CA1100884A - Device for increasing the compliance of a speaker enclosure - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention is a device for use in combin-ation with a loudspeaker system, that includes a speaker enclos-ure, in order to effectively enlarge the volume of the speaker enclosure. The loudspeaker system also includes a vibratable cone. The device for effectively enlarging the volume of the speaker enclosure includes a gas having a gamma less than 1.4 and the product of its density and the square of the speed of sound therein less than the same product for air and a bag which is formed from a soft, pliable material for enclosing the gas within the speaker enclosure and which is adapted to seal the gas therein. The device also includes an acoustically trans-parent and porous cocoon which is disposed about the bag so that it surrounds completely the bag and an acoustical padding which is disposed adjacent to the sidewalls of the speaker enclosure and which is adapted to enclose the acoustically transparent and poxous cocoon. The decive may also include a device for gener-ating the gas by heating a fluid in its liquid phase so that the fluid changes to its gas phase. The device for heating the fluid may either be a resistive electrical element which is dis-posed within the speaker enclosure or fibrous, sound absorbent material which is disposed within the bag. The device is placed in the speaker enclosure in back of the vibratable cone in order to increase the compliance that the vibratable cone sees.

Description

1 ~ACKGROUND OF THE INVENTION

Field of the Invention , .
The present invention relates to a loudspeaker system and more particularly to a device for effectively enlarging the volume of a speaker enclosure of the loudspeaker system in order to increase the compliance that a vi~ratable cone of the loud-speaker system sees.
Description of the Prior Art The fundamental physics of the pressure variations in a sound wave are discussed in Mechanics, Heat and Sound ~y Francis ~eston Sears wherein on page 498 he sets out the follow-ing equation:
v - ~y ~ .. (p/ P ~, where v is the speed of sound in a gas, ~y) is the ratio, Cp/Cv of the specific heat of the gas at a constant pressure~ Cp, to the specific heat of the gas at a constant volume, Cv, p is the -pressure of the gas, and (p) is the density of the gas.
In Applied Acoustics, Harry F. Olsen and Frank Massa discuss a ~ack enclosed cone of a loudspeaker system on pages 197 and 198, the following is an excerpt from that discussion:
"In general cone speakers are used with ~oth sides of the cone open so that radiation into the air ~, ~' , 8~34 1 takes place from both sides. For certain uses, 25 for

2 example a standard source of sound for microphone cali-

3 bration, reverberation measurement, it ic desirable to

4 enclose the speaker mechanism in a box and thus confine t~e radiation from the cone. The important factor in 6 this syst~ is the stiffness introduced by the box. The 7 net result of this added stiffness is an attenuation of 8 the low-frequency response. A specific exam~le will 9 illustrate the important factors in this system."
In the specific example, the velocity of the 11 cone is given by the equation:
12 x = fM
13 M + jwm +
14 where fM = 31i = B(flux density in the air gap) 1 (length 1~ of wire in the voice coil) i (current in the voice coil), rM i 16 radiati~n resistance, m = mass of the cone, voice coil and 17 air load, C~ = CMl + CM2 = CMl ~compliance of center and 18 suspension system of the cone) + CM2 (compliance of the box 19 enclosing the back of the cone), CM2 - V (volume of the box)/
A (the square of the area of the cone) (p) (the density of 21 air) c2 (the square of the velocity of sound in air).
22 Another excerpt from Applied Acoustics follows:
23 'From a consideration of the equation it will be 24 seen that above the resonant frequency the velocity of the system is inversely proportional to the frequency; there-26 fore, since rM is proportional to the square of the 27 frequency the power output will be independent of the 28 frequency. Below the resonant frequency the velocity is 1 limited by the compliances CMl and C~12 and the velocity 2 of the cone is practically proportional to the ~requency, 3 which means that the response is rapidly attenuated with 4 decreasing frequency. Therefore, the low-frequency response limit will be determined by the respmamce fre- `
6 quency of t~e system. If CM2 is large compared to CMl, 7 then the compliance of the box will not materially affect 8 the response and the action will be practically the same 9 as that w~ th ~oth sides open to the air. If the resulting volume when the condition is satisfied is too large and 11 cumbersome, then the system must be altered. Since the mass 12 of the cone is practically proportioral to the area and 13 its inte ac~ion with the compliance CM2 is inversely 14 proportional to the square of the area, we can reduce the resonance frequency by reducing the area of the cone. Of 16 course, in general this will reduce the efficiency of the 17 system."
18 U. S. Patent No. 2,986,229, entitled Loudspeaker 19 Enclosures, issued to J. Carter Perkins on May 30, 1961, teaches a loudspeaker enclosure ,hat includes a rectangular 21 housing having a top, a bottom, a back, a pair of sides 22 and a front. The`front has a first port for a speaker and 23 a second port which is disposed below the first port. The 24 loudspeaker enclosure also includes a first planar baffle which is disposed between the sides- and which extends from 26 the front above the second port partway toward the back and 27 a second planar baffle which is disposed between the sides 28 and which extends from the rear edge of the first baffle 1 partway to the top. The loudspeaker further includes a 2 third planar baffle which is disposed above the firs~
3 port and which extends from the upper edge of the second 4 baffle partway toward the front. The patent relates generallv to the loudsoeaker enclosures of the sound absor-6 bent-walled, tortuous-path type sold under the trademark 7 "Acoustical La~yrinth."
8 U. S. Patent No. 3,923,119, entitled Sound 9 Pressure Box issued to George Joseph Frye on December 2, 1975, teaches a sound pressure box in which sound-absorbing 11 material is placed therein. U. S. Patent No. 3,923,118, 12 entitled Acoustic Baffle for Deep Submergence, issued to 13 Carl R. Johansen on ~ecember 2, 1975, teaches a multi-14 layered acoustic baffle which has a plurality of layers of fume-produced silicon dioxide with at least one layer 16 of high-density, viscoelastic material having a high 17 acoustic shear loss coefficient and being sandwiched 18 between the layers of silicon dioxide.
19 U. S. Patent No. 3,945,461, entitled Sound Speaker System, issued to Ralph J. Robinson on March 23, 1976, 21 teaches a sound speaker system whose volume in back of its 22 vibratable cone is varied by the telescoping of the two 23 cylinders which form the speaker enclosure. U. S. Patent 24 No. 3,941,207, ~ntitled Loudspeaker Assembly, issued to Robert E. Croup on March 2, 1976, teaches a loudspeaker 26 enclosure that includes at least two layers of corrugated 27 material on its inner surface.
28 U. S. Patent No. 3,938,617, entitled Speaker

-5-.
, :

1 Enclosure issued to Lonnie R, Forbes on February 17, 1976, teaches a loudspeaker enclosure that is sufficiently thin so as that it may be supported from a vertical wall and concealed by a wall mounted picture disposed forwardly thereof. The spea~er enclosure includes a plurality of elongated spaced baffles secured to a baffle sheet to form a labyrinth in order to contin-uely divert the sound waves thereby effectively increasing the volume of the speaker enclosure. However, the fundamental phys-ical relationships discussed by Olsen and Massa in their book Applied Acoustics are still applicable and therefore they caused - the speaker enclosure of this type to be inefficient, U.S. Patent No. 3,917,914, entitled Loudspeaker, issued to Rollin James Parker on November 4, 1975, teaches a loudspeaker of a moving magnet type which has a vibratable paper cone with ~ -an opening at the apex in which a cobalt-rare earth ring magnet is positioned.
U.S. Patent No, ~,797,766, entitled Loud Speaker, issued to Herbert W, Sullivan on July 2, 1937 an air tight enclosure containing an acoustic diaphragm is provided with a ~0 membrane substantially permeable to mechanical vibrations, but substantially impermeable to the gaseous medium on either side of the membrane. The acoustic diaphragm vibrates in a gaseous medium which is heavier than air and in which sound travels at a slower speed than in air. The characteristic imped-ance of the diaphragm in the gaseous medium and the acoustical capacitance is lower than when compared to those prevailing in air~

., . ~. .

1 U. S. Patent No. 4,004,094, entitled Enclosure 2 System ~or Sound Generators issued to James H. Ott on 3 January 1~, 1977, teaches a device for use in an enclosure 4 associated with an audio speaker which permits relatively large volume changes within the enclosure as a result of

6 relatively small pressure changes so that relatively small

7 enclosures can be effective as enclosures of larger volume

8 The device reduces the energy required from the speaker to

9 change the volume of the interior of the enclosure. Pressure perturbations caused by the movement of the vibratorily 11 driven me.~rane of the sound producing device cause alternate 12 condensation and vaporization of the composition to minimize 13 backpressure.
14 U. S. Patent No. 3,867,996, entitled Speaker Enclosure, issued to Nils Lou on February 17, 1976, teaches 16 a speaker enclosure that includes a sound absorbent material 17 behind the vibratable cone. U. S. Patent No. 3,948,347, 18 entitled Acoustical Panel, issued to Richard A. Rutledge on 19 April 6, 1976, teaches a sandwich type acoustical panel that includes a frame, a plastic skin of thin, limp, flexible, 21 air~tight and water-tight protective material disposed 22 within the frame and adapted to contain a fibrous matrix.
23 Loudspeakers presently in use are of the acoustical 24 suspension design and reouire a large amount of power. There is a movement toward more efficient loudspeakers because 26 individuals are demanding more volume with less distortion 27 than typical low efficiency speakers can deliver with 28 amplifiers of moderate size. The super-powered amplifiers ~l~U~34 1 in home music systems require several hundred watts of 2 amplifier power in order to achieve faithful reproduction 3 of modern recordings with low efficiency speaXers. These 4 speakers are unable to hold up under this deluge of amplifier power and they eventually break down. There is 6 therefore a need for a high efficiency speaker that will 7 require less power for lifelike reproduction of modern 8 recordings.

9 SUr~l`qARY OF T~E INVENTION
In view of the foregoing factors and conditions 11 characteristic of the prior art it is an object of the 12 present inven,ion to provide a device for a loudspeaker 13 svstem that increases the effective volume for low 14 frequencies o. its speaker enclosure.
It is another object of the present invention to 16 provide a device for a loudspeaker system that increases the 17 compliance that its vibratable cone sees for a particular 18 volume of its speaker enclosure.
19 It is still another object of the present inven-tion to provide a device for a loudspeaker system that 21 increases the acoustical path of its speaker horn.
22 It is yet another object of the present invention 23 to provide a device for a loudspeaker system that enables 24 the speaker enclosure to be reduced in volume and to still retain its efficiency at low frequencies.
26 In accordance with an embodi~ent of the present 27 invention a device for use in combination with a loudspeaker 28 system, that includes a speaker enclosure, in order to . .

U8~34 1 effectively enlarge the volume of the speaker enclosure 2 has been described. T~e loudspeaker system also includes 3 a vibratable cone. The device for effectively enlarging the volume of the speaker enclosure includes a gas having a gamma less than 1.4 and the product of its density and 6 the square of the speed of sound therein less than the 7 same product for air and a bag which is formed from a soft, 8 pliable material for enclosing the gas within the speaker 9 enclosure and which is adapted to seal the gas therein. The device also includes an acoustically transparent and porous 11 cocoon which is disposed about the bag so that it surrounds 12 completely the bag and an acoustical padding which is 13 disposed aajacent to the sidewalls of the speaker enclosure 14 and which is adapted to enclose the acoustically transparent and porous cocoon. The device may also include a device 16 for genera~ing the sas by heating a fluid in its liquid 17 phase so that`the fluid changes to its gas phase. The 18 device for heating the fluid may either be a resistive 19 electrical element which is disposed within the speaker enclosure or fibrous, sound absorbent material which is 21 disposed within the soft, pliable membrane. The device is 22 placed in the speaker enclosure in back of the vibratable 23 cone in order to increase the compliance that the vibratable 24 cone sees.
The features of the present invention which are 26 believed to be novel are set forth with particularity in 27 the appended claims.
28 Other objects and many o~ the attendant advantages _g_ . I

' 8t34 1 will be more readily appreciated as the same beco~.es 2 better understood by reference to the following detailed 3 description and considered in connection with the accom-4 panying drawing in which like reference s~mbols designate like parts throughout the figures.
6 DESCRIPTIO~ OF TEE _RA~ING
7 FIG. 1 is a perspective view of a loudspeaker 8 system that has a speaker enclosure of the vented box 9 type and that has a device for e fectively enlarging the volume of the speaker enclosure which is constructed in 11 accordance with the principles of the present invention.
12 FIG. 2 is a side elevational view of the speaker 13 enclosure of FIG, 1 showing the device for effectively 14 enlarging-its volume.
FIG. 3 is a vertical cross-sectional view of the 16 speaker enclosure of FIG. 1 taken along line 3-3 of FIG. 2.
17 FIG. 4 is a side cross-sectional view of a 18 speaker enclosure of the sealed box type.
19 FIG. 5 is a vertical cross-sectional view of the speaker enclosure of FIG. 4 taken along line 5-5.
21 DESCRIPTION OF THE PREFERRED EMBODIME~T
22 The present invention can best be understood by 23 reference to a description of its preferred embodiment and 24 to the showings in the drawing. The invention is an impro~ement for use in combination with a loudspeaker 26 system 10 shown in FIG. 1 to effectively enlarge the volume 27 of a speaker enclosure 11 so that its vibratable cone 12 28 sees a-larger compliance than it sees without the improvement.

`` 11~8~4 1 Referring now to FIG. 2, the speaker enclosure 2 11 includes a top 13, a ~ottom 14, a pair of sides 15 and 3 a front 16 having a speaker port 17. The loudspeaker 4 system 10 includes a vi~ratable cone 18 having an open apex which is disposed within the speaker port 17. The 6 side cross-sectional view of the speaker enclosure 11 also 7 reveals that its back 19 has a port opening 20 and a pair 8 of parallelly disposed planar baffles 21. A standard 9 loudspeaker which is described in U. S. Patent No. 3,917,914, entitled Loudspeaker, issued to Rollin James Parker on 11 November 4, 1975 and which is also described in any one of 12 a nu~ber of patents covering loudspeakers includes in 13 addition to the vibratable cone 12 a ring magnet 22 posi-14 tioned in its peripheral circular surface within and in 15 solid contact with the apex of the vibratable cone 12, a ; .
16 magnet circuit 23 positioned within the ring of t~e magnet 17 22, ~ut not in direct contact therewith, with the magnet 22 18 extending interiorly of the vibratable cone 12, an electro-19 magnetic coil 24 mounted on the magnetic circuit 23 and a device 25 for positioning the vibratable cone 12, the ring 21 magnet 22, the magnetic circuit 23 and the electromagnetic 22 coil 24 whereby the ri~g magnet 22 and the vibratable cone 23 12 are vibratable'in response to an electrical signal 24 impressed on the electromagnetic coil 24.
~ Still referring to FIG. 2 the improvement to the 26 loudspeaker system 10 is the use of a gas 30 that has a 27 gamma (y), Cp~Cv, less than 1.4 and the product of its 28 density (p~ and the square of the speed of sound (c), (P) c2, '- ' ' 8~4 1 less than the same product for air. The gas 30 is enclosed 2 within the speaker enclosure 11. T~e enclosed volume of 3 a vented loudspeaker system 10 of the Helmholtz resonator 4 type will determine the low frequency conversion efficiency of the loudspeaker system 10 if the bandwidth is held 6 constant. An increase in t~e effective volume therefore 7 will result in an increased efficiency in practice because 8 the optimum volume of such an enclosure is very large 9 relative to practical realizations. Such an increase in the effective volume is o~tained in adiabatic compression 11 by use of an enclose~ vapor with a gamma less than 1.4.
12 The gas 30 is enclosed in a bag 31 which is formed from a 13 soft, pliable mem~rane and which is adapted to seal the 14 gas 30 thexein. The bag 31 is generally formed from a lamina,ed nylon or some other polymeric material. The bag 16 31 is disposed behind the vibratable cone 12 within the 17 speaker enclosure 11 and fills most of the inner volume 18 thereof. Referrin~ to FIG. 3 in conjunction with FIG. 2 19 one can see how the bag 31 is disposed within the speaker enclosure 11. ~he bag 31 is disposed within an acoustically 21 transparent and porous cocoon 32 which completely surrounds 22 it. The acoustically transparent and porous cocoon 32 is 23 disposed ~ithin an acoustical padding 33 which covers the 24 top, the ~ottom, the sides and the front of the enclosure 11 so that it completely surrounds the acoustically trans-26 parent and porous cocoon 32.
27 The acoustically transparent and porous cocoon 28 allows the sound pressure waves to act upon the bag 31 over 1 its entire surface ~y preventing the bag 31 from 2 laying flat against the hard interior of the enc]osure 3 11. The fact that the bag 31 is formed from a soft, 4 plia~le mem~rane necessitates the use of the cocoon 32, but the use of the soft, pliable bag enables one 6 to reduce to the practice the theoretical advantages 7 of the loud speaker taught in U. S. Patent No. 2,797,766, 8 entitled Loud Speaker, issued to Herbert ~Y. Sullivan on 9 July 2, 1957 and the enclosure system taught by U. S.
Patent No. 4,004,094, entitled ~nclosure System for 11 Sound Generators, issued to James ~. Ott on January 18, 12 1977. One of the reasons that the cocoon 32 is neces-13 sary is that the bag 31 in the cocoon 32 does not block 14 the vent or duct in a vented cabinet. A second xeason lS is that it is virtually impossible to seal a vibratable 16 piStGn and cabinet from gas diffusing out therefron~
'17 for more than a few hours. A third reason is that the 18 cocoon 32 allows uniform cabinet pressure to impinge on 19 all surfaces of the bag 31. The fourth reason is that the bag 31 would vibrate, chafe and rupture if it came into 21 contact with the sharp edges of the cabinet. The 22 cocoon 32 does not allow the bag 31 to come into contact 23 with the edges of the cabinet.
24 Still referring to FIG. 2 in conjunction with FIG. 3 the bag 31 is adapted to enclose a fluid 34 26 which is in its li~uid phase and a fibrous, sound 27 absorbent material 35 disposed within the bag 31 for 28 heating the fluid 34 by excitation and changing the `' ~

1 fluid 34 from its liquid phase to its gas or vapor 2 phase.
3 Referring now to FIG. 4 in conjunction with 4 FI5, 5 a cross-sectional view o a second speaker enclosure 61 has a bag 62 in a cocoon 63 within 6 acoustical padding 64. The speaker enclosure 61 has 7 a resistence element 66 which serves the same function 8 as tAe fibrous~ sound absorbent material 35 in FIG. 2.
9 According to F. K. Moore, author of rEffect of P~adiative Transfer on a Sound Wave in a Gas ha~ing near One", in the Physics of Fluid, Volume 9, Number 1, 12 page 70, January, 1966 weak sound waves propagating in 13 rather transparent gas which is at a sufficiently high 14 temperature are dispersed by radiative heat transfer.
In Fluid D~namics of Multiphase Systems, S. L. Soo .
16 describes a gas-liquid system and condensation on page 17 309 thereof. According to Mr. Soo the speed of sound 18 in a gas i~ water system is slower than in pure water.

19 Mr. Soo also stated that the speed of sound in a mixture of gas and liquid is much slower when compared 21 with that in either the gas or the liquid because the~

22 gas acted as a weak spring which is coupled mechanically 23 to the water as a large mass. Referring to the refer-24 ence from Applied Acoustics the compliancQ of the speaker enclosure is determined by one divided by the 2~ product of the square of the speed of sound in the 27 gas and the density of the gas. Therefore the compliance 28 is increased as a result of the lower speed of sound.

~l~U-~4 1 The vapor is generated in the soft, pliable membrane 2 31 by the sound, w~ich is ~enerated by the movement of 3 the vibratable c~ne 12, being absorbed by the fibrous, 4 sound absorbent material 35 and thereby becoming energy in the form of heat which causes the fluid 34 in its 6 liquid phase to change to its va~or or gaseous phase.
7 In one of the preferred embodiment the fibrous 8 sound absorbent material 35 is fiberglass which is loosely 9 contained in the ~ag 31. The fluid 34 is Freon*ll which has a ~oiling point near room temperature, 74.8F.
11 - The gas 30 or fluid 34 which may be used may be a 12 flurocarbon such as Freon 11*, Freon*113 and Freon*114 or any 13 gas or fluid which has a gaseous state with a gamma less 14 than 1.4, such as carbon~dioxide (C02), and a compliance factor whicn is larger than the compliance factor for air 16 [1/(~)c21. In TABLE I, which is set out below, several 17 appropriate compounds are listed with their physical 18 properties that maXe them useful in the present invention.

Compound ~oiling Density Speed of Gamma Compliance Point Sound Factor ,~ , . . f . .
Freon 11 74.8 F 5.86 lit. 469 sec 1.137 1.29 x 106 Freon*113 117 " 7.38 " 390 " l.Q80 1 12 n 23 - - -- __ Freon*114 38.2 " 7.83 " 411 " 1.084 1.32 Air 1.29 " 1130 " 1.4 1.65 "

26 From the foregoing it can be seen that a device for 27 effectively enlarging the volume of a speaker enclosure has 28 been described. The device is used in combination with a *Trade Mark 1 loudspeaker system to increase the compliance that a 2 vibratable cone sees. Furthermore, it should be noted that 3 the schematics of the device have not been drawn to scale 4 and that distances of and between the figures are not to be considered significant.
6 Accordingly, it is intended that the foregoing 7 disclosure and showings made in the drawing sha'l be 8 considered as illustrations of the principles of the 9 present invention.

Claims (4)

WHAT IS CLAIMED:

1. In a loudspeaker system an improvement for use in combination with a loudspeaker system which includes:
a. a vibratable cone having an open apex; and b. a speaker enclosure having a top, a bottom, a back, a pair of sides and a front, the front having a speaker port wherein the vibratable cone is mechanically coupled so that it may vibrate in response to an electrical signal, said improvement is a device for effectively enlarging the volume of the speaker enclosure in order to increase the compliance that the vibratable cone sees with said device comprising:
a. a gas having a gamma value less than 1.4 and the product of its density and the square of speed of sound therein less than the same product for air; and b. a bag formed from a soft, pliable material for enclosing said gas within the speaker enclosure and adapted to seal said gas therein;
c. an acoustically transparent and porous cocoon disposed about said bag so that it surrounds completely said bag;
and d. an acoustical padding disposed adjacent to the top, the bottom, the back, the sides and the front of the enclosure and adapted to enclose said acoustically transparent and porous cocoon.

2. In a loudspeaker system an improvement for use in combination with a loudspeaker system according to claim 1 wherein said gas is produced by heating a fluid in its liquid phase and said device also comprises means for heating said fluid.

3. In a loudspeaker system an improvement for use in combination with a loudspeaker system according to claim 2 wherein said means for heating said fluid is a fibrous, sound absorbent material disposed within said bag and adapted to heat said fluid by excitation created by sound pressure changes.

4. In a loudspeaker system an improvement for use in combination with a loudspeaker system according to claim 2 wherein said means for heating said fluid is a resistive electric element which is disposed within said bag and which utilizes externally supplied electrical energy.