US 3247925 A
Description (OCR text may contain errors)
April 26, 1966 ca. E. WARNAKA 3,247,925
LOUDSPEAKER Filed March 8, 1962 h "N" g INVENTOR. /5 W United States Patent Ofitice 3,247,925 Patented Apr. 26, I966 9 2 LQUDSPEAKER Glenn E. Warnalra, Erie, Pa., assignor-to Lord Corporation, a corporation of Pennsylvania Filed Manfi, 1962, Ser. N0.'17:8,469 6 Claims. (Cl. 18131) This invention is intended to-improve theefiioiencyof low frequency loudspeakers by exciting bending waves in a light weight, stifif panel which remains essentially sta tionary, except for the bending waves. In one form, such a panelmay comprise a rigid core such as honeycomb or foam sandwiched between and bonded to steel skins.
In the drawing, FIG. 1 is a sectional side elevation of a loudspeaker, FIG. 2 is a section on line 22 of FIG. 1, FIG. 3 is a front view of the speaker, FIG. 4 is .a front view of a modification, and FIG. 5 is a diagrammatic view illustrating the principle of operation.
Referring first to FIG. 5, there is shown an edge view of a panel 1 subjected to atransversebending wave causing sound to be radiated from the panel. This sound radiation is due to the alternating pressure disturbances created in the adjacent air, as indicated by arrows-2., 3, 4 by the motion of the bending Wave on the pa el. The alternating pressure disturbances thus set up constitute a sound wave. With this mode of excitation, sound cannot .be elliciently radiated if the bending Wave speed is much below the speed of sound in air. When the speed of transverse bending waves on the panel is equal to or greater than the speed of sound in air, sound may be radiatedfrom the panel efficiently. The efficiency is comparable with that of cone speakers in the mid audio range (eg. 1000 cps.) and far exceeds that of cone speakers at low bass frequency (e.g. 60-100 c.p.s.). H
In a homogeneous panel, transverse bending waves are propagated at a speed dependent on the excitation frequency. This relationshipis given by theequation where C is the bending wave speed, W is 21r times the excitation frequency, B is the bending stiffness of the panel per unit Width, and m is the mass of the panel per unit area. Thus, the most important characteristic of a panel for sound radiation is .not it-s overall bending stiffness, but the bending stiffness of its cross section.
At some excitation frequency the bending wave length will exactly equal the trace wave length (or projected wave length) or the exciting wave. This means that a crest of the exciting wave will exactly coincide with the crest-of a'bendingwave, andthe transverse bending Wave speed equals the speed of soundin air. At this cond tion all the-energy of theexciting wave, except for that small portion lost by damping in the panel and at the edges of the panel and byreflection, will be transferred to the bending wave. The lowest frequencyforwhich this wave coincidence can-occur is called the criticalfrequency. For homogeneous panels the critical frequency is given by Where a is the speed of sound in air in./sec., m is mass of the panel per unit area in lbs. sec. /-in. andB is the bending stiffness per unit width or the cross-sectional stilfness in lbs. in. /in. -When wave coincidence occurs, thepanelbecomes transparent tothe exciting wave.
The transmission loss through the panel drops to a very low level, and the exciting wave may pass through the panel readily andbe efiicientlyradiated from the other side as a sound wave.
The 10nd speaker makes use of theefficient coupling between transverse bending .waves and sound waves in air which exists at and above the critical frequency of the panel. The panel is driven by a conventional voice coil arrangement, in which 5 indicates a permanent magnet fixed to the supporting irarne 6 and 7 indicates the voice coil electrically connected by wires 8 to the audio output and mechanically connected at 9 to a panel .10. Theoritical frequency of the panel should be adjusted to be the lowest bass note to be produced by the speaker for best .results. However, acceptable results can be obtained with some loss in efiiciency if the critical frequency is in the lowbass range but is somewhat higher than the lowest bass note to be produced. In this case, fairly efficient couplin can still exist even ifthe transverse bending Wave speed is slightly below the speed of sound in air.
As an example f-or reproduction from c.p.s. upward,
the ratio for the panel should be equal to or less than 2.8 l0 sec./in. This can be obtained by using a 4 thick paper honeycomb core 11 bonded to steel skins 12, 13 each 0.010" thick. The critical frequency of the panel may be adjusted in two ways. First, different critical fre quencies can be obtained .by changing the square root of the ratio of the mass of the panel to its bending stiffness econdly, the voicecoil and magnet assembly can be set at different angles to the panel] In'thisway, the tracewavelength can be changed and'different critical frequencies will be produced.
In order to reproduce the lowest bassmusical notes, the ratio i Y should be as small as possible. Lightweight honeycomb panels and panels employing similar spacing type times as ,well as .rigid plastic foams which have very low mass and high stiffness are well suited for'us'e in this type of loudspeaker. Any type of panel construction used should, of course, have low internal damping to insure maximum efficiency.
Conventional cone and piston type speakers pump large volumes of air to produce sound. Thismeans that they mustbe capable of rather large motions, particularly at lowifrequencies, to be efficient sound radiators. With the wave coincidence type of rad'iaton'however, the panel 3 in a pumping action, the energy will be taken away from the bending waves.
There are two basic types of edge termination which will prohibit overall motion while permitting the proper cross-sectional distortion to occur. The most obvious is a rigid clamping method, shown in FIG. 1. In this case, the inner and outer peripheral edges of the panel are bolted or cemented to a rigid frame 14. In the modification of FIG. 4, the panel is supported by a soft elastic suspension. In this case, the natural frequency of the suspension should be much lower in frequency than the lowest frequency to be produced by the radiator. The panel would be well isolated and would not be influenced by the motion imparted by the voice coil. In this modification, brackets 15 are attached to the corners of the skins 12, 13 and are connected to the supporting frame by soft springs 16. The natural frequency of such a suspension would be only a few c.p.s. This is so soft that the panel could be easily displaced by external disturbances. Large displacement of the panel would degrade the performance of the loudspeaker, and damage the magnet and voice coil assembly. With the soft suspension of FIG. 4, other common methods for guiding voice coils could be used. In the case of the rigidly attached panel of FIG. 1, no special guiding is necessary, since the panel is stiff enough to locate the voice coil relative to the magnet.
Of course, it is also important that the edge terminaltion used does not add significant damping to the system which would tend to attenuate the bending waves.
The panel may be part of the enclosure or cabinet for the associated record player, radio, or television such as top, bottom, sides, or the panel may be part of a Wall or ceiling of a room.
It is not necessary that the dimensions of the panel 10 be as large as the lowest sound waves to be radiated. The important characteristic is the low ratio of the unit mass to the unit cross sectional stiffness.
The efficiency of cone type loudspeakers is generally rather poor. As the quality of the cone type speaker, measured by its frequency response range is increased, its efficiency is generally still further reduced. The coincidence effect radiator does not require large power consumption in order to increase its frequency coverage. Efficient radiation is obtained as long as the bending wave speed is equal to or greater than the speed of sound in air. This occurs for all frequencies above the critical frequency. As a result, the coincidence effect radiator is much more efiicient than conventional cone speakers of comparable quality.
A panel made of a 1 /2" thick core of paper honeycomb with .005" thick steel skins has a-critical frequency of 209 c.p.s., and, consequently,
Such a panel would give useable performance down to 100 c.p.s. Research has shown that the radiation efficiency of a panel does not decrease as rapidly below coincidence as the radiation efficiency of a cone type loudspeaker below its resonance frequency. This means that better efficiency and greater frequency coverage can be obtained more easily with a panel radiator than a cone type loudspeaker.
The drawings show only flat panel construction. This need not be the case. It is only desired that the shape of the speaker introduce no discontinuities or sharp bends which would tend to produce large reflections, attenuations, or distortions of the bending waves.
What is claimed as new is:
1. In a loudspeaker, a frame, a voice coil and magnet assembly supported on the frame, a panel, panel supporting means for supporting the panel in fixed relation to the frame prohibiting over all motion of the panel relative to the frame while permitting cross sectional distortion substantially equal to or less than 2.8 10' sec./in. where m is the mass per unit area in lbs. secF/in. and B is the cross sectional stiffness in lbs. in. /in.
2. In a loudspeaker, a frame, a voice coil and magnet assembly supported on the frame, a panel, panel supporting means for supporting the panel in fixed relation to the frame prohibiting over all motion of the panel relative to the frame while permitting cross sectional dist-ortion throughout the panel at least up to the edges of the panel, and means mechanically connecting the coil to the panel to bend the same, the ratio of the mass per unit area to the cross sectional stiffness of the panel being such that the speed of the bending wave in the panel at a low bass frequency is substantially equal to the speed of sound in air.
3. The loudspeaker of claim 2 in which the panel supporting means consists of means rigidly connecting at least one edge of the panel to the frame and in which the ratio of the mass per unit area to. the cross sectional stiffness of the panel is substantially equal to or less than i a r 2 7.54X10 sec.,in.
where m is the mass per unit area in lbs. secP/in. and B is the cross sectional stiffness in lbs. ind/in? in order that the vibration of the panel at audio frequencies equal to or greater than the critical frequency fc determined by the equation a m B are substantially confined to transverse bending waves of speed C equal to or greater than the speed a of sound in air determined by the following equation:
substantially equal to or less than 2.8 10- sec/in. where m is the mass per unit area in lbs. sec. /in. and B is the cross sectional stiffness in lbs. in. /in.
5. In a loudspeaker, a frame, a panel, panel supporting means for supporting the panel in fixed relation to the frame prohibiting over all motion of the panel relat 1ve to the frame while permitting cross sectional distortion throughout the panel at least up to the edges of the panel, the ratio of the mass per unit area to the cross sectional stiffness of the panel being such that the speed of the bending wave in the panel at a low bass frequency is substantially equal to the speed of sound in air, and audio means connected to the panel to bend the same.
6. In aloud speaker, 21 frame, a panel, panel supporting means for supporting the panel in fixed relation to the frame prohibiting overall motion of the panel relative to t e frame While permitting cross sectional distortion throughout the panel at least up to the edges of the panel,
said panel having a ratio References Cited by the Examiner UNITED STATES PATENTS Hutchison 181-32 Hopkins 181-32 Thomas 181-31 Lane 181-31 Boudette 179-1155 Du Puy 179-115.5
12/1936 Pierce 181-31 12/1936 Pierce 181-31 5/ 1956 Pace. 7
5/1962 Hegeman 181-31 11/1963 Barlow 181-32 12/1963 Wood 18131.1
FOREIGN PATENTS 1/ 1930 Great Britain. 7/1963 Great Britain.
OTHER REFERENCES Publication: Rigidity of Loudspeaker Diaphragms by D. A. Barlow Wireless World, vol. 64, N0. 12, Decem- 1 ber1958,179-181 (F), (pp. 564-569). LEO SMILOW, Primary Examiner.
W. L. LYNDE, Examiner.