US3553392A

US3553392A - Electrodynamic sound radiator

Info

Publication number: US3553392A
Application number: US711313A
Authority: US
Inventors: Arthur Liebscher; Nils Harry Ljungman
Original assignee: Electronics Inc of Pennsylvania
Current assignee: Electronics Inc of Pennsylvania
Priority date: 1968-03-07
Filing date: 1968-03-07
Publication date: 1971-01-05
Anticipated expiration: 1988-01-05

Abstract

A sound radiator which includes a coil driven at audible frequencies and a magnet structure having an air gap within which the coil is moveable. The coil and magnet structure are attached to a body of stabilized foam plastic, such as polystyrene, polyurethane or bonded fibrous materials which may be in block, panel or other shape.

Description

United States Patent inventors Appl. No.

Filed Patented Assignee Arthur Liebschcr Jenkintown;

Nils Harry Ljungman, Merion, Pa. 7 l l ,313

Mar. 7, 1968 Jan. 5, 1971 Electronics Inc. of Pennsylvania Willow Grove, Pa.

a corporation of Pennsylvania ELECTRODYNAMIC SOUND RADIATOR 1 Claim, 12 Drawing Figs.

US. Cl 179/ 115.5, 181/32 Int. Cl H04r 9/00 Field ol'Search 179/1 15.5; 181/32, 31.1

[56] References Cited UNITED STATES PATENTS 3,351,719 11/1961 Schoengold 179/115.5 3,366,749 l/1968 Ries 179/l15.5 3,430,007 2/1969 Thielen 179/115.5 FOREIGN PATENTS 931,080 1/1960 GreatBritaim. 181/32 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas L. Kundert AttorneyZachary T. Wobensmith, 2nd

ABSTRACT: A sound radiator which includes a coil driven at audible frequencies and a magnet structure having an air gap within which the coil is moveable. The coil and magnet structure are attached to a body of stabilized foam plastic, such as polystyrene, polyurethane or bonded fibrous materials which may be in block, panel or other shape.

PATENTEUJAN slsn 3,553,392

' SHEET 1 UF 3 INVENTORS ARTHUR L/E'BSCHER N/LS HARRY LJUNGMAN A TTORNE Y PATENTED JAN 5 IBYI SHEET 2 OF 3 INVENTORS ARTHUR L IEBSCHEA /V/L$ HARRY L JU/VGMA/V PATENTEDJAN SL971 v 3,553,392

' sumsurs IIVVE/VTORS ARTHUR L/EBSCHE'R NH. 5 HA RR) L JU/VGMA N ATTORNEY ELECT RODYNAMIC SOUND RADIATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to massive sound radiators and more particularly to a wall of stabilized foam plastic or the like to which vibrations in an audible range are imparted, the body serving as a sound radiator without apparent movement.

2. Description of the Prior Art It has heretofore been proposed to provide speakers of various types, i.e., electrodynamic, electromagnetic, and electrostatic, with cones or diaphragms, but none of these covers the entire range from H, base to 20 KH treble without a distinct focal point of sound origin, distortion, or deficient output.

The electrostatic speakers, due to their design, are very limited in the amplitude of their displacement and since the diplacement varies:

I f= ma. where:

f= force on the diaphragm an also m= mass of diaphragm and also I w= angular frequency in radians per second All waveshapes can be broken into Fourier components and the force I will be a sum of sinusoids.

Since the-amplitude of the displacement varies as the inverse square of the frequency, and since the electrostatic speaker cannot displace from its quiescent position greatly, it can only respond to high frequencies.

Conversely, the electrodynamic speaker, because of the mass and'flexible structure of its cone, cannot respond very well to high frequencies.

At low frequencies when the cone of the electrodynamic speaker must make large excursions as indicated by the above mathematical analysis the cone must be very light in mass to allow the system to comply with a complex wave form.

However, this lightness also makes the cone structurally weak by decreasing its rigidity.

I, The rigidity F of'a structural section is measured by the sectional modulus:

Then

dI= dA.y

where A area of section, and a y =distance from axis of inertia to sectional element dA f d1= f y dA I 2 I LydA Obviously, for well behaved shapes, I grows much faster than y max, since y is the measure of I, not y. Therefore, by concentrating the mass about the center line, as in the case of a paper, solid plastic cone or diaphragm, or molded bead foam, the moment of inertia is very low, and therefore, the section modulus Z, which is the measure of rigidity, is very low.

If the rigidity of a cone or diaphragm is low, the element will break up structurally thereby causing distortion.

Both of the speaker types just referred to, disperse sound from a limited point source, which gives the presentation of an orchestra, even in stereo, an unnatural directional quality, emanating from discrete speakers.

SUMMARY OF THE INVENTION In accordance with the present invention, stable foams of synthetic plastic material and which are capable of conducting rather than absorbing sound energy, such foams including those of polystyrene and polyurethane, are utilized as an element or elements to conduct, radiate and disperse sound energy in such a manner as to alleviate the foregoing limitations of contemporary speakers. A body of such stabilized foam when coupled with acoustic driving means, such as electrodynamic voice coils, has been discovered to have striking characteristics, including the transmission of high audio frequencies with very high fidelity and low attenuation, as well as very great coupling of the acoustic energy from the foam to the air because each cell in the foam acts as a sound radiator and disperser. These characteristics can be utilized in both small and large foam elements, and the large foam elements may function structural members to provide walls or screens. Such a wall may also function as a motion picture screen.

It is accordingly the principal object of the present invention to provide a sound radiator which is simple in construction and highly effective in its performance over the audio range.

It is a further object of the present invention to provide a sound radiator employing, as one component, a stabilized foam of synthetic plastic which can be widely varied in size and shape thereby permitting a wide variety of applications in differing environments.

It is a further object of the present invention to provide a simplified sound radiator which only requires three elements, the foam wall, coil and magnet, so that manufacture and assembly of the unit is very simple and economical.

Other objects and advantageous features of the invention will be apparent from the description and claims.

BRIEF DESCRIPTION OF THE DRAWING The nature and characteristic features of the invention will taken in connection with the accompanying drawings forming part thereof, in which:

FIG. 1 is a view in perspective of one embodiment of the invention;

FIG. 2 is a fragmentary vertical sectional view, taken approximately on the line 2-2 of FIG. 1;

FIG. 3 is a vertical sectional view taken approximately on the line 3-3 of FIG. 2;

FIG. 4 is a view in perspective of another embodiment of the invention;

FIG. 5 is a rear elevational view of another embodiment of the invention for three dimensional stereo effect;

FIG. 5A is a diagrammatic view illustrating the use of a four track tape for separately energizing four drivers for three dimensional stereo effect;

FIG. 6 is a plan view of another embodiment of the invention with a curved wall and a pair of drivers;

FIG. 7 is a plan view ofan embodiment ofthe invention with a straight wall and a single driver;

FIGS. 8A, 8B and 8C are diagrammatic views showing the directional output characteristics in terms of relative loudness of a typical speaker heretofore available at different frequency levels; and

FIG. 9 is a diagrammatic view showing the distance at which a listener must be located to properly hear high fidelity stereo with the prior systems.

It should, of course, be understood that the description and drawings herein are illustrative merely, and that various modifications and changes can be made in the structure disclosed without departing from the spirit of the invention.

1 Like numerals refer to like parts throughout the several views.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to FIGS. 1 to 3 of the drawings, in the embodiment there illustrated, a body 10 is provided which serves for acoustic conduction, radiation and dispersion. The body 10 is of a stabilized open or closed pore foam'of synthetic plastic, polystrene or polyurethane, having been found to be particularly suitable.

The body 10 may be of the shape shown but is not limited to that shape.

In a particular instance, the body 10 can be substantially rectangular, about 10 feet on one side, about 8 inches thick and about feet long.

The body 10 is driven by any suitable driver 11 shown exaggerated as to size, and preferably by an electrodynamic coil 12 cemented as at 13 at one end of the body 10 and the coil 12 in turn operates in a magnetic flux gap 14 provided by an annular magnet 15 Surrounding} 9nt ,..n9!s2 29; .-lh setuc pole piece Ibis secured to a magnetic responsive mounting plate 17 by a screw 18 and the mounting plate 17 is secured to front plate 19 by screws 20.

The coil 12 is preferably a helical coil of wire which may be of aluminum wire and enamel coated for insulation. The coil 12 being held in its helical form in any desired manner such as by an epoxy resin coating. The coil 12 has its return lead 120 preferably cemented longitudinally along the outside of the coil, the leads (not shown) being connected to any suitable audio input.

The magnet assembly including the

magnets

15 and 16 and the plate 17 may be supported in any desired manner at any fixed location, such as by a frame 22 of conventional type, depending upon the place and manner of use. The frame 22 advantageously can be secured to the body 10 by an adhesive 25,

such as an epoxy resin, embedded into the body 10 in diverging openings 26 like tree roots.

While the body 10 in FIGS. 1, 2 and 3 has been described as of relatively large size, it may be a beam, slab or wall, composed of one or more blocks or strips, stacked and/or adhesively connected along their edges, where the entire member acts as a conductor, radiator and disperser of sound energy.

The body 10 may concurrently serve as a motion picture screen being particularly advantageous in that it avoids the short comings of the present screens with their perforations which are required for sound transmission but which impair the light reflective quality of the screen. With the body 10 serving as a motion picture screen the projection equipment required will be reduced in cost because of the better light reflective properties. Not only is the reflectivity increased but the sound transmission is not impeded by a partially closed motion picture screen. With the foam screen also there is a blending of channels thereby synthesizing the position of the original sound in the proper position on the screen.

Because of the high structural strength of foams employed herein, especially of the polyurethane group, the wall or screen can be used as a str uctural member in the building where it is housed.Therein, the cost of the building is greatly 5 reduced. Structural members of high insulating and compressive qualities are marketed by various large'manufacturers.

The wall surface of the block lflzmay'eonsist of .cavities and undulations which act as=resonators;and.'waveguidesfoi};the frequencies within the audio spectrum. Complex mathematical shapes such as hyperbolic paraboloids, ellipsoids,' paraboloids of revolution complement theacoustical properties of the foam and room. I 1 11- I a The foam acts as a mechanical delay line for high frequencies, thereby propagating said high frequencies 'th'roughoutthe material instead of acting as a mechanical low pass-filter, which is the case in the conventional cone in an electrodynamic speaker. The foam transmits without alterations very much alike an all pass sonic and ultrasonic or wave guide filter. Since the body 10 is of large dimensions, e.g. a number of feet in length and width, the surface couplingof the high frequency sound to air is very effective, which allows every bubble structure on the surface of the foam to act as a radiator. The cut off frequency is far above the audible range. With the foam body also, at low frequencies the entire beam vibrates without apparent movement, thereby providing the low bass frequencies.

From Vibration Theory & Applications, by Thompson pp 274-276.

where:

E: modulus of elasticity W= weight w n=mode, and

g= gravitational constant, 32.2 ft./sec.

It will thus be seen that the low frequencies of bass are propagated as well as the high treble by the foam acting both as a tweeter and a woofer.

The sound is propagated from all surfaces of the body, thereby providing a wall of sound, instead of a point source, which greatly increases the presence and realism of reproduction. The wall acts as its own baffle for the low bass frequencies. Speakers previously available required an ancillary baffle oflaigdimensions, or a folded horn or cabinet or air loaded sealed or drone horn cabinet in order that low frequency responses could be heard. This is because the compressional wave created by the front of the speaker cone is canceled by the rarefaction from the back surface which results in a greatly attenuated bass.

Instead of acting like a rigid structure, as do prior designs, the wall of sound as herein described is a mechanical delay line where the compressions and rarefactions take place in the compressional wave propagating through the mass. Therefore, the structure as a whole does not compress and rarefy the'air as in the conventional devices, but transmits the sound to the air over the entire area, by the transfer of the energy from the foam via the cellular resonators which comprise'the'surface of the mechanical delay line of the foam.

With the body or wall 10 as is herein described and with spaced drivers the wall has the ability to blend the channelsof a stereo reproduction system supplied through 'a pair of drivers thereby, synthesizing the spacial displacement of music of an entire orchestra wherein, numerous-discrete sound sources each, stereophonically-gives the illusion of being located in position directly relative to the corresponding sources of origin.

point sources and since the directionality of all speakers 'becornes greater at high frequencies, a listener, under usual.

Since all prior methods of reproduction are substantially 21rR T sin or where:

Ra= ratio of the pressure for an angle and to the pressure for an angle a= J== Bessel function of the first order R=radius of a circular piston a=angle between the axis of the circle and the vline joining the point of observation and the center of the circle X wave length I The characteristics of a typical prior speaker are illustrated in FIGS. 8A, 8B and 8C. In these diagrams the relative loudness output is shown in terms of direction for frequencies of 500 H, I500 H and 12000 H, respectively.

From this it will be noted that departures of more than from direct alignment with the speaker severly reduces thev ability to hear the higher ranges, such as 12000 H, and higher.

The distance at which a listener must be located in order to hear high fidelity stereo is indicated in FIG. 9.

d=5 csc 5 d= 5X 11. 47= 62. 35 feet 60 feet Accordingly, if the speakers are feet apart, which is recommended stereo practice, in order to experience any appreciable effect a listener will have to be positioned at least 60 feet away to be able to hearany high frequency stereo, otherwise, if the listener is closer the stereo high fidelity experiencewill be reduced by 6 or more decibels.

However in the case of the wall of sound such as is available with the

walls

10, 10a, 10b, the radiation from the wall is a QLK L a 2a 2a 02 p 03: by 6Z where =scalar velocity potential V= ratio of adiabatic to isothermal specific heat p= total pressure (static plus excess) p=static or original density or this may be written where v=velocity of propagation for a plane wave VPs .for simple harmonic wave traveling in the positive T sense the solution of the above is where w= 21rf s displacement measured in radians Therefore, the attenuation as a function of distance is much smaller than for a conventional speaker (a point source) which decreases its intensity as the inverse square of the distance, and this is only if no attenuation is present in the 10 dispersive medium as air is highly dissipative.

In the structures of the present invention all the high frequency components are present anywhere the listener happens to be in front of the wall.

The frequency response of the wall is flat to 50 kHz. and

then falls off at 90 kHz.

The phase distortion of audio in prior sound system is especially detrimental during transient condition.

As is well known in engineering practice, the phase shift becomes appreciable long before the so called 3 decibel cutoff frequency is reached. At the 3 decibel point, representing the conventional limit of the pass band, the phase shift is 45. For a 10 phase shift distortion which is already appreciable the attenuation from midrange is only 0.2 db. Therefore, the prior designs which claim 20,000 Hz upper 3 db point already have at 12,000 Hz a phase shift distortion, which is excessive, and which does not occur with the wall action of the present invention.

The foregoing discussion may seemingly be inconsistent with the so called Ohms auditory law, that the ear tends to analyze the compounds of a complex wave regardless of the phase relations. However, there is a definite phase relation which will produce the greatest loudness and another which will produce the least loudness. For example, an harmonic in 5 the actuating sound may reinforce or cancel an aural harmonic. (Stevens & Davis, Hearing Wiley).

Since the phase characteristic of a system is intimately relatedto the amplitude response by the so-called phase rule the transient response will vary greatly depending on the phase characteristic which can cause markedly audible phase distortion.

By placing drivers 11 at excessive heights in horizontal planes along the radiator overly high fidelity vertical stereo will result in a two dimensional stereo effect.

Referring now to FIG. 4 a rectangular panel body 100 is shown which in a specific construction may be about 10 feet long, 2 /2 feet wide and 8 inches thick and may have its long dimension either vertical or horizontal and with a plurality of electromagnetic drivers 11 spaced inwardly Erick BT13? about 14 inches and centered to provide a multiple input for stereo action. Any desired support which does not unduly restrain the body 10a may be used. The body 10a can be integral or composed of a plurality of blocks in contact with each other and with or without adhesives joining at their meeting edges.

Referring now to FIG. 5 in the embodiment there illustrated the body 10b which may be the same or larger than the panel 10a, and of a plurality of blocks if desired, has four drivers 11 mounted thereon, spaced say 10 feet apart horizontally and about 4 or 5 feet apart vertically. The blending of the sound as horizontally and vertically propagated in the wall is shown by circles concentric with the respective drivers 11. This arrangement, with a four track stereo tape 35, as illustrated diagrammatically in FIG. 5A, provides a three dimensional stereo effect because of the multiple level as well as the horizontal spacing of the drivers 11.

Referring now to FIG. 6, the blending of the sound in a wall 10c with a curvature between its ends and with spaced drivers 11 is illustrated. The locations of two listeners at 40 and 41 with blended but different sound distribution thereto is shown.

In FIG. 7 the sound propagation horizontally along a wall 10d from a driver 11 is indicated and from this it will be seen that the true relationship and position such as that of various instruments is illustrated regrouped for listener at location 42 in realistic space proximity of the original.

For purposes of explanation the tape 35 is shown as having four tracks 36 and pick up heads 37 through a suitable multiple channel amplifiers 38 energize each of drivers 11. I

It will thus be seen that a simple but effective sound radiator has been provided with which results not heretofore attainable are accomplished.

We claim:

I. A sound radiator comprising:

a wall of stabilized rigid foam of plastic material;

Claims

1. A sound radiator comprising: a wall of stabilized rigid foam of plastic material; a driver having a relatively movable driving and driven members; said wall having a continuous uninterrupted face portion with which both of said members are in secured engagement; and one of said members comprising a helical coil and the other of said members comprising magnetic members having an air gap within which said coil is movable.