1956 E. M. VILLCHUR SOUND TRANSLATING DEVICES Filed March 15. 1954 FEG. 2

ENCLOSURE VOLUME CU. F'T

PTIMUM RESONANT FREQUENCY 0F LOUDSPEAKER \AX` O B RESONANT FREQUENCY) CYCLES/SEC.

IN VEN TOR.

@7W 44. mi@

United States Patent SOUND TRANSLAT ING DEVICES Edgar M. Villchur, Woodstock, N. Y., assigner to Acoustic Research, lne., Cambridge, Mass., a corporation of Massachusetts Application March 15, 1954, Serial No. 416,207

10 Claims. (Cl. 18131) This invention relates to sound translating devices and particularly to loudspeakers of the dynamic or movingcoil type. The term loudspeaker refers to a complete electro-acoustic device intended to translate periodic current to sound, comprising both the means for converting periodic electrical energy into mechanical energy, and the means for coupling said mechanical energy to the air into which the loudspeaker radiates.

One object of this invention is to improve the linearity of the force-displacement relationship in the moving system of a loudspeaker, thereby decreasing amplitude distortion of the reproduced signal. The expression moving system of a loudspeaker refers to that system comprising the voice-coil, the diahragm, the elastic supporting means supporting or suspending the aforesaid voice-coil and diaphragm from the frame, and the acoustical elements that influence the efective mass or stiffness of the foregoing mechanical portion of the moving system. In accordance with the main feature of this invention, the radiating diaphragm and driving voice-coil of a loudspeaker are so mounted and suspeended, that the elastic restoring force of the moving system is substantially derived from an enclosed gaseous body totally enveloping the rear of the diaphragm, rather than from the mechanical suspending members of the moving system. In order to achieve this result the suspending members are made in such a way, as described in further specifications, that their elastic stiffness is small or negligible in comparison to the elastic stiffness of the enclosure, but the total elastic restoring force applied to the moving system is not changed from its optimum value. The existence of such an optimum value of elastic restoring force in a loudspeaker of given structure will be explained hereinafter.

A further object of this invention is to improve, in a loudspeaker, the uniformity of frequency response at the lower frequencies. In accordance with this object, means are provided for applying most of the elastic restoring force of the moving system of the loudspeaker to the whole of the surfaces of the diaphragm through the acoustical stiifness of a totally enclosed body of air, in contrast lto the present-day practice of applying such elastic restoring force to the apex and rim of a conical diaphragm through the mechanical stiffness of elastic susending members. The application of the restoring force to the Whole of the diaphragm surfaces tends to keep the diaphragm rigid in the lower frequencies, thereby achieving the aforesaid object. The increased diaphragm rigidity in the lower frequencies also opposes the formation of sub-harmonic vibrations spurious to the input signal.

Still another object of this invention is to reduce the required cubic volume of the mounting device of a loudspeaker of the type which employs a totallyenclosed cabinet. Means are provided for achieving acoustical isolation between the front and rear surfaces of the diaphragm of a loudspeaker by a small, sealed enclosure placed around the rear of the diaphragm, the ,elastic stiffness of the enclosed air compensating for the lack of elastic stiifness in the suspending members of vthe moving ICC system, and the volume of said enclosure so regulated and co-ordinated with the characteristics of the electro-mechanical transducing mechanism that the total stiifness and restoring force of the moving system is neither higher than nor lower than optimum.

These and other features of the invention will be apparent from the following specifications, which are associated with drawings in which:

Fig. 1 is a front View of a loudspeaker embodying the present invention.

Fig. 2 is a section, taken on line 2-2 of Fig. 1, of the same loudspeaker.

Fig. 3 is a front view, on a larger scale, of the flexible suspending member 6 for the small end of the diaphragm.

Fig. 4 is a graph, in which the relationship between cabinet volume and resonant frequency in a loudspeaker embodying this invention is compared to the same relationship in a present-day conventional totally enclosed loudspeaker of equivalent diaphragm diameter and moving mass.

Before describing the foregoing structures and graphs in detail, it will be well to set forth certain theoretical considerations regarding a moving-coil loudspeaker.

One of the major sources of amplitude distortion in present-day moving-coil loudspeakers, and often the greatest single source of such distortion, is the non-linear force-displacement relationship in the moving system created by the mechanical elastic suspensions, especially at large diaphragm excursions. The diiculty of designing such suspensions for uniform stiffness over the excursive path is increased by the fact that, in present-day loudspeakers, suspension compliance in the direction of diaphragm travel cannot be made as large as would be desirable from the point of view of linearity. One of the functions of such present-day diaphragm suspensions is to provide a necessary elastic restoring force to the moving system of the loudspeaker, in order to counter-balance the mass reactances of the diaphragm, voice-coil and air load to the proper degree.

As is well known, the principal resonant frequency of a loudspeaker is that freqdency at which the total mass reactance and total compliance reactance of the vibratory moving system become equal, according to the relationship:

Fwrrn (l) where Other factors being equal, it is desirable to make the resonant frequency of the loudspeaker as low as possible because of the attenuation of response below the resonant frequency. There is, however, an optimum resonant frequency for a given loudspeaker, and if the resonant frequency is lowered from this optimum the performance is degraded rather than improved.

The existence of a lower limit to the choice of suitable suspension stiffness and resonant frequency, in the design of a given loudspeaker, is associated with the fact that voice-coil excursion for the same acoustical power radiation is an inverse function of frequency. Excursion and frequency are inversely proportional in any constant velocity vibratory system, while the radiation resistance seen by the loudspeaker diaphragm decreases progressively below the frequency of ultimate air load resistance. Constant power radiation from the diaphragm of a directradiator loudspeaker thus requires an approximately quadrupled voice-coil excursion for each lower octave in the bass portion of the frequency spectrum, a requirement which is fulfilled by the mass-controlled nature of the moving system above the resonant frequency.

Since the excursion of a speaker voice-coil approximately quadruples withl each lower bass octave down to the resonant frequency, for a speaker with a given power rating and electromagnetic and mechanical structure, the resonant frequency cannot be lower than that value which keeps the voice-coil excursions within the arca of linear' ux, and within bounds before the suspensions bottom7 or reach their elastic limit.

The values of suspension stiffness and resonant frequency of a given loudspeaker must not, therefore, be lower than those values which keep the voice-coil excursion within its proper limits for all signals within rated power. Such limits are determined by the length of the uniform magnetic path in relation to the mechanicoacoustical efliciency of the loudspeaker. If the suspension stiffness and resonant frequency are decreased from the above-mentioned values, the voice-coil will be driven out of the path of uniform magnetic flux or strike the flange of the pole piece on low frequency, high amplitude signals. The resultant amplitude distortion is far worse than the alternative attenuation below a higher resonant requency, an attenuation which protects the moving system against low-frequency excursions of undesirable magnitride.

ln addition to the foregoing considerations concerning the relationship between amplitude distortion and the choice of resonant frequency for a given loudspeaker, the influence of such choice on bass frequency response must also be taken into account. Too low a resonant frequency, while extending the low frequency limit of response at the expense of increased amplitude distortion, may also produce attenuation of response in the frequency region of optimum resonance and above.

Various proposals have been made in the literature to reduce the effect of non-linearity in diaphragm and voicecoil suspensions by designing such suspensions with very high compliance and negligible restoring force. Because of the foregoing considerations relative to allowable voicecoii excursion and reinforcement of bass response, however, such proposals cannot be and have not been carried out in commercial practice. Present-day loudspeakers are generally designed for resonant frequencies in the frequency region between 30 and 15() cycles per second. Loudspeakers whose magnetic structures allow limited linear voice-coil excursion, and whose mechanico-acoustical etiiciency is low by reason of the employment of a direct-radiator diaphragm of small area, are purposely and necessarily assigned resonant frequencies in the higher range, while loudspeakers in which the magnetic structures provide for large voice-coil excursion, and which have increased mechanico-acoustical efliciency by virtue of large diaphragm area or special acoustical couplers to the air, are properly ,designed for resonant frequencies in the lower range.

lt is recognized that, at the lower frequencies, it is desirable for the loudspeaker diaphragm to move rigidly, as a unit since diaphragm llexure and break-up introduce sub-harmonics of the input signal fundamental, and irregularities in the frequency response curve as is well known.

The two main structural features of the loudspeaker illustrated in Fig. l and Fig. 2, in necessary combination, are:

l. The rear of the loudspeaker diaphragm 1 is totally enclosed by a suitably air-tight and rigid acoustically sealed enclosure 2, in the preferred design filled with Fiberglas or some other equivalent sound absorbing material 3, the cubic volume of said enclosure being determined by the optimum acoustical compliance and elastic restoring force required by the moving system according to the following relationship:

where CAzthe compliance of the enclosure, in cm./dyne.

V=the interior cubic volume of the enclosure, in cm3.

p=the density of the enclosed medium, in gm./cm.3.

c=the velocity of sound in the enclosed medium, in

cm./sec.

S=the area of the diaphragm, in cm2.

ano;

2. The diaphragm 1 and voice-coil 4 of the loudspeaker are mechanically suspended by a rim-suspending member 5 and a voice-coil suspending member 6 from a frame 7, said members designed for the sole functions of maintaining the moving system mechanically centered, and providing little or no restoring force to the moving system in quantitative comparison with the restoring force provided by the acoustical pressures on the surfaces of said diaphragm and for completing the acoustical sealing of the enclosure. These structural features will now be discussed in detail.

It is Well known that an enclosed body of air has elastic properties. In the structure of Figs. l and 2 the elastic restoring force provided by the enclosed body of air is applied to the diaphragm surfaces for excursions in both directions; when the diaphragm moves back the compressed air of the enclosure exerts a greater pressure on the rear surface of the diaphragm than the pressure exerted on the front surface of said diaphragm by the free atmosphere, and on forward excursions the rarefied air in the enclosure exerts less pressure on the rear surface of the diaphragm than the pressure exerted on the front surface of the diaphragm by the free atmosphere. The index of the optimum restoring force and enclosure volume for a given loudspeaker is the optimum resonant frequency, at which the amplitude distortion characteristics and bass frequency response of the loudspeaker are most favorable.

The enclosure 2 is constructed of wood or other strong, rigid gas impervious material, preferably reinforced by braces 7 as shown. The existence of even small air leaks at the enclosure joints will significantly deteriorate the performance of the loudspeaker. The following measures are therefore taken to prevent air leakage from the enclosure: a gasket S, made of cardboard or other suitably soft material, and cut to match the opening through which the front of the loudspeaker diaphragm communicates to the free atmosphere, is placed between the removable balile 9 and the enclosure sides as an acoustical seal; the loudspeaker electrical leads 10 are passed to the outside of the enclosure through binding posts 11 rather than through open holes; and the enclosure sides are joined by glue 12 or fastened by other known methods that produce an acoustical seal.

The aforementioned Fiberglas filling the enclosure reduces the necessary volume of enclosed air for a given acoustical compliance, since the acoustical compliance of the enclosure is inversely proportional to the square of the velocity of propagation of sound in the enclosed mcdium, and the Fiberglas reduces said velocity. The Fiberglas also acts as an acoustical damping element, reducing the Q of the moving system and preventing the formation of standing wave resonances in the enclosure. rl`he Fiberglas is prevented from getting into the small spaces in which the voice-coil moves by a protective layer of cheesecloth 3.

While a completely gas-tight seal for the enclosure is theoretically desirable, in a practical construction it is apparent that the speaker cone would function in a manner analogous to an aneroid barometer in response to variations in atmospheric pressure. The voice-coil would accordingly be continuously biased from its predetermined optmum position in the magnetic eld due to atmospheric changes. Accordingly, a small amount of ,.5 bleeding or air leakage is permitted to exist between the inner and outer portions of the speaker enclosure in accordance with this invention. The amount of leakage is sucient only to equalize long-tirne variations due to atmospheric conditions but is insufficient to affect the inherent elastic stiffness of the enclosed air cavity for voicecoil excursions occurring when reproducing sound. In other words, the enclosure is designed to provide an acoustic seal, that is, one which does not allow significant air leakage over a period corresponding to a half-cycle of the lowest frequency encountered.

The enclosure of this invention differs functionally from the conventional totally enclosed loudspeaker cabinet in that, in the present invention, the acoustical stiffness of the enclosed air is regulated and used as an integral and necessary characteristic of the loudspeaker, while in the conventional totally enclosed loudspeaker cabinet the acoustical stiffness of the enclosed air is a necessary evil, and reduced to values as small as possible in order to avoid raising the resonant frequency of the moving system. To achieve such end, the volume of the conventional totally enclosed loudspeaker cabinet is made as large as practical, with no upper limit in an attempt to approach an infinite baffle. This functional difference has the quantitative result that the optimum volume of the enclosure used in the preferred embodiment of this invention is reduced, from the minimum volume of a conventional totally enclosed cabinet designed as part of an equivalent present-day loudspeaker, by a factor of four. While conventional loudspeakers have in the past been designed with relatively small totally enclosed cabinets, such design was at the expense of raising the resonant frequency of the loudspeaker well above its optimum value and of introducing bass attenuation, at 12 db/ octave in terms of pressure, below the new resonant frequency. Even if the conventional electromechanical transducing mechanism of said conventional small-enclosure loudspeaker is chosen on the basis of possessing a low resonant frequency unmounted, so that the final resonant frequency, using the small enclosure, is not too high for relatively good bass response, the added expense and care involved in the manufacture of such a conventional low-resonance transducting mechanism is wasted, and advantage cannot be taken of the increased length of the uniform magnetic path and other features associated with conventional loudspeaker transducer mechanisms of low resonant frequency.

An experimental model of the structure illustrated in Fig. 1 was constructed, and it was found that an enclosure of 2.4 cubic freet inside dimensions was required to produce an optimum resonant frequency of 5() cycles per second when employing a loudspeaker transducer having a diaphragm of nominal 12-inch diameter, and possessing a moving mass of typical present-day value. A comparable totally enclosed conventional loudspeaker of 5G cycles per second resonant frequency normally requires a cabinet volume of more than ten cubic feet.

Figure 4 illustrates the relationship between cabinet volume and resonant frequency in the experimental model of the present invention, plotted as curve A, compared to the same relationship in a conventional totally enclosed type of loudspeaker of equivalent moving system characteristics, plotted as curve B. It will be seen that for the conventional system, the curve of cabinet volume vs. resonant frequency approaches the line representing optimum resonant frequency asymptotically, While for the present invention the curve of cabinet volume vs. resonant frequency crosses the line representing optimum resonant frequency; the principle demonstrated by curve A applies to allloudspeaker transducers constructed according to this invention, whether or not the characteristics of the particular loudspeaker correspond to the values used in Fig. 4. The cabinet volume of the loudspeaker of this invention thus has an optimum value rather than a minimum value.

While the design of mechanical suspending members for the function herein set forth may take various forms,

one form thereof is illustrated in Fig. 2 and Fig. 3. The annular rim suspending member S supporting the large end of the diaphragm is made of a compliant, gas impervious flexible material such as closely woven cloth, soft leather, or similar material, which is bellied out or pleated for maximum freedom and minimum stiffness. A requirement of said rim suspending member is that it be substantially air-tight, to confine the acoustical pressures produced by the diaphragm.

The voice-coil suspending member 6 is shown in crosssection in Fig. 2, and in front view in Fig. 3. Said voicecoil suspending member 6 may be constructed of ber, cloth, or similar material, and does not have to be airtight since the dust cover 13 shown in Fig. 1 and Fig. 2 effectively seals the acoustical path from the rear surface of the diaphragm, through the voice-coil supporting member 6, and thence to the outside air. The voice-coil suspending member 6 of Fig. 2 and Fig. 3 is of the conventional corrugated disc type, altered by the introduction of staggered concentric slots 14. An alternative voice-coil suspending member may be made identical in shane to the conventional corrugated disc type, but composed of thinner material.

The acoustical seal between the front and rear surfaces of the diaphragm 1 is thus established and defined by the sides of the enclosure 2 joined by glue 12 or fastened by other suitable methods, the gasket 8 between the enclosure sides and the baille 9, the annular rim suspending member 5, the diaphragm 1 itself, and the dust cover 13.

ln the experimental model of the loudspeaker of this invention it was found convenient, in the light of the unavoidable elastic stiifness of mechanical suspensions which keep the voice-coil properly centered, to use a resonant frequency of twelve cycles per second for the electromechanical transducer mechanism without its enclosure. The transducer employed was conventional but was modified to provide the described type of gas-tight compliant rim and voice-coil suspensions, and was designed for an optimum resonant frequency, according to the considerations described above, of approximately 50 cycles per second. Referring to Equation l for determining resonant frequency, it will be seen that, compared to an equivalent present-day loudspeaker with the same moving mass, the compliance of the mechanical suspensions of a transducer constructed in accordance with the present invention is increased by a factor of sixteen. The electromechanical transducer mechanism of the loudspeaker of this invention, Without its enclosure, is unsuitable for mounting in conventional enclosures or baies of any type. Similarly, the enclosure of this invention is not suitable for use with any conventional loudspeaker electromechanical transducer mechanisms of size comparable to that of the transducer mechanism used in the invention.

When the transducer mechanism of this invention is combined with the enclosure 2, the resonant frequency of the moving system of the loudspeaker is raised to its optimum value, since the total compliance of the vibratory moving system is then determined by the net effect of the mechanical and acoustical compliances, according to the relationship:

where C=total net compliance. CMzmechanical compliance of suspending members. CA=acoustical compliance of enclosure.

The high mechanical compliance is thus offset by low acoustical compliance. In the experimental model of the loudspeaker of this invention the resonant frequency was raised to 50 cycles per second by the enclosure of air. The value of C in the formula for resonant frequency previously referred to is decreased sixteen times by employing the enclosure according to this invention, and the total stiffness and restoring force of the moving system is approximately 94% acoustical and only 6% mechanical. The effect of any non-linearity remaining in the mechanical suspending members is thereby rendered insignifcantly small.

The description of the experimental loudspeaker built in accordance with this invention, and of the illustrated embodiment, is illustrative only, and is not intended as a limitation of the invention.

The frequency response, in terms of pressure, of the experimental loudspeaker embodying this invention was measured in the low frequency range, under open-field conditions, and found to be within :4:15 db of reference level between forty cycles per second and four hundred cycles per second. The amplitude distortion of pure tones, in the region between forty cycles and one hundred cycles and at a given power input, was compared with the amplitude distortion of a control loudspeaker identical in every way to the said working model, except for the fact that the elastic restoringJ force of the moving system of the control loudspeaker was derived from the suspending members thereof, and the acoustical coupling device was of the type known as infinite baflie. The amplitude distortion of pure tones in the aforesaid frequency region, produced by the working model of this invention, was lower by several times than the distortion produced by said comparison loudspeaker.

I claim:

l. A sound translatingy device comprising a transducer of the type in which a voice-call and associated diaphragm, supported from a frame, execute excursions in the presence of a magnetic field, elastic supporting means for supporting said voice-coil and diaphragm in centered relationship with respect to the frame, said elastic supporting means providing a tiuid seal between the forward and rear surfaces of the diaphragm, and establishing, in combination with the moving mass of the transducer, a resonant frequency according to the relationship FT is the resonant frequency of the transducer in free air, in cycles/sec.

M is the moving mass of the transducer', in grams CM is the compliance of the elastic supporting means, in

cm./dyne;

means for applying an elastic restoring force to the diaphragm. comprising an essentially fluid-tight enclosure containing said voice-coil and diaphragm and defining, in connection with the rear surface of the diaphragm and the elastic supporting means attached thereto, a sealed cavity containing an elastic, fluid medium, the stiffness reactance of the moving system comprising the acoustical stiffness reactance of the enclosed iiuid medium and the mechanical stiffness reactances of the elastic supporting means for the voice-coil and the elastic supporting means for the diaphragm, according to the relationships Carl-Cn where C is the total compliance, in cm./dyne (reciprocal of stiffness) of the moving system CA is the compliance of the enclosure, in CDL/dyne and V C1A-1962s? where V is the interior cubic volume of the enclosure, in cm.3

S is the area of the diaphragm, in cm.2

c is the velocity of sound in the enclosed medium, in

ctn/sec.

p is the density of the enclosed medium, in gin/cm3;

the magnitude of the stiffness reactance of said enclosed fluid medium constituting more than half of the total stiffness reactance of the moving system, the remaining stiffness reactance being that of the said elastic supporting means for the voice-coil and diaphragm, and the combined stiffness reactance of said elastic supporting means and said enclosed iluid medium establishing, in combination with the mass reactance of the moving system, a natural resonant frequency according to the relationship 1 FTE=- Cari-CA where FTE is the resonant frequency of the transducer when contained in the enclosure.

2. The structure set forth in claim l, in which the stitfness reactance of the enclosed fluid medium constitutes more than seventy-five percent of the total stiffness reactance of the moving system.

3. The structure set forth in claim l, in which the stiffness reactance of the enclosed fluid medium constitutes more than ninety percent of the total stiffness reactance of the moving system.

4. The structure set forth in claim l, in which the elastic supporting means for the diaphragm comprises an annular cloth rim, attached to the large end of the diaphragm and to that part of the frame adjacent thereto, providing an acoustical seal between said diaphragm and said enclosure.

5. Pfhe structure set forth in claim l, in which said enclosure is filled with porous sound-absorbent material.

6. The structure as set forth in claim l, in which the natural frequency of said transducer in free air is in the subsonic region below 20 cycles per second.

7. A sound translating device having a natural frequency corresponding to the lower end of the range of frequencies to be reproduced uniformly, comprising a transducer of the type in which a voice-coil and associated diaphragm execute excursions in the presence of a magnetic field in response to applied electrical signals, means suspending said diaphragm and voice-coil in said eld comprising a compliant, essentially gas impervious material, the mass and compliance of said transducer having a characteristic natural frequency in free air which is at least one octave below the natural frequency of said sound translating device, means for raising said transducer natural frequency to an operating value corresponding to said defined natural frequency of said translating device and for exerting an elastic restoring force on said diaphragm comprising a rigid walled, essentially gas-tight enclosure having an opening sealed by said diaphragm and suspending means whereby the front and rear surfaces of said diaphragm are acoustically isolated from each other, the cubic volume of said enclosure being that value which will raise the natural frequency of said transducer to said operating value in accordance with the relationship FTE:

where 8. The structure as set forth in claim 7, in which the References Cited in the le of this patent elastic supporting means for the diaphragm comprises an UNITED STATES PATENTS annular cloth rim, attached to the large end of the dlaphragm and to that part of the frame adjacent thereto, 119 66564 Schlenker July 17 1934 providing an acoustical seal between said diaphragm and 5 214691773 Knowles May 10 1949 said enclosure- 2,688,373 Olson Sept. 7, 1954 10. The structure as set forth 1n claim 7, 1n which Hoadley: Publication, Radio dfn/News? Pages 52 the natural frequency of said transducer in free air is in 10 53 54 84 86 December 1951 Page 54 Fig 5 is ep the subsonic region below 20 cycles per second. tinent P Martin: Publication, Service, Fig. 2 is pertinent. Dec. 1953.

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