Low frequency loudspeaker system

Jan. 5, 1965

S. R. RICH

3,164,221

LOW FREQUENCY LOUDSPEAKER SYSTEM

Filed June 6, 1961

Sheets-Sheet 1

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FIG.2

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FIG.4

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INVENTOR.

STANLEY R. RICH BY _ „

ATTORNEY

Jan. 5, 1965

Filed June 6, 1961

s. r. rich 3,164,221

LOW FREQUENCY LOUDSPEAKER SYSTEM 2 Sheets-Sheet 2

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ATTORNEY

United States Patent Office

3,164,221

Patented Jan. 5, 1965 .

3,164,221 LOW FREQUENCY LOUDSPEAKER SYSTEM Stanley R. Rich, West Hartford, Conn., assignor, by mesne assignments, to Alfred H. Rosen, Newton, Mass.

Filed June 6, 1961, Ser. No. 115,171 13 Claims. (Cl. 181—31)

This invention relates to loudspeakers for the reproduction of musical sounds, and more particularly to improvements in low frequency loudspeaker systems.

A basic problem in loudspeaker design arises from the fact that, for a given radiator diaphragm, the radiation resistance per unit area of diaphragm decreases as the diaphragm size or area becomes smaller compared to a wavelength in air of the sound being radiated. This problem becomes particularly acute at the lowermost frequencies in the low audio frequency range, that is, at frequencies below approximately 200 cycles per second. For example, the velocity of sound in air at ordinary room temperatures (about 60°-80° F.) being about 1100 feet- 20 second, the wavelength (λ) of sound at 100 c.p.s. is about 11 feet, and at 40 c.p.s. about 27.5 feet. At 200 c.p.s. λ—5.5 feet (approximately). Obviously, even a large loudspeaker having an effective radiating diaphragm area 12-15 inches in diameter is barely λ/4 across at 200 c.p.s. 25 Thus, the radiation resistance of such loudspeakers falls off sharply at frequencies below about 200 c.p.s.

I am aware that numbers of solutions have heretofore been proposed to the problem of increasing the capability of loudspeakers in the low frequency range. One of these 30 employs a so-called “corner horn,” in which a driver unit is coupled to a horn, of folded configuration, located to employ a corner of a room as its mouth. This approach effectively increases the size of the mouth of the radiating element, but it sets a stringent limit to speaker placement 35 (in corners only), and is bulky and space consuming. A self-contained folded horn configuration, not so limited as to placement, would be even more bulky.

Bass reflex cabinets, making use of the Helmholtz resonator, with a duct or port that serves useful phase control functions, are also used. Included here are the “acoustic labyrinth” and similar schemes to back-load a speaker and sometimes radiate from the rear to reinforce the direct radiation at the low frequency end of the audible spectrum. Conventional cone-type speakers are employed in these cabinets. These cabinets tend to peak selectively at certain frequencies, and are also quite bulky. They are also complex to construct.

One obvious approach, which has enjoyed only limited acceptance, employs the brute force technique of using extremely large cone-shaped diaphragms, often 18 inches, and sometimes in excess of 30 inches in diameter. These require proportionately large enclosures, such as infinite baffles, or back-enclosed cabinets. Aside from the obvious facts that such loudspeakers are too large and heavy 65 for use in places other than public assembly places and the largest of homes, there is also the problem of preventing such large diaphragms from having their own resonances at various frequencies, especially those above the low frequency range. 60

Still another approach to solution of this problem is that which is described in Villchur Patent No. 2,775,309, wherein the back of a cone-type speaker is acoustic loaded by the stiffness of an enclosed air mass to resonate at a prescribed low audio frequency. This system employs 65 cones having diameters of 12 inches or less and obviously accepts low radiation resistance as an applicable condition, as do those employing bass reflex cabinets and, to a lesser extent, those employing cones 30 inches in diameter. The Villchur system is further characterized by requirement for an extremely limp suspension for, and extremely long excursions of the cone, in order to achieve its objec2 tive of improved low frequency response. Extremely large excursions of the radiator diaphragm are necessary in and are designed into nearly all of these low radiation resistance systems.

It is an object of the present invention to. provide a loudspeaker system having improved capabilities in the low audio frequency range, namely at frequencies under 1000 cycles per second, and particularly at frequencies under 200 cycles per second. A particular object of the 10 invention is to increase the radiation resistance of low frequency loudspeaker systems without simultaneously requiring a substantial increase in size of the resulting system or introducing any requirement as to speaker placement. Another object of the invention is to provide 15 such an improved low frequency loudspeaker system which requires only small excursions of the radiator diaphragm to operate effectively. A further object of the invention is to provide such an improved low frequency loudspeaker system which can employ as a driver unit a commercially available cone-type loudspeaker of lesser capabilities and modest cost. Another object of the invention is to provide a low frequency loudspeaker system which has improved and highly faithful low frequency response throughout the entire range of low audible frequencies and is substantially devoid of resonances and distortions throughout said frequency range. Another specific object of the invention is to provide a low frequency loudspeaker system having improved transient response. Additional objects are to provide, low frequency loudspeaker systems having improved operational characteristics which are not complex in structure, may be manufactured of readily available materials, will be interchangeable with other loudspeakers in sound reproduction systems, and can be priced competitively with other “high fidelity” loudspeaker systems.

According to the present invention a substantially fully enclosed column of air (or other gas) which is short relative to the wavelength of sound in it at any frequency in the low audio frequency range, is used to couple a first 40 or driver diaphragm to a second, or radiator diaphragm. This short column of gas is substantially incompressible under these conditions, so that the resulting system is dynamically the acoustic analogue of a hydraulic force transmitting system, such as a hydraulic jack. As a fur45 ther improvement, this column of gas may be endowed with a configuration such that it includes an acoustic inductance which assures isophasal radiation from the radiator diaphragm. The radiator diaphragm is thus parasitically driven from the driver diaphragm. The area of the 50 radiator diaphragm can be the same as or different from that of the driver diaphragm; when the area of the radiator diaphragm is larger than the area of the driver diaphragm, the invention has a number of unique advantages, noteworthy among which is an increase in radiated power. The driver diaphragm can be the cone of a commercially available loudspeaker, and the radiator diaphragm can have any desired shape, including that of a planar rectangular sheet, or a dome-shaped sheet. The radiator diaphragm can easily be designed to be substantially devoid of self-resonance at any frequency in the entire range of low audible frequencies and throughout a substantial frequency range immediately higher than the low audible frequency range.

I prefer to employ a radiator diaphragm having an area larger than that of the driver diaphragm since then, as will be shown, the radiation resistance of the over-all system is increased over that of the driver diaphragm alone in proportion to the ratio of the larger area to the smaller area. The radiator diaphragm can be made a few 70 to several hundred times larger in area than the driver diaphragm. The present invention then has advantages similar to those of a very long horn, but in an extremely

3,184,221 short length, without any of the geometric or acoustic disadvantages of a low frequency horn.

Other and further objects and features and advantages of the invention will become apparent from the following description of certain embodiments thereof. This de- 5 scription refers to the accompanying drawings, wherein:

FIG. 1 is a side-sectional view of an embodiment of the invention which illustrates the general principles thereof;

FIG. 2 is a section along line 2—2 of FIG. 1;

FIG. 3 is a cross-sectional view of one structure useful iq for the radiator diaphragm;

FIG. 4 is a cross-sectional view of another suitable radiator diaphragm structure;

FIG. 4A is a cross-sectional view of a dome-shaped radiator diaphragm;

FIG. 5 is a side-sectional view of a second embodiment of the invention;

FIG. 6 is a front view of FIG. 5; and

FIG. 7 is a top-sectional view of a third embodiment of the invention.

Referring to FIGS. 1 and 2, the system there illustrated includes a rigid-wall cabinet 10 having a top wall 11, bottom wall 12, back wall 13, side walls 14 and 15 and a front wall 16 having a rectangular aperture 17 occupying the major portion thereof. An intermediate wall 18, 25 parallel to and between the front and back walls 16 and 13, respectively, and having an aperture 19 occupying a minor portion thereof, is supported from the top, bottom and side walls 11, 12, 14 and 15, respectively, and divides the interior of the cabinet into two chambers 21 and 22, 30 respectively. A driver transducer 23, comprising a driver diaphragm element 23.1 supported in a first flexible compliance frame 23.2 in the aperture 19 in the intermediate wall 18, completely hermetically seals that aperture. A radiator diaphragm 25, which is substantially rigid and 35 aperiodic, as will be hereinafter more particularly described, is supported in a second flexible compliance frame 26 in the aperture 17 in the front wall 16, completely hermetically sealing that aperture. The first chamber 21 is thus totally enclosed with respect to the regions outside 40 it, and can communicate acoustically with outside regions only through motion of the driver diaphragm 23.1 and the radiator diaphragm 25. A mass of air, or other gas, 24, is contained in the first chamber 21. The second chamber 22, at the rear of the driver transducer 23, may be vented to the exterior through an orifice 27; it is otherwise totally enclosed. The walls of the cabinet, including the intermediate wall 18, are made of a rigid material, such as plywood % or more thick. Other materials which are sufficiently stiff, thick and dense, or otherwise constructed substantially to prevent the transmission therethrough of sound energy, may be used, if desired.

The driver transducer 23 is generally illustrative of any suitable driver transducer. A commercially available moving-coil cone-type electrodynamic loudspeaker may be employed, as will be more fully explained in connection with FIGS. 5 and 6 hereof.

Referring now particularly to the totally enclosed first chamber 21, the distance “d” between the periphery of the radiator diaphragm 25 and the periphery of the driver diaphragm 23.1 is small compared to the shortest wavelength λ in the operating frequency band of the system. More particularly, this distance “d” is, for example, onequarter or less of such wavelength. The driver diaphragm element 23.1 of the transducer 23, during operation in this frequency band, drives the larger radiator diaphragm 25 through the gas 24 enclosed in the first chamber 21. Under the condition stated above the gas 24 is substantially noncompressible with respect to elastic wave motion therein at all frequencies in the operating frequency band, that is, at all frequencies below and including the frequency corresponding to half the aforesaid shortest wavelength λ. This phenomenon is physically practical in the band of frequencies from somewhat below 20 c.p.s. to above 1000 c.p.s., the upper limit depending on how small the distance “d” is made. The result of this phenomenon is that the dynamic pressure exerted by the smaller driver diaphragm 23.1 on the enclosed volume of gas is applied uniformly to and throughout the entire interior of the enclosed volume of gas 24, and consequently to the entire inside surface of the walls of the sealed first chamber 21, including the entire inside surface of the larger radiator diaphragm 25. This phenomenon is the same, under the dynamic conditions stated above, as that which exists when static pressure is exerted upon an enclosed volume of a liquid, as expressed by Pascal’s law—to wit: The pressure in a confined, noncompressible liquid at rest, due to the application of external forces, has the same value throughout the body of liquid. By virtue of this phenomenon, which I have discovered to be true under dy15 namic conditions rendering a gas also noncompressible, the smaller driver diaphragm 23.1 is uniformly coupled acoustically to the larger radiator diaphragm 25 by the acoustical stiffness of the intervening gas 24. The radiator diaphragm may be regarded as “parasitically excited” 20 by the driver diaphragm through the effectively noncompressible gas. In addition, as will be explained in detail hereinafter, the mass of the enclosed volume of gas 24 and the mass of the radiator diaphragm 25 are added dynamically to the mass of the driver diaphragm 23.1, thus lowering the effective resonant frequency of the driver diaphragm element 23.1.

The radiator diaphragm 25 is, as is mentioned above, substantially rigid and aperiodic. It is supported in the aperture 17 by a flexible compliance frame 26, which may for example be a tape of woven cotton attached, as by a suitable cement (not shown), at or near one edge to· the periphery of the diaphragm and at or near the other edge to or near the periphery of the aperture 17, as is shown and described in greater detail in connection with FIGS. 5 and 6. The compliance frame 26 should be flexible at all frequencies in the audio frequency range of the system, and should offer the minimum resistance to piston-like vibration of the radiator diaphragm 25, while at the same time supporting the radiator diaphragm in the opening 17, and preserving the hermetic seal of the first chamber 21. A woven fabric (e.g., cotton) tape, attached as described above, and coated or impregnated with a nonhardening material, such as latex rubber, to provide a hermetic and highly flexible seal, will achieve these purposes. The radiator diaphragm 25 is driven by acoustic pressure waves ⁴⁵ applied with uniform dynamic pressure throughout its entire inner surface, and therefore vibrates acoustically as a rigid piston throughout its entire frontal area, that is, the area facing a listener. Being substantially rigid and aperiodic, the radiator diaphragm develops no flexural waves of its own, but acts as a true flat piston diaphragm. Additional advantageous features of the radiator diaphragm 25 will be described below in connection with FIGS. 3 and 4, which illustrate preferred embodiments thereof.

The configuration illustrated in FIGS. 1 and 2 combines ⁰⁰ two physical phenomena which cooperate to provide a net gain in radiated power, in a small space. Considering first the relative motions of the driver diaphragm 23.1 and the radiator diaphragm 25, these motions can be described by the relation:

⁶⁰ X₁A₁=X₂A₂ (Relation One) where:

Xi is the piston-like displacement of the driver diaphragm 23.1 in the direction toward or away from the front ⁶⁵ wall 16;

X₂ is the corresponding piston-like displacement of the radiator diaphragm 25 in the same direction;

Aj is the area of the driver diaphragm 23.1; and A₂ is the area of the radiator diaphragm 25.

It is clear, then, that the actual displacement of the larger radiator diaphragm is smaller at any operating frequency than the corresponding displacement of the smaller driver diaphragm, in accordance with the ratios of the 75 areas of the smaller to the larger diaphragm. That is, if

3,164,221

4^=34_1; then Xi=3X₂, just as occurs under steady-state conditions in a hydraulic jack. There is thus a diminution in amplitude of motion as we go from the smaller diaphragm 23.1 to the larger diaphragm 25, which of itself would reduce the net power output of the system, if not for the fact that the increase in radiation resistance contributed by the larger radiator diaphragm 25 more than overcomes the effect of reduction in displacement from Xi to X₂, and attributes a net gain of power output to the system, as will now be explained.

_ At low frequencies, that is, frequencies at which the circumference of a radiating diaphragm or piston (2tt/·, where “r” is the radius) is small compared with the wavelength λ in air of the sound being radiated, the impedance of the air, per unit diaphragm or piston area doing the radiating, is:

„ pc 4ir²r² . ,8rap + 3 (Relation Two) where:

2tt/;

/=the frequency of the sound being radiated;

P=air density (grams/cm.^s);

c=velocity of sound (cm./sec.);

r=radius of circular piston or diaphragm (cm.); and /-—wavelength (cm.)

The first term is radiation resistance R_a, per unit area, and the second is reactance. Since radiated power is proportional to radiation resistance we can, for our present purposes, ignore the second, or reactance, term. Thus:

Sa=-5—Γ7- (Relation Three)

J Λ⁴

This is the expression for the radiation resistance per unit area of a circular diaphragm or piston having a circumference which is small compared to λ, and it is sufficiently accurate to apply to a square or nearly square piston or diaphragm under the same condition.

The acoustic power output, P, of a radiating piston or diaphragm is always defined by the relation:

(flrX. 2 &amp; ) (Relation Four) where:

P is expressed in ergs/sec.;

R‘_a is the total radiation resistance of the diaphragm or piston; and (Ϊ) is velocity of motion of the piston or diaphragm, expressed in cm./sec.

The term R^!_a is the product R_&amp;-A, for any given diaphragm, where:

R_a is the radiation resistance per unit area; and A is the area of the diaphragm.

Further:

Α=τγγ², for a circular diaphragm or piston and, for practical purposes in our present case, we can assume this relationship to apply to a close degree of approximation for a square or nearly square piston or diaphragm.

Hence, we can set:

Ρ—ΐι,~τ·^![ (Relation Five) for circular and for square and nearly square piston and diaphragm radiators. Relation Five reduces to:

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Zd-rM

P=Kr²· } (Relation Six) where K is a constant substituted for the expression /PC 4tt³\ ₅ Thus we can state:

Pq.j.2.,.2/_ J (Relation Seven)

It is to be noted that the factor r² occurs twice in Relajθ tion Six in the product which determines P, and only once in Relation Three in the product which determines R_a. The increase in the radiation resistance per unit area which is due to the increase of radius from the smaller. piston 23.1 to the larger piston 25 is exactly opposed by the _lg decrease in displacement X₂ of the larger diaphragm 25 as compared with the displacement Xj of the smaller diaphragm 23.1. Thus, in Relation Seven we can cancel the term /dx\²

XjdtJ and one of the terms r², leaving:

Par² (Relation Eight)

It is thus shown that, in the present invention, an in^2o crease in the radius of the radiator diaphragm 25 as compared with the radius of the driver diaphragm 23.1 results in an increase in the radiated power which is proportional to the square of the ratio of the two radii. This increase in radiated power is thus also proportional to the ratio ^o0 of the areas of the two pistons or diaphragms. While the foregoing analysis has been made in terms of circular pistons or diaphragms, it is also true, to a close degree of approximation for square or nearly square pistons or diaphragms which are approximately equivalent to a ⁰⁰ circle of the same area, and in the case where one piston or diaphragm is round and the other is square or nearly square. By the expression “square or nearly square,” I mean a piston or diaphragm in which the length and width have a ratio of 1:1 to approximately 2:1. For example, I have verified this conclusion in the case where a rectangular radiator diaphragm 25, which was 18 inches long and 12 inches wide, was driven by a circular coilcone type loudspeaker having an overall diameter of 8 inches, and again where the latter had an over-all diam45 eter of 12 inches, and have found that this conclusion, namely that PaA, is true to a close degree of approximation when only the ratio of the areas is considered. It is a unique characteristic of the present invention that this relationship holds true while simultaneously the displace50 ment of the larger radiator diaphragm is smaller in amplitude than that of the smaller driving element; this reduction in the amplitude of displacement of the radiator diaphragm, of itself, endows the invention with additional advantages contributing to high-fidelity sound re55 production, which will be explained below.

The invention introduces the possibility of employing for the radiator diaphragm 25 materials having advantages which are not easily used in cone-type reproducers. As has already been shown, the radiator diaphragm 25 60 is driven with dynamic pressure applied substantially uniformly all over its inner surface, in effect according to Pascal’s law. It is thus inherently capable of behaving as a true “isophasal” radiator, unlike a cone driven by a voice coil concentrating driving force at its apex, which 65 inherently tends to flex under the driving force, especially as its aperture is increased. FIGS. 3, 4 and 4A illustrate, in cross section, radiator diaphragms which I have built and used as the radiator diaphragm 25 and as the corresponding element 45 in FIG. 5.

In FIG. 3, a sheet of light-weight plastic material 30, for example, a sealed-cell polystyrene foam, approximately one-quarter inch to one-half inch in thickness t,” provides the basic element of a radiator diaphragm. By itself, the sheet 30 is capable of exhibiting flexural reso73 nances at one or more frequencies in the low frequency

3,164,231 &lt;7 a band (that is, the frequency band from below approximately 20 c.p.s. to approximately 1000 c.p.s.). One solution to this problem is to provide a bulge in the sheet 3®, that is, to make it dome-shaped, as is shown in FIG. 4A, sufficient to assure that the lowest frequency of such self-resonance will be greater than 1000 c.p.s. This is a successful solution, but it may be undesirable in practice in certain cases, in that it increases the space requirement of the system. Another solution, preferred because it does not introduce such an increased space requirement, is to coat one or both of the surfaces 31 and 32 with a vibration dissipative material 33 adhering to the surface and providing damping resistance to flexural motion of the sheet 38. A layer or coat of rubber-like material which resists cross-linking, hardening and oxydizing with age may be used as the vibration dissipative material 33. I have used polyisobutylene. Other useful materials are acrylics, thiokols, vinyls and rubbers. This material can be painted or sprayed on the sheet 30, or the sheet can be dipped in the vibration dissipative material.

The vibration dissipative material can be made more efficient for damping flexural vibrations if it is combined with another element to place it in shear with respect to flexural motion in the sheet 30. FIG. 4 illustrates a suitable structure, in which radiator diaphragm sheet 30.1, similar to the sheet 3® in FIG. 1, is coated on one side 32.1 with vibration dissipative material 33.1, and a layer of fibrous paper 34 similar to soft facial tissue, covers the exposed surface of the dissipative material. In practice, the vibration dissipative material occupies the interstitial space between the fibers of the paper. The opposite surface 31.1 of the sheet 30.1 can be similarly treated, if desired.

Radiator diaphragms according to FIGS. 3 and 4 are substantially free of flexural resonances at frequencies below approximately 1000 c.p.s. They may be used flat or planar, or curved, for example, dome-shaped, as desired. I have thus provided a true piston-type radiator which possesses sufficient rigidity to be used as a loudspeaker, and a system for driving such a radiator as a true isophasal radiator.

A radiator diaphragm made of a sheet 38 or 30.1 of sealed-cell polystyrene foam 18 inches long, 12 inches wide and ¹/4 inch thick, coated with vibration dissipative material as shown in FIG. 3 or FIG. 4 weighs approximately 90 grams. The volume of the radiator diaphragm is approximately 885 cubic centimeters. The density of the diaphragm is then:

90/885=0.1 gram/cc. (approximately)

This is only 100 times the density of air.

Alternatively, one might employ balsa wood in place of the sealed-cell polystyrene foam described in connection with the sheets 30 and 30.1. Generally speaking, any low density rigid sealed-cell plastic foam, or equivalent material, may be employed.

The mass loading of the radiator diaphragm 25 (FIG. 1) lowers the resonant frequency of the driver diaphragm 23.1. As is well known, the frequency of resonance of a cone-type speaker in free space is calculated from the relation:

. _ 1 /Stiffness 1 [~S Mass ~2TV M where:

S is the stiffness of the suspension (dynes/cm.); and M is the mass of the cone (grams).

This is generally in the region 40-80 c.p.s., in average 12-i.nch loudspeakers. The frequency of resonance can be lowered by increasing the effective mass, or lowering the effective stiffness, of the speaker system.

Referring to FIG. 1, we recall that the mass of the radiator diaphragm 25 is coupled to the driver diaphragm 23.1 by an effectively noncompressible fluid 24 in the first chamber 21. Hence, the mass of the radiator diaphragm is added directly to that of the driver diaphragm along with the mass of the gas 24 contained in the first chamber. This lowers the resonant frequency of the sys5 tern to the region 10-30 c.p.s. The second chamber 22, at the rear of the driver diaphragm 23.1, may be vented through aperture 27 to hold the stiffness of the system to a minimum, consistent with well-known principles of using a cabinet at the rear surface of the driving transducer. 10 Alternatively, the vent aperture 27 may be omitted, as desired: it is an advantage of the present invention that the dimensions and geometry of the second, or rear chamber 22, are not critical.

My invention achieves improved transient response, and 15 virtual elimination of distortion due to transients. By “good transient response” I mean the ability of a loudspeaker system to reproduce a pulse without distortion of shape or addition of tails or hangovers following the main pulse. If a pulse is applied to the voice coil of a 20 cone-tyne loudspeaker, the cone will continue to oscillate on its own after removal of the driving force at the end of the pulse, This is due to the fact that the cone is a complex assembly of moving elements excited by the voice coil but not necessarily rigidly coupled to it. Shorter, of 25 B.B.C. (“Transient Response of Loudspeakers,” B.B.C. Research Report 1944-49) states that resonant elements loosely coupled to the moving coil continue to oscillate after the driving signal is removed and will radiate at their own natural frequency for a time that is propor30 tional to the Q of the resonating elements. This observation is further complicated by the fact that paper cones have unique discontinuities, inhomogeneities, configurations of surface slivers and other elemental parts which may resonate on their own when shock excited by a 35 transient. Thus, in a paper cone-type speaker the application of a signal pulse to· such an element will result in the addition of a transient tail to the end of the pulse, in which the frequency of the oscillations is the natural frequency of the resonant element and not the frequency of 40 the applied signal. This is the source of the individual characteristics of even the best paper cone-type loudspeakers. In the present invention, in addition to lowering the resonant frequency of the cone and coil assembly from the 40-80 c.p.s. region to the 10-30 c.p.s. region, 45 ^thf'Prevision of my radiator diaphragm 25 or 45 which is substantially aperiodic in the operating frequency band (up to approximately 1000 c.p.s.), in a system which tightly couples this diaphragm to the driver diaphragm 23.1, virtually eliminates transient distortion in this fre₅θ quency band.

The radiator diaphragm 25 executes small excursions at even the lowest audible frequencies, the largest excursions being approximately one-eighth inch. In a case where the driver diaphragm 23.1 has an area one-third 55 that of the radiator diaphragm, the driver diaphragm 23.1 will execute excursions three times as great as those of the radiator diaphragms. These are ail small excursions, compared to excursions executed by speakers in the small enclosure heretofore available. As a result, the system ₆₀ of the invention eliminates, for practical purposes, the sources of amplitude distortion and frequency modulation distortion which accompany loudspeaker systems employing cones driven at large excursion amplitudes. Frequency modulation distortion, in particular, is reduced 05 by reducing the amplitude of diaphragm motion in the very low frequency ranges.

FIGS. 5 and 6 illustrate a loudspeaker system according to the invention, employing a commercially available cone speaker 43 as the driver transducer, and having all 70 the advantages and features which have been described with reference to FIGS. 1 and 2. A rigid-wall cabinet 50 has a top wall 51, bottom wall 52, back wall 53, side walls 54 and 55, and a front wall 56 having a rectangular aperture 57 occupying the major portion thereof. An 75 intermediate wall 58 located between the front wail 56

8,164,221 and the back wall 53, and having an aperture 59 occupying a minor portion thereof, is supported from the top, bottom and side walls 51, 52, 54 and 55, respectively, and divides the interior of the cabinet into two chambers 41 and 42, respectively. A radiator diaphragm 45, simi- 5 lar to the radiator diaphragm 25 in FIGS. 1 and 2, is supported in a woven fabric flexible compliance frame 46 in the aperture 57 in the front wall 56. This compliance frame may be coated with a flexible rubber-like coat (not shown) similar to the layer 33 on the diaphragm 10 3©, for example, to assure that the aperture 57 is hermetically sealed by the diaphragm 45. The driver transducer 43 is shown only schematically, and has a coneshaped driver diaphragm 43.1 supported in the second chamber 42 by a rim 43.2 from the intermediate wall 15 58 surrounding the aperture 59 therein. A voice coil (not shown) of usual form, serves to drive the cone diaphragm. A vent aperture 47 may be provided in one wall of the second chamber 42, if desired. Each chamber encloses a gas (such as air), which is not shown. 20

The embodiment of FIGS. 5 and 6 operates in essentially the same manner as that of FIGS. 1 and 2. The distance “di between the confronting surfaces of the radiator diaphragm 45 and the intermediate wall 58 in FIG. 5 is smaller than the corresponding dimension (not identi- 25 fled) in FIG. 1 for the reason that the FIG. 5 embodiment is intended to operate in a low audio frequency band having a higher upper-frequency limit than the FIG. 1 embodiment. As in the case of the dimension “d,” in FIG. 1, this distance “dj” is maintained small compared 30 to half the shortest wavelength λ in the operating frequency band of the system; more particularly, “d,” is for example in the present embodiment %o or less of such shortest wavelength. If the upper-frequency limit is 200 c.p.s., λ—5.5 feet (approx.) at ordinary room tempera- 35 ture and di=7^j=0.28 feet in air, or about 3.5 inches. 40

If the upper frequency limit is 1000 c.p.s., λ=1.1 feet (approx.) in air at ordinary room temperature, and &lt;2ι=Λ=0·0δδ feet or about % inch, at the most. I have built systems according to FIGS. 5 and 6, intended for operation in the low frequency range up to 1000 c.p.s. in which the distance “dj” is Ya inch.

The aperture 59 in the intermediate wall 58 is smaller ₅θ than the design aperture of the driving transducer 43 so that the portion of the wall 58 covering the cone diaphragm 43.1 pressure loads this diaphragm during operating, and air moves through the aperture 59 at correspondingly increased pressure. The distance “dj” is so short _5g(for any wavelength in the operating low frequency band) that the space between the confronting surfaces of the radiator diaphragm 45 and the intermediate wall 58 functions as an annular-shaped acoustic choke or inductance surrounding the intermediate wall aperture 59, θθ throughout this frequency band; this acoustic inductance prevents sound wave radiation from the intermediate wall aperture 59 toward the periphery of the front wall aperture 57 (a distance “s” away in FIG. 5) from actuating the edge portions of the radiator diaphragm 45 out of θpliase with the mid portion thereof. This assures that the radiator diaphragm will be driven substantially isophasally throughout its entire radiating area.

It will be understood, of course, that, in operation, the low frequency system of the present invention is in- γθ tended to be used together with a suitable transducer or transducers for the higher frequency ranges, and with a suitable cross-over network which will assure that only signals in the low frequency range for which a particular low frequency system is designed will be applied to it. 75

Suitable dimensional specifications for loudspeaker systems which I have built according to FIGS. 5 and 6 are as follows:

The driver transducer 43 was an 8-inch permanent magnet cone speaker of a commercially available type; the second chamber was 20 inches high between the inner surfaces of the top wall 51 and the bottom wall 52, 14 inches wide between the inner surfaces of the side walls 54 and 55, and 5 inches deep between the inner surfaces of the back wall S3 and the intermediate wall 58; the aperture 59 in the intermediate wall 58 was a round hole 5 inches in diameter; the aperture 57 in the front wall 56 was 19 inches high and 13 inches wide; the radiator diaphragm Was a rectangle 18 inches high and 12 inches wide; the compliance rim 46 was made of woven cotton tape 1 inch wide, cemented to both the periphery of the radiator diaphragm 45 and to the front wall 56 at the periphery of the aperture 57 with a suitable cement such as a nonhardening and non-oxidizing rubber cement, as described above; this diaphragm was otherwise one of those described in connection with FIGS. 3, 4 and 4A; the distance “dp was Ya inch; the walls 51, 52, 53, 54, 55, 56 and 58 were all made of % inch thick plywood; and, air was used as the gas in the chambers 41 and 42.

The radiator diaphragm 45 may be planar as is shown in FIG. 5. Alternatively, if desired, the radiator diaphragm can be given a dome-shape (not shown) as is mentioned above. If it is desired to cover the front of the radiator diaphragm with a grille cloth, the cabinet side, top and bottom wails 54, 55, 51 and 52, respectively, can be extended forward to provide a grille-cloth mounting which will enable it to clear the radiator diaphragm.

While it will be clear that my low frequency loudspeaker system can be made of such small dimensions that its placement in a room presents no problems, as is set forth in the objects of this invention, the fact should not be overlooked that the second chamber (42 in FIG. 5 or 22 in FIG. 1) can have any desired shape, including a triangular shape enabling a unit according to the invention to be set deeply into the corner of the room. FIG. 7 illustrates such an embodiment of the invention, in top section. This figure is a modification of FIGS. 5 and 6, in which only the configuration of the second chamber 42 is modified. All other parts are the same as in FIGS. 5 and 6, and bear the same reference characters. To a subassembly comprising the intermediate wall 58, front wall 55, radiator diaphragm 45, compliance rim 46, and driver transducer 43, enclosing the first chamber 41, there is peripherally mounted a pair of angularly related side walls 63‘ and 64 confining with triangular-shaped top and bottom walls (not shown) a triangular-shaped second chamber 62. A vent aperture 67 (similar to apertures 27 in FIG. 1 and 47 in FIG. 5) may be provided in one or the other of the top and bottom walls, if desired. This embodiment of the invention may be located snugly in the corner of a room, if desired.

It will also be appreciated that the above-mentioned subassembly can be provided without the wall parts defining the second chamber 62, for mounting in an infinite baffle, such as the wall or ceiling of a room, if desired, or to be built into a cabinet having a compartment which will constitute a suitable second chamber. Embodiments of my invention can easily be provided for such “custom” installations, as is evident from FIG. 7.

The embodiments of the invention which have been illustrated and described herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also within the spirit

S 1 R4 QO and scope of the invention, and it is intended that this invention include all modifications and equivalents which fall within the scope of the appended claims.