US3275758A

US3275758A - Sound reproducing apparatus

Info

Publication number: US3275758A
Application number: US226703A
Authority: US
Inventors: Bryan Samuel; Walter B Udell
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-09-27
Filing date: 1962-09-27
Publication date: 1966-09-27
Anticipated expiration: 1983-09-27

Description

Sept. 27, 1966 BRYAN T L 3,275,758

SOUND REPRODUC ING APPARATUS Filed Sept. 27, 1962 5 sheets-$11861, 1

0 39 E 80 OPEN 32/ U1 6 e0 40 Z 0 4 no S7. 8% 20 2E l I l l a 3 5 E3 FREQUENCY IN excuse/sec.

INVENTORS SAMUEL BRYAN WALTER B. UDELL S A/$5 6W ATTORNEY 2/ Fig. 2

Sept. 27, 1966 Filed Sept. 27, 1962 Distortion of I30 Cycles/ Second s. BRYAN ETAL 3,275,758

SOUND REPRODUCING APPARATUS 5 Sheets-Sheet 2 D at R Open Circuited D of R Short Circuited 500m Frequency In Cycles/Second SAMUEL BRYAN WALTER B. UDELL ATTbRNEY Sept. 27, 1966 s. BRYAN ETAL 3,275,753

SOUND REPRODUCING I APPARATUS Filed Sept. 27, 1962 5 Sheets-Sheet 5 iii Fig, 10

R t ohms constant Relative Phase Anglein Degrees 8 8 40 6O 80 I00 I20 I40 I60 Frequency in Cycles /Second Fig. l/ 23 9 lao- Q= C .s 60- Um 8 L' S4on I I F! I I l f I l l 40 5O 6O 90 I00 I10 I I I I Frequency in Cycles/Second VENTQRfi SAMUEL BRYAN WALTER B.UDELL Mew ATTORNEY United States Patent 3,275,758 SOUND REPRODUCING APPARATUS Samuel Bryan, Silver Spring, Md., and Walter B. Udell, Philadelphia, Pa, assignors, by mesne assignments, to Walter B. Udell, Philadelphia, Pa.

Filed Sept. 27, 1962, Ser. No. 226,703 14 Claims. (Cl. 179-416) This invention relates generally to sound reproducing apparatus, and more particularly relates to loudspeaker systems of the acoustical resonator phase inverter type. The acoustical resonator is early described in one practical form in the United States patent to Bobb, No. 2,059,- 929, as a device for substantially reducing the low frequency resonances associated with a loudspeaker mounted in an openbacked cabinet to thereby improve the distortion characteristics of the loudspeaker system. Basically, the resonators described may be considered as loudspeakers without a voice coil and magnet structure in which the cone movements induced at the mechanically resonant frequencies are damped out by viscous structures coupled to the apex of the cone. The resonators are of course energy absorbing devices operative within a predetermined frequency band to materially lower the Q of the mechanical system resonances of the loudspeaker system. In general, and as described in the aforesaid patent, several resonators of appropriately selected resonant frequencies and Qs are required to properly damp a broadly resonant system or one exhibiting spaced resonance peaks.

This very same type of acoustical resonator in a closed cabinet has been more recently treated by Olson, in his book entitled, Acoustical Engineering, in which it is pointed out that this arrangement, designated as a drone cone, is superior to the phase inverter or Helmholtz resonator type of enclosure commonly known as the bass reflex cabinet. In general, the advantages are said to derive from the fact that the effective port area can be as large as that of the active or driving cone, which is much larger than that possible with an open port, and that the phase and amplitude of particle motion are the same over the entire drone cone, a condition which is not even approximated by an open port. As a consequence the drone cone system is capable of wider frequency range with greater acoustical output than is possible with an open port because a lower particle velocity at higher pressure results together with substantial reduction of the losses incurred due to phase shift across the open port.

The present invention differs from the resonator systems above described in that while the latter are strictly passive mechanical dissipation systems, the present invention provides selectively controllable dynamic damping of the driving cone which results in a marked reduction of distortion as compared with that of the passive dissipation systems together with augmented bass frequency outputs. Accordingly, it is a primary object of this invention to provide a novel loudspeaker system which incorporates therein a driving cone and acoustical resonator mechanically resonant throughout the resonant frequency range of the system, with the acoustical resonator also providing electro dynamic damping of the driving cone.

Another object of this invention is to provide a novel loudspeaker system as aforesaid wherein only a single resonator is employed which has the same mechanical resonance spectrum and Q as the driving cone.

Still another object of this invention is to provide a novel loudspeaker system employing an acoustical resonator in an electro-acoust-ical velocity sensitive feedback loop with the driving cone of the system which varies the damping on the driving cone in accordance with the velocity of the amplitude excursions of the latter.

Yet another object of this invention is to provide a Patented Sept. 27, 1966 ice novel loudspeaker system as aforesaid which includes means for selectively altering the phase characteristic of the resonator to change the phase angle difference between the driving cone and the resonator.

A further object of the invention is to provide a novel loudspeaker system of the phase inverter type in which the system may be tuned electrically to selectively alter the acoustic output thereof.

The foregoing and other objects of the invention will become clear from a reading of the following specification in conjunction with an examination of the appended drawings, wherein:

FIGURE 1 illustrates in perspective a loudspeaker system according to the invention with the enclosure back removed to reveal a loudspeaker and a resonator mounted therein;

FIGURE 2 is a front elevation of the loudspeaker system of FIGURE 1 .as would be seen when viewed along line 2-2 thereof;

FIGURE 3 is a horizontal sectional view through the loudspeaker system of FIGURE 2 as would be seen when viewed along line 33 thereof, the loudspeaker and resonator being shown in elevation;

FIGURE 4 is an axial diametric section taken through a typical dynamic loudspeaker of the permanent magnet field type;

FIGURE 5 is an enlarged fragmentary view of the corrugated edge surround of the loudspeaker cone of FIG- URE 4- enclosed in the phantom circle thereof;

FIGURE 6 is a graph of distortion generation variation with change in resonator resistive electrical damping for two power levels at a constant frequency;

FIGURE 7 is a graph showing the distortion reduction achievable with the dynamically damped resonators according to the invention as compared with the previously known passive resonators.

FIGURE 8 is a graph of relative phase shift versus frequency between the driving loudspeaker cone and the resonator device for two different conditions of operation;

FIGURE 9 is a representational showing of a resonator according to the invention in a modified electrical circuit;

FIGURE 10 is a somewhat linearized graph of relative phase shift between the loudspeaker cone and the resonator as a function of frequency resulting from utilization of the circuit of FIGURE 9; and

FIGURE 1'1 is a plot of capacitance variation in the circuit of FIGURE 9 as a function of frequency for three selected degrees of constant phase shift between the cone of the driving loudspeaker and that of the resonator.

In the several figures, like elements are denoted by like reference characters.

Turning now to the drawings consider first FIGURES l to 5 in which there will be seen a loudspeaker system including an enclosure 20 having an apertured front wall 21 to which are mounted in peripherally sealed fashion a loudspeaker 2'2 and a resonator 23, the resonator being in actuality another loudspeaker of the same kind as the loudspeaker 22. The voice coil terminals of a loudspeaker 22 are connected to a pair of external terminals 24 mounted to the enclosure side wall 25 by a pair of leads 26. The voice coil terminals 27 of resonator 23 are connected in a closed loop through rheostat Rs by leads 28, the rheostat being physically mounted to the enclosure top wall 29 with its shaft projecting therethrough and terminated externally by knob 30. The interior of the enclosure is suitably padded, although this is not shown for purposes of clarity, and the enclosure is sealed up by securing the back 31 to the top, bottom and side walls of the enclosure 20 in any convenient manner to provide a substantially airtight interior chamber.

As seen in FIGURE 4 the resonator 23, and of course the active driver 22 also, is a permanent magnet dynamic loudspeaker having a field magnet 32 formed to set up a radial magnetic field Within which the turns of a voice coil 33 wound on cylindrical tube 34 are disposed for axial motion transverse to the lines of magnetic flux of the field. The voice coil tube 34 is secured to the apex of cone 35 which latter is currugated at its outer periphery as best seen at 36 in FIGURE and is peripherally clamped to the rim of the speaker frame by annular compression gasket 37. The number, depth and flexibility of the corrugations 36 and the spider (not shown) determine the compliance or acoustical capacitance of the loudspeaker, while the mass of the cone 35 and voice coil structure 33-34 determine the acoustical inductance of the loudspeaker.

It should be recognized at this point that the resonator according to this invention diflers from the passive resonators described by the prior art in that it includes a voice coil and magnet structure and does not utilize viscous mechanical damping elements other than the inherent mechanical dissipative damping provided by the 'cone surround 36 and spider. With the voice coil 33 open circuited, the resonator 23 behaves merely as a mechanically resonant system on which the magnet 32 has no effect whatever, and is as a practical matter not present. The vibratory system of the resonator 23 is mechanically resonant over a limited frequency band tional to the amplitude of the driving cone oscillations.

The mechanical suspension system of the cone 35 does of course provide damping which is a function of excursion of the cone, relatively little damping occurring at small excursions with progressively increasing resistance to larger excursions. The reason for this is that the suspension system is relatively compliant at center position, becoming less and less compliant with displacement from center position. This is of course the same way in which the passive type of resonator behaves, the damping provided by the damping elements of the resonator increasing with increased displacement of the resonator cone from its center position.

It is at this point that the operation of the resonator according to the invention departs from the operation of previously known passive resonators because the voice coil 33 is in fact not left in an open-circuited condition, but is instead connected in a closed series loop with network elements which cause the resonator to become a velocity responsive damping device and to thus anticipate large amplitude excursions before large displacements occur and react quickly to dynamically damp the same while the excursion is still of relatively small amplitude. The passive type of resonator is incapable of this action because it is not velocity responsive, but responds only to amplitude changes which must first occur.

The resonator action is analogous to dynamic braking of an electric motor and takes place because the induced vibrations of the resonator cone drive the voice coil 33 across the lines of magnetic flux in the air gap of field magnet 32. A motional is thus generated which appears across voice coil terminals 27 and is of such polarity as to tend to produce a current giving rise to a thrust on the voice coil which opposes the motion which produces it, in accordance with Lenzs law. The amplitude of the motional is of course proportional to the field strength in the gap and to the velocity of voice coil motion. With the voice coil terminals 27 open circuited there is no current flow and no electromagnetic counterforce to the mechanically induced vibrations is generated. When, however, the voice coil circuit is closed through network elements to be described, a current flows which brakes the motion of the resonator cone and the driving cone by converting the mechanical energy of cone motion into electrical energy and then dissipating the electrical energy by doing work in creating a magnetic field and driving current through resistive elements.

As will be subsequently seen in more detail the relative phase between motion of the driving cone and the resonator cone is controllable within limits by proper choice of the network elements connected in closed circuit with the resonator voice coil 33. This is an important feature of the invention for the following reason. It will be recalled that it has been shown by others that one of the major advantages of the passive drone cone phase inverter over the open port inverter is that the phase of particle motion is the same over the entire drone cone. While this is true it is not necessarily of any real value unless the phase of particle motion is that which is desired relative to the particle motion of the driving cone. For this reason careful consideration must normally be given to the design of passive resonators and to the enclosures with which they are to be used. In particular the volume of the enclosure must be chosen sufiiciently large to provide the required acoustical capacitance to produce the desired phased output from the drone cone. To a considerable extent the present invention provides the ability to properly phase the resonator with enclosures of substantially reduced size, and provides the ability to electrically tune a loudspeaker system in an enclosure rather than requiring the physical reconstruction of the enclosure to vary the acoustical capacitance thereof.

FIGURES 6, 7 and 8 show the results achieved by resistance loading of the resonator voice coil as a function of the magnitude of Rs, while FIGURES 9, 10 and 11 relate to phase control of the resonator cone by the introduction of reactive elements into the resonator voice coil circuit. The data illustrated was obtained by measure ments made with a loudspeaker system of the physical type shown in FIGURES 1 to 3 and having the following characteristics.

Enclosure internal dimensions=27" x 14" x 4 /2" Enclosure internal volume with loudspeakers 0.8"ft. Driver 22 and resonator 23:

Jensen Mfg. Co. type P12NL loudspeaker 12" nominal diameter 8-ohm nominal voice coil impedance Free air cone resonance -30 cycles/ second System resonance cycles/second.

It should be noted that the system resonance is substantially higher than the free air cone resonance of the driver 22 and resonator 23 due to the extremely small internal volume of the enclosure which was chosen for several reasons. First, because small volume enclosures are coming increasingly into public demand and improved results achieved with such enclosures are ofimmediate interest to the general public. Second, because the use of a small enclosure raises the system resonance to a frequency where the radiation resistance of the driver is realtively good so that effective air loading of the driver cone is obtained through the resonance region. This latter consideration makes it feasible to electrically tune the resonator voice coil circuit below the system resonance to provide better in-phase coupling of the resonator cone to the driver cone, resulting in better loading of the driver cone at lower frequencies which produces augmented bass response and lower distortion.

FIGURE 6 illustrates schematically the electrical circuit of the resonator voice coil which comprises the closed series loop including voice coil inductance L, voice coil resistance R motional generator 38 and rheostat Rs. The magnitude and phase of the current pro duced in the voice coil circuit by generator 38 is variable within limits determined by the magnitude of Rs, which latter can be varied between a high resistance value and short circuit. The effect of so varying Rs is plotted for a signal of 130 cycles/second at constant power inputs to the driver 122 of milliwatts and 500 milliwatts.

The high resistance values of Rs prevent any appreciable current flow in the voice coil circuit and simulate the condition of the open-circuited voice coil wherein the only damping operative is that due to the mechanical re sistances of the cone suspension and is comparable to the passive type of resonator. The short circuit condition of Rs permits a substantial current to flow in the voice coil circuit, which current effects heavy electromagnetic damping on the resonator cone and hence on the driving cone. The significant reduction in total generated distortion which results is shown in the graph of FIGURE 6 together with the variation in distortion as a function of Rs between its short circuit and effectively open circuit conditions.

The graph of FIGURE 7 compares the total distortion generated by the loudspeaker system when the resonator voice coil is open circuited with that generated when the voice coil is short circuited over the frequency range of mechanical resonance of the resonator for three levels of electrical power input to the voice coil of driver 22. The open circuit distortion is in all cases higher than the short circuit distortion, and for the 150 milliwatt curve,- which is a normal average power input level to be expected in a residential sound system it is seen that the open circuit or passive resonator distortion at 130 cycles is 55% higher than the short circuit or dynamically damped resonator distortion. The higher power curves show the same general condition but to a lesserdegree as the power input to the driver is increased. The significance of the curves of FIGURE 7 is not that there is a significant distortion peak at the resonant frequency of the system, but that a material improvement in the distortion characteristics of loudspeaker systems can be effected by dynamic damping, and that this improvement over the passive type of resonator is effective under conditions where a current flows in the resonator voice coil.

FIGURE 8 shows the effective phase shift between the cone motion of the driver 22 and the cone motion of the resonator 23 for an open-circuited resonator voice coil and with the voice coil shunted by ten ohms, the line 39 designating the open-circuit phase and the line 40 designating the resistive shunt phase. The improvement in phase response of the resonator with its voice coil shunted is obviously not due to a speed-up of resonator cone motion since the effect of the induced current is to produce a counter motion of the cone, but is in fact due to the heavy damping produced on the driver cone which prevents it from excursing to the extent formerly possible without the electromagnetic damping of the resonator. It should also be noted that the slope of line 40 is less than that of line 39 so that the phase shift with the shunted resonator voice coil is more gradual. The slope of line 40 may be considerably lowered by decreasing the voice coil shunt resistance, thus further improving the phase characteristic.

Turning now to FIGURE 9 it is observed that the resonator 23 is shown with a voice coil circuit modified from that of FIGURE 6 by the inclusion therein of a series capacitor C. The effect of the capacitor C is graphically illustrated in FIGURE 10 which is a series of phase shift characteristics of the type shown in FIGURE 8, and shows theeifect of various values of capacitance C in the circuit of FIGURE 9 with Rs held at ten ohms. It is observed that increase in capacitance causes the in-phase frequency to shift downward, a capacitance of 75 microfarads effecting a twenty cycle downward shift so that at 63 cycles there still exists a significant in phase component which augments the bass response whereas without the capacitor C there is no such in phase component since th ephase shift has reached 90. Below 63 cycles there is an actual outphasing condition without the capacitor which causes the response .to drop off quite rapidly. As in the case of phase line 40 of FIGURE 8 reduction of Rs lowers the slope of the phase lines of FIGURE 10, and listening tests confirm a marked increase of fundamentals in the bass frequency output be tween 40 cycles and cycles. No audible difference was discernible in the output above 100 cycles with the capacitor C in the circuit or short circuited.

The effect of the capacitor as a phase shifting element is in converting the voice coil circuit from an inductive circuit exhibiting a current lag due to the inductance L of the voice coil to a capacitive circuit in which the induced current can build up rapidly. In the previously mentioned case comparing a short circuited capacitance with one of 75 microfarads it is seen from FIGURE 10 that a desired phase shift of almost 40 is achieved. Effective resonator action is achieved in the describedsystem at frequencies substantially below the resonance range of the system because of the tight coupling between the cones of the driver 22 and resonator 23 due to the acoustical stiffness of the enclosure at low frequencies resulting from the very small enclosed air volume. This is not true however for frequencies substantially above the system resonance. It should be noted that independently of phase considerations the electrodynamic damping is always operative because motion of the resonator cone produces a current flow and power dissipation as earlier set forth.

Phase shifts in the direction opposite to that produced by series capacitance C may be caused by using a series inductor L in place of the capacitor C, as shown in dashed line in FIGURE 9 and resulting in phase lines as generally indicated in FIGURE 10 by dashed line L"=a. For special purposes combination inductionalcapacitance networks might be utilized.

, FIGURE 11 shows lines of constant phase shift plotted as a function of frequency and series capacitance from which other phase lines may be constructed in FIGURE 10 for particular values of capacitance not shown thereon.

If desired, a plurality of dynamically damped resonators may be utilized, and these resonators may be of the same size or different sizes, and of the same or different size than the driver. Moreover, each may be selected or designed to have a free air cone resonance frequency either the same as or different from that of the driver and other resonators. Additionally, they may be selectively electrically tuned by utilizing different amounts of external series resistance in their voice coil circuits, with or without reactive elements of selected reactance magnitudes.

Having now described our invention in connection with particularly illustrated embodiments thereof, variations and modifications thereof may now occur from time to time to those persons normally skilled in the art without departing from the essential scope or spirit of our invention, and accordingly it is intended to claim the same broadly as well as specifically as indicated by the appended claims.

What is claimed as new and useful is:

1. In a system for radiating sound energy, first vibratory means for propagating sound energy and means for coupling the same to an actuating source, a baffle for said vibratory means, frequency selective second vibratory means acoustically coupled to said first vibratory means, and electrodynamic means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom, said electrodynamic damping means including means for selectively varying within limits the relative phase relation between the motions of said first and second vibratory means.

2. In a system for radiating sound energy, first vibratory'means for propagating sound energy and means for coupling the same to an actuating source, a baffie for said vibratory means, second vibratory means acoustically coupled to said first vibratory means, electrodynamic means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom, and means electromechanically coupled to said second vibratory means for selectively varying Within limits the relative phase relation between the motions of said first and second vibratory means.

3. In a system for radiating sound energy, first vibratory means for propagating sound energy and means for coupling the same to an actuating source, a batile for said vibratory means, second vibratory means acoustically coupled to said first vibratory means, velocity responsive means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom, and means electromechanically coupled to said second vibratory means for selectively varying within limits the relative phase relation between the motions of said first and second vibratory means.

4. In a system for radiating sound energy, first vibratory means for propagating sound energy and means for coupling the same to an actuating source, second vibratory means, an enclosure having apertured wall partions to which both of said vibratory means are mounted so that one surface of each of said vibratory means is contacted by the atmosphere outside of said enclosure and the vibratory means are acoustically coupled to one another Within the enclosure by means of the air volume contained within the enclosure, each of said vibratory means being characterized by a free air vibratory resonance frequency substantially lower than the resonance frequency range of said vibratory means mounted in the enclosure, and velocity responsive damping means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom.

5. In a system for radiating sound energy, first vibratory means for propagating sound energy and means for coupling the same to an actuating source, second vibratory means, an enclosure having apertured wall portions to which both of said vibratory means are mounted so that one surface of each of said vibratory means is contacted by the atmosphere outside of said enclosure and the vibratory means are acoustically coupled to one another within the enclosure by means of the air volume contained within the enclosure, each of said vibratory means being characterized by a free air vibratory resonance frequency substantially lower than the resonance frequency range of said vibratory means mounted in the enclosure, and velocity responsive damping means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom, said velocity responsive damping means including means for selectively controlling within limits the degree of damping desired.

6. In a system for radiating sound energy, first vibratory means for propagating sound energy and means for coupling the same to an actuating source, second vibratory means, an enclosure having apertured wall portions to which both of said vibratory means are mounted so that one surface of each of said vibratory means is contacted by the atmosphere outside of said enclosure and the vibratory means are acoustically coupled to one another within the enclosure by means of the air volume contained within the enclosure, each of said vibratory means bein characterized by a free air vibratory resonance frequency substantially lower than the resonance frequency range of said vibratory means mounted in the enclosure, and velocity responsive damping means electromechanically coupled to said second vibratory means for damping the amplitude excursions of said first vibratory means by selectively absorbing the sound energy therefrom, said velocity responsive damping means including means for selectively varying Within limits the relative phase relation between the motions of said first and second vibratory means.

7. In a system for radiating sound energy, first and second loudspeakers each of which has a vibratile diaphragm, an enclosure housing said loudspeakers with the latter mounted to said enclosure so that one side of the diaphragm of each of said loudspkears faces out of the enclosure through aperture means in the enclosure and the other sides of the diaphragms are acoustically coupled to one another by the air volume within the enclosure, means for coupling said first loudspeaker to an actuating source, said second loudspeaker being of the magnetic field type and having a voice coil coupled to its vibratile diaphragm so that the turns of the voice coil are movable transversely to the lines of magnetic induction of the field, and means closing the voice coil circuit to permit an induced current to flow therethrough whenever the vibratile diaphragm of said second loudspeaker is set in motion.

8. The system for radiating sound energy as set forth in claim 7 wherein said means closing the voice coil circuit to permit an induced current to fiow therethrough includes an electrical resistor.

9. The system for radiating sound energy as set forth in claim 7 wherein said means closing the voice coil circuit to permit an induced current to flow therethrough includes a selectively variable electrical resistor.

10. The system for radiating sound energy as set forth in claim 7 wherein said means closing the voice coil circuit to permit an induced current to flow therethrough includes a capacitor.

11. The system for radiating sound energy as set forth in claim 7 wherein said means closing the voice coil circuit to permit an induced current to flow therethrough includes an inductor.

12. In a system for radiating sound energy, first and second loudspeakers each of which has a vibratile diaphragm, an enclosure housing said loudspeakers with the latter mounted to said enclosure so that one side of the diaphragm of each of said loudspeakers faces out of the enclosure through aperture means in the enclosure and the other sides of the diaphra-gms are acoustically coupled to one another by the air volume within the enclosure, the air volume Within the enclosure being sufficiently small to raise the resonant frequency range of the system substantially above the free air resonant frequency of said second loudspeaker to thereby increase the acoustical stiffness of the enclosure and the acoustical coupling between the said vibratile diaphragms below the resonant frequency range of the system, means for coupling said first loudspeaker to an actuating source, said second loudspeaker being of the magnetic field type and having a voice coil coupled to its vibratile diaphragm so that the turns of the voice coil are movable transversely to the lines of magnetic induction of the field, and means closing the voice coil circuit to permit an induced current to flow therethrough whenever the vibratile diaphragm of said second loudspeaker is set in motion.

'13. The system for radiating sound energy as set forth in claim 12 wherein said means closing the voice coil circuit to permit an induced current to flow therethrough includes an electrical resistor.

14. The system for radiating sound energy as set forth in claim :12 wherein said means closing the voice coil circuit to permit an induced current to flow therethrough includes a capacitor.

References Cited by the Examiner UNITED STATES PATENTS 1,734,944 ll/1929 Green 179180 1,988,250 1/1935 Olson 179l16 X 2,489,862 11/1949 Cook 179--180 3,202,773 8/1965 Tichy l79-18O KATHLEEN H. OLAEFY, Primary Examiner. WALTER L. LYNDE, Examiner. S. H. BOYER, L. A. WRIGHT, Assistant Examiners.

Claims

6. IN A SYSTEM FOR RADIATING SOUND ENERGY, FIRST VIBRATORY MEANS FOR PROPAGATING SOUND ENERGY AND MEANS FOR COUPLING THE SAME TO AN ACTUATING SOURCE, SECOND VIBRATORY MEANS, AN ENCLOSURE HAVING APERTURED WALL PORTIONS TO WHICH BOTH OF SAID VIBRATORY MEANS ARE MOUNTED SO THAT ONE SURFACE OF EACH OF SAID VIBRATORY MEANS IS CONTACTED BY THE ATMOSPHERE OUTSIDE OF SAID ENCLOSURE AND THE VIBRATORY MEANS ARE ACOUSTICALLY COUPLED TO ONE ANOTHER WITHIN THE ENCLOSURE BY MEANS OF THE AIR VOLUME CONTAINED WITHIN THE ENCLOSURE, EACH OF SAID VIBRATORY MEANS BEING CHARACTERIZED BY A GREE AIR VIBRATORY RESONANCE FREQUENCY SUBSTANTIALLY LOWER THAN A RESONANCE FREQUENCY RANGE OF SAID VIBRATORY MEANS MOUNTED IN THE ENCLOSURE, AND VELOCITY, RESPONSIVE DAMPING MEANS ELECTROMECHANICALLY COUPLED TO SAID SECOND VIBRATORY MEANS FOR DAMPING THE AMPLITUDE EXCURSIONS OF SAID FIRST VIBRATORY MEANS BY SELECTIVELY ABSORBING THE SOUND ENERGY THEREFROM, SAID VELOCITY RESPONSIVE DAMPING MEANS INCLUDING MEANS FOR SELECTIVELY VARYING WITHIN LIMITS THE RELATIVE PHASE RELATION BETWEEN THE MOTIONS OF SAID FIRST AND SECOND VIBRATORY MEANS.