U.S. patent number 3,553,392 [Application Number 04/711,313] was granted by the patent office on 1971-01-05 for electrodynamic sound radiator.
This patent grant is currently assigned to Electronics Inc. of Pennsylvania. Invention is credited to Arthur Liebscher, Nils Harry Ljungman.
United States Patent |
3,553,392 |
Liebscher , et al. |
January 5, 1971 |
ELECTRODYNAMIC SOUND RADIATOR
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.
Inventors: |
Liebscher; Arthur (Jenkintown,
PA), Ljungman; Nils Harry (Merion, PA) |
Assignee: |
Electronics Inc. of
Pennsylvania (Willow Grove, PA)
|
Family
ID: |
24857576 |
Appl.
No.: |
04/711,313 |
Filed: |
March 7, 1968 |
Current U.S.
Class: |
381/152; 381/426;
181/167 |
Current CPC
Class: |
H04R
5/02 (20130101); H04R 7/04 (20130101); H04R
9/066 (20130101); H04R 2205/022 (20130101) |
Current International
Class: |
H04R
5/02 (20060101); H04R 9/00 (20060101); H04R
7/00 (20060101); H04R 7/04 (20060101); H04R
9/06 (20060101); H04r 009/00 () |
Field of
Search: |
;179/115.5
;181/32,31.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Claims
We claim:
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.
Description
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 20H.sub.z base to 20 KH.sub.z 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 displacement
varies: ##SPC1##
All waveshapes can be broken into Fourier components and the force
f will be a sum of sinusoids. and since: ##SPC2##
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.
The rigidity F of a structural section is measured by the sectional
modulus: ##SPC3##
Obviously, for well behaved shapes, I grows much faster than y max,
since y.sup.2 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 be
more readily understood from the following description 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 of an embodiment of the 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.
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 20 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
a center pole piece 16. The center pole piece 16 is 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 12a 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 structural member in the building where it is housed.
Therein, the cost of the building is greatly reduced. Structural
members of high insulating and compressive qualities are marketed
by various large manufacturers.
The wall surface of the block 10 may consist of cavities and
undulations which act as resonators and waveguides for the
frequencies within the audio spectrum. Complex mathematical shapes
such as hyperbolic paraboloids, ellipsoids, paraboloids of
revolution complement the acoustical properties of the foam and
room.
The foam acts as a mechanical delay line for high frequencies,
thereby propagating said high frequencies throughout the 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 coupling of 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.
##SPC4##
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 of large dimensions, 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 channels of 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.
Since all prior methods of reproduction are substantially point
sources and since the directionality of all speakers becomes
greater at high frequencies, a listener, under usual circumstances,
never hears a true high fidelity reproduction of stereophonic
sound.
The equation for a circular piston (cone of a speaker) is
##SPC5##
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.sub.z 1500
H.sub.z and 12000 H.sub.z respectively.
From this it will be noted that departures of more than 5.degree.
from direct alignment with the speaker severly reduces the ability
to hear the higher ranges, such as 12000 H.sub.z and higher.
The distance at which a listener must be located in order to hear
high fidelity stereo is indicated in FIG. 9. ##SPC6##
Accordingly, if the speakers are 10 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 hear any high frequency stereo, otherwise, if the
listener is closer the stereo high fidelity experience will 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 plane wave
which propagates uniformly through out the area without the point
source type directionality of a cone type radiator. Since the
channels of the stereo blend in the wall before being radiated as a
plane wave, the natural positions of the instruments or other
original sources of sound are duplicated. Therefore, the positions
of the true sources are duplicated with fidelity as well as
propagated in a plane wave, thereby giving the listener a full
spectrum of audio no matter where he is located.
The general equation for a source of sound is ##SPC7## where
##SPC8## where ##SPC9## for simple harmonic wave traveling in the
positive T sense the solution of the above is ##SPC10## where
.omega.= 2.pi.f
.chi.= 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 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.degree.. For a
10.degree. 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 30.degree. 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 Ohm's 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 the actuating sound may
reinforce or cancel an aural harmonic. (Stevens & Davis,
"Hearing" Wiley).
Since the phase characteristic of a system is intimately related to
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 10a is shown which
in a specific construction may be about 10 feet long, 21/2 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 from the ends 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.
It will thus be seen that a simple but effective sound radiator has
been provided with which results not heretofore attainable are
accomplished.
* * * * *