U.S. patent number 3,816,672 [Application Number 05/323,574] was granted by the patent office on 1974-06-11 for sound reproduction system.
Invention is credited to Herbert Gefvert, Keith Peter.
United States Patent |
3,816,672 |
Gefvert , et al. |
June 11, 1974 |
SOUND REPRODUCTION SYSTEM
Abstract
A sound system for reproducing the full frequency spectrum of
recorded sound throughout a listening environment at its natural
volume and in the same spatial relationships as the originally
created sound includes a multidirectional main audio driver
utilizing the convex area of a conical shell diaphragm integrated
with an acoustically reflective throating inertial regulator disk
to provide spatial acoustic coupling between the audio driver and
the atmosphere that approximates a live performance. The regulator
disk is a rigid reflective plate mounted in a plane which forms an
interior angle between 45.degree. and 75.degree. with the axis of
the conic diaphragm. The plate may be flat, or shaped
hyperbolically or exponentially. It may be either integral with the
conical diaphragm assembly or separate and attached by external
means. A bass equalization device comprising a tube of controlled
inertance having an acoustically resistive network inserted therein
amplifies the output from the convex radiating surface of the audio
driver and equalizes the bass and upper frequency audio power
output. It also serves to cancel the out-of-phase upper frequency
sound waves generated by the concave radiating surface. The bass
equalization device and the inertance regulator disk control the
resonant frequency and inertance of the system. The bass frequency
output faces a cabinet enclosure equalized without resonance so
that 12 through 512 Hz. is generated without spurious harmonic
partials or parasitic frequency components. Horizontal-radial and
vertical dispersion speakers, comprising high frequency response
speakers, are vertically oriented and face a conic, hyperbolic or
exponential throating and inertance regulated dispersion surface
for a full spatial distribution of the upper frequency audio
output. The sound reproduction system may further be described as
an inertially regulated sonic transducer. By controlling the
diaphragm velocity and acoustic expansion rate audio reproduction
can be recreated that approximates the spatial perspective of a
live performance.
Inventors: |
Gefvert; Herbert (Winnetka,
IL), Peter; Keith (Deerfield, IL) |
Family
ID: |
26730840 |
Appl.
No.: |
05/323,574 |
Filed: |
January 15, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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52590 |
Jul 6, 1970 |
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Current U.S.
Class: |
381/352; 181/144;
381/160; 381/432; 381/354; 381/346; 181/199 |
Current CPC
Class: |
H04R
1/345 (20130101); H04R 1/26 (20130101) |
Current International
Class: |
H04R
1/26 (20060101); H04R 1/34 (20060101); H04R
1/22 (20060101); H04R 1/32 (20060101); H04r
009/06 () |
Field of
Search: |
;179/115.5R,1E,16A,1D
;181/31R,31A,31B |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Attorney, Agent or Firm: Neuman, Williams, Anderson &
Olson
Parent Case Text
This is a continuation, of application Ser. No. 52,590 filed July
6, 1970 and now abandoned.
Claims
What is claimed is:
1. A system for reproducing sound throughout a listening
environment including:
a. a hollow flexible main audio driver including a conical
radiating member having a small and a large end axially aligned
with each other, and defining an inner concave sound radiating
surface and an outer convex sound radiating surface;
b. driving means directly coupled to said small end for
mechanically driving said radiating member;
c. a relatively rigid reflective member essentially the same
diameter as the large end of the conical radiating member disposed
at an angle to the axis of said conical radiating member, adjacent
said small end, the angle between the outer surface of said
radiating member and the reflective member being between about
45.degree. and 75.degree.;
d. tubular sound conducting conduit and bass equalizing means
cooperatively mounted to receive sound energy from the concave
inner surface of the large end of the conical driver so that the
outer convex surface of said conical radiating member is freely
exposed for direct electroacoustic coupling to the surrounding
environment and the reflective member, the inner concave surface of
said conical radiating member being isolated from the surrounding
environment by said conduit;
e. said outer surface and said reflective member cooperating to
direct sound waves produced at the outer surface of said radiating
member radially outwardly therefrom.
2. A sound reproducing system as claimed in claim 1 wherein said
reflective member is a circular, plane disk.
3. A sound reproducing system as claimed in claim 2 wherein said
reflective member has a diameter between three/fifths and
seven/fifths of the diameter of said large end of said radiating
member.
4. A sound reproducing system as claimed in claim 3, in which said
reflective member is formed with a surface of revolution coaxial
with the axis of said radiating member.
5. A sound reproducing system as claimed in claim 4, wherein the
intersection of said surface of revolution with a plane including
said axis is a curve.
6. A sound reproducing system as claimed in claim 1, wherein said
base equalization means comprises a closed tube having a length
substantially greater than its width, such that the resonant
frequency of said tube is a function of said tube length, and
acoustic network means supported within said tube and dividing said
tube into two separate lengths for controlling the acoustic
response of said bass equalization device.
7. A sound reproducing system as claimed in claim 6 in which the
acoustic distance from said radiating member to the end of said
tube is approximately one/quarter of the wavelength of the resonant
frequency of said radiating member.
8. A sound reproducing system as claimed in claim 7 in which the
inner surface of said tube is lined with an acoustically absorbent
material adapted to attenuate high frequency sound waves generated
by the inner surface of said radiating member.
9. A sound reproducing system as claimed in claim 8 in which said
sound absorbent material is a porous polyurethane material.
10. A sound reproducing system as claimed in claim 6 in which said
acoustic network means is positioned within said tube at a distance
from the large end of said radiating member of between three/fifths
and two/thirds of the length of said tube.
11. A sound reproducing system as claimed in claim 6 in which said
acoustic network means comprises a plurality of layers of
microporous acoustic damping material in a sandwich array such that
said network provides acoustic resistance to the passage of low
frequency sound waves generated by the inner radiating surface
response and extending the overall frequency response range of said
bass equalization device.
12. A sound reproducing system as claimed in claim 1 including an
electroacoustic transducer for reproducing high frequency sound
waves of 2,500 Hz. and above, and a dispersion member facing said
transducer, said transducer and dispersion member cooperating to
disperse said high frequency sound waves radial outwardly
therefrom.
13. A sound reproducing system as claimed in claim 12 wherein said
dispersion member is formed with a surface of revolution coaxial
with said electroacoustic transducer.
14. A sound reproducing system as claimed in claim 13, wherein the
intersection of said surface of revolution with a plane including
the axis of said dispersion member is a circular arc.
15. A sound reproducing system as claimed in claim 12 in which said
electroacoustic transducer includes a permanent magnet and said
dispersion surface has an apex, said apex being mechanically
coupled to said magnet so that said magnet supports said dispersion
surface in substantially coaxial relationship with said
transducer.
16. A sound reproducing system as claimed in claim 12, including an
absorbing member surrounding said dispersion member and absorbing a
portion of said high frequency sound waves dispersed in a plane
transverse to the axis of said transducer.
17. A system as claimed in claim 1 for reproducing sound throughout
a listening environment comprising an audio driver and a bass
equalization device, said equalization device comprising a tube
having an open end juxtaposed with said driver and a length
substantially greater than its width, such that the resonant
frequency of said tube is a function of the tube length, and
acoustic network means disposed within said tube and dividing said
tube into two separate lengths for controlling the acoustic
response of said bass equalization device.
18. A sound reproducing system as claimed in claim 17 in which the
distance from said driver to the opposite end of said tube is
approximately one/quarter of the wavelength of the resonant
frequency of said driver.
19. A sound reproducing system as claimed in claim 17 in which said
acoustic network means is positioned within said tube at a distance
from said driver of between three/fifths and two/thirds of the
length of said tube.
20. A sound reproducing system as claimed in claim 17 in which said
acoustic network means comprises a plurality of layers of
microporous acoustic damping material in a sandwich array such that
said network provides acoustic resistance to the passage of low
frequency sound waves generated by said driver and extending the
overall frequency response range of said bass equalization device.
Description
FIELD OF THE INVENTION
This invention is directed to a sound reproduction system and more
particularly to a combination of unique electroacoustic transducers
which cooperate to accurately reproduce sound throughout a
listening environment in essentially the same geometric spatial
profile as it is originally created.
BACKGROUND OF THE INVENTION
The controlling and limiting factor in sound reproduction is an
electroacoustic transducer, or loudspeaker, whose major element is
a diaphragm which though limited in physical size, must transmit
sound across an unlimited space within its environment. Sound is
transmitted by the action of the vibrating diaphragm in setting air
molecules in motion. The resulting motion of the air molecules is a
function of the speed and distance of each excursion by the
diaphragm of the transducer. The diaphragm moves back and forth
under the control of electric signals received by the transducer.
The diaphragm thus creates wave fronts of air particles, or sound
waves, which travel through the atmosphere to the listener. The
formula that expresses the relation between the inertance of the
atmosphere and the diaphragm is:
Z = F/V, where
Z is the inertance of the atmosphere;
V is the velocity of the vibrating diaphragm; and
F is the acoustic force impinged upon the atmosphere.
When the acoustic force is produced by a vibrating diaphragm, the
inertance Z is defined by:
Z' = Z/S or P/V, where
S is the area of the diaphragm;
P is the pressure of the atmosphere on the diaphragm; and
V is the velocity of the vibrating conical diaphragm.
To increase the output efficiency and the fidelity of sound
reproduction, means must be employed to control and limit the
amount of atmosphere (i.e., the number of air molecules) the
diaphragm must face. If the diaphragm faces an atmosphere that is
too limited, the molecular motion or wave front created by each
excursion of the diphragm is limited and compressed, and the audio
output fidelity is extremely poor, the sound being characterized by
a megaphone or tunnel effect. If the radiating diaphragm faces an
infinite atmosphere unlimited as to length, breadth and width, then
the effect of the motion imparted by the diaphragm to the air
molecules is quickly dispersed and the audio output is low,
expecially in the high frequency ranges where the diaphragm
movements are short and rapid.
The number and design of acoustic transducers necessary to create a
sound system which is capable of accurately reproducing sound in
any environment depends on two factors: (1) the spatial
distribution of the acoustic output of each loudspeaker of the
sound system, and (2) the efficiency and fidelity of sound
reproduction of each speaker. Sound systems, as presently
developed, ordinarily utilize at least two speakers to reproduce
the full range of frequencies: a deep throated cone type speaker
for low frequency sound reproduction and an almost straight-sided
speaker for higher frequencies. A single speaker designed to
reproduce a broad sound spectrum is capable of contributing little
high frequency sound reproduction. This is due especially to the
mass of the speaker cone which results in a relatively low resonant
frequency and an inability of the speaker diaphragm to move quickly
enough to reproduce high frequency sound. Further, the deep throat
of the speaker cone attenuates the higher frequencies before they
can project to the outer atmosphere.
A defect present in speakers designed specifically for high
frequency reproduction is that they fail to compensate for the fact
that high frequencies are more subject to obstructions than low
frequencies. Most high frequency sound waves are created in a
relatively limited area near the apex of the high frequency speaker
cone; this is because at high frequencies the diaphragm must move
in and out so quickly that in fact only the area of the cone near
the apex or center moves a significant distance. Nor do high
frequency sound waves disperse easily through a listening
environment; the upper range frequencies have a tendency to
propagate in concentration along the axis of the loud speaker cone,
resulting in a narrow beam of generated sound. Thus most of the
listening area of a given room is off the speaker axis and the
sound reproduction as heard at these positions is limited.
Conventional low frequency speakers also have difficulty in
accurately reproducing the frequencies for which they are designed.
The speaker voice coil and diaphragm combination must be capable of
reversing its direction of travel under the control of an input
signal having considerable variation in amplitude and frequency.
Most conventional low frequency speakers cannot fulfill this
requirement because of the large mass of the diaphragm used to
achieve a low resonant frequency. As a consequence, the output has
a muddy or rounded effect at low frequencies due to the failure of
the voice coil to instantaneously reverse its direction in response
to changing input signals.
The sound system of this invention undertakes to present reproduced
sound to the ear in the same geometric spatial profile as it was
originally created without making compromises in tonal quality. It
is known that sound is projected outwardly from various instruments
and vocalists in classifiable patterns. These are broadly:
1. A hemisphere (e.g., stringed instruments and percussion
instruments);
2. A narrow conical beam (e.g., brass instruments and voices);
3. A briad conical beam (e.g., woodwinds); and
4. A radial pattern (e.g., guitars).
A properly designed sound system should incorporate a number of
acoustic transducers such that the combination is capable of
approximating the spatial distribution pattern of the recorded
sound, the several transducers cooperating to make up for the
deficiencies of the individual transducing elements.
True sound reproduction also requires that the volume of the
reproduced sound be the same as that originally recorded. But
speakers operate at varying levels of efficiency depending on the
use for which they are designed, ranging from relatively efficient
tweeters to inefficient midrange and low frequency reproducers.
Further it is known that sound radiates from both the concave and
convex surfaces of the speaker diaphragm; but the sound radiated
from the rear or convex diaphragm surface must be attenuated in
order not to cancel out the in-phase direct radiation. Prior
attempts to utilize radiation from the rear of a diaphragm employ
baffles in which the sound waves radiated from the rear are
reflected to reinforce the bass frequencies transmitted from the
front surface of the diaphragm. Such baffles require a diaphragm of
considerable mass to provide the strength necessary to dampen
out-of-phase sound waves reflected by the baffle directly back to
the speaker. Such baffles are also ordinarily of great size and may
introduce many out-of-phase components into the output, unless the
range of reflected frequencies is limited by complex design.
In the sound system disclosed herein, which is capable of
efficiently reproducing recorded sound at the same volume and in
the same acoustic profile as it is recorded, each transducer is
designed for a specific frequency range or purpose, resulting in a
decrease in the number of design compromises in each transducer,
and improved overall sound reproduction. Moreover, in the main
audio driver of this invention, the acoustic radiation from both
the front and rear surfaces of the speaker contributes to the total
output of the sound system, resulting in improved efficiency of
sound reproduction.
SUMMARY OF THE INVENTION
The present invention provides means for reproducing recorded sound
in the same spatial profile and volume level as it was originally
created. Means are included to compensate for the varying
reproduction characteristics of the electro-acoustic transducers in
a multi-element sound system, thus recreating the full frequency
spectrum as it was recorded.
The sound reproduction system of this invention includes a
multidirectional main audio driver whose conic diaphragm is
structurally supported so that the outer, convex surface of the
cone is the electroacoustic coupling element between the electrical
signals and the listening environment. Using the outside or convex
area of the acoustic driver's cone provides equalized audio
dispersion over a 360.degree. sound projection pattern, as opposed
to the 30.degree. projection pattern from the concave area of a
conventionally mounted speaker cone.
Prior to this time a major problem in using the convex or outside
area of the cone has been the decreased audio power output
therefrom. Although below 3,500 Hz, the audio output of the convex
area of the conic diaphragm is equal in intensity to that from the
inside or concave area, a loss in the higher frequencies occurs due
to the lack of horn coupling or throating on the outside area of
the cone. To supply the acoustic throating ordinarily provided by
the inside area of a conventionally mounted speaker cone, a
reflective surface is mounted in or near the plane of the apex of
the main driver and relative to the direction of oscillation of the
main driver's diaphragm so that an acoustic throat is formed having
the reflective surface as one side and the outside of the main
driver conic diaphragm as the other side. This unique design
restores the loss in high frequency output which ordinarily results
from broadcasting from the concave surface of the main driver's
conic diaphragm.
A bass equalization device may be used in combination with the main
audio driver of this invention to provide bass response equal in
audio intensity to the response across the midrange and high
frequency spectrum, as well as to acoustically cancel the
out-of-phase components of the upper frequency spectrum. The
equalization device is a tube whose resonant frequency and overall
acoustic response characteristic are carefully controlled. The
resonant frequency is a function of the length of the tube. The
acoustic response of the tube throughout the frequency range of
interest is controlled by inserting an acoustic network into the
tube. The acoustic network controls the acoustic resistance and
ultimately the acoustic response of the tube, expecially in the
frequency spectrum including the resonant frequency of the
equalization device. The result of using the network is to generate
a more linear dynamic audio output as well as to extend the lower
limit of audible frequency response. In a further refinement the
walls of the tube may be lined with an acoustic damping material
chosen for its absorbency of high frequency waves, providing
damping of out-of-phase components of the high frequency audio
output from the main driver's concave or inside surface. The audio
output from the concave surface of the main driver is acoustically
coupled to one end of the tube of the bass equalization device. The
resonant frequency of the equalization device is so chosen that the
device amplifies the low frequency components of this audio input,
eliminating the need for a conventional cone-type woofer. Due to
the low resonant frequency and the damping materials used in
constructing the tube, the upper frequency components are sharply
attenuated, eliminating the problem of out-of-phase components of
high frequency sound output.
Four objectives are fullfilled by the base equalization device of
this invention:
1. The bass output is linearized between 16 Hz. and 256 Hz.,
resonance peaks being eliminated;
2. The output is generated over a 360.degree. radial pattern;
3. the acoustic power output is equal to that of the main driver
generator of the system; and
4. the out-of-phase components of the upper frequency spectrum
produced by the concave or inside area of the main generator
diaphragm are effectively damped out.
To reproduce more accurately instruments which project their sound
in a horizontal radial pattern in the upper frequency spectrum, a
horizontal radial high frequency dispersion unit may be added to
the system. The dispersion unit includes in combination a high
frequency acoustic transducer and a hyperbolic dispersion surface.
As used herein, a dispersion surface is a substantially conically
shaped surface having flared upper edges, the slope at which the
cone flares being scientifically determined to distribute the
acoustic output from the radiating surface of the associated
transducer in a 360 degree radial pattern. hyperbolic or
exporential surfaces of revolution may be employed instead of the
conical surface. The axis of the dispersion surface is generally
parallel to the axis of the high frequency transducer.
A vertical radial dispersion unit may be added to the sound system
of this invention to more completely reproduce the hemispherical
outward sound patterns of certain other instruments. The dispersion
unit consists of a high frequency responsive transducer having a
dispersion surface cooperating therewith to disperse the high
frequency waves in the hemispherical sound patterns creaded by
certain instruments.
IN THE DRAWINGS
FIG. 1 is a vertical section taken through a center line of a sound
system embodying the present invention;
FIG. 2 is a perspective view of the sound system of FIG. 1, mounted
on a base containing the bass equalization device, and illustrating
a sound directing shield used in cooperation with the horizontal
radial tweeter;
FIG. 3 is a perspective view of the sound system of FIG. 1 with the
bass equalization device removed;
FIG. 4 is a graph depicting the acoustic output from the bass
equalization device of the sound system of FIG. 1, over its
effective operating range;
FIG. 5 is a schematic diagram of the main audio driver and its
associated inertance regulator disk, illustrating their angular
relationship and direction of their audio output;
FIG. 6 is a vertical section of the bass equalization device of the
sound system of FIG. 1, illustrating the separation of the sound
reproduction system into two sections, one including the main audio
driver and the other including the high frequency dispersion
speakers;
FIG. 7 is a section of a bass equalization device having two
folds;
FIG. 8 is a horizontal section of the bass equalization device of
FIG. 7, taken along the line 8--8; and
FIG. 9 is a horizontal section of the equalization device of FIG.
6, taken along the line 9--9, in which the device is folded in
three planes to produce a more compact structure.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a view in vertical section of a preferred embodiment of
the sound system of this invention.
The main multidirectional audio driver 2 of the sound system is
shown having the axis of driver 2 mounted in the vertical plane,
the driver being supported in this position by supporting frame
structure 4. Membranes 6 and 8 hold the diaphragm 9 of audio driver
2 centered in the supporting framework 4 but leave it free to move
axially. The apex 10 of diaphragm 9 is flexibly connected by the
membrane 6 to a supporting shoulder 11 of framework 4. The shoulder
11 supports a regulator disk 12 (shown more clearly in FIG. 3)
which cooperates with diaphragm 9 to provide a throating effect and
amplify the acoustic output of driver 2 as is clearly explained
below. The outer end 14 of the diaphragm 9 of generator 2 is
flexibly connected by membrane 8 to supporting framework structure
4. The main driver generator 2 is otherwise of essentially
conventional construction, and includes a magnetic core 16 which
may be permanently magnetized to generate a constant magnetic
field.
A voice coil 18 wound on a tubular member 20 extends into the air
gap of the magnetic circuit formed by magnetic core 16. The voice
coil 18 carries an exciting current which varies in amplitude and
frequency to represent the volume and frequency of the sound to be
reproduced. As a result of the flexible support provided by
membranes 6 and 8 the entire diaphragm 9 is free to move in
response to the exciting current applied to the voice coil 18 and
thus act as an electroacoustic coupling device between the electric
circuit signals and the surrounding atmosphere of the listening
environment. The diaphragm 9 thus radiates sound waves to the
surrounding atmosphere in response to the exciting current in the
voice coil.
Utilizing the convex cone area surface 21 of the diaphragm 9 to
provide electroacoustic coupling results in a 360.degree. sound
radiation pattern surrounding the sound source and filling the
listening environment, as opposed to the limited 30.degree. sound
pattern projected by the concave cone used in the prior art. The
acoustic output of such prior art cone type loud speakers
propagates through the listening area with decreasing intensity as
the angle from the axis of the cone increases.
The acoustic energy from the outer or convex side 21 of the
diaphragm 9 is the same as the energy from the inside surface 22,
and below 3,500 Hz. There is however, a measurable loss in high
frequency output from the convex surface 21 due to the elimination
of horn coupling or throating on the outside or convex area 21 of
the diaphragm 9. This throating effect is ordinarily provided by
the two sides of the conventional horn megaphone type speaker; the
outward projection of midrange and higher frequencies is a result
primarily of a resulting horn coupling and outward emission, rather
than the piston action of the cone itself. In the preferred
embodiment disclosed herein, the throating action is provided by
using the surface 21 of the moving diaphragm 9 as one side of the
acoustic throat necessary to provide adequate audio coupling
between generator and atmosphere, and reflecting regulator disk 12
as the other side of the throat.
In the preferred embodiment disclosed herein, regulator disk 12 may
properly be termed an inertial transducer in that it controls the
inertance of the main audio driver system. It does this by
controlling the amount of air which faces the conic diaphragm, thus
limiting the number of free air molecules which must be set in
motion to reproduce sound. In the preferred embodiment, the
regulator 12 has a disk shape, the outer diameter of which is the
same as the diameter of the large end of the conic diaphragm 9. It
may, however, range from three-fifths to seven-fifths of the
diameter of the large end of conic diaphragm 9. The regulator disk
12 is shown mounted on supporting shoulder 11 in a plane
perpendicular to the generator's axis and at or near the apex 10 of
the generator 2, as shown in FIG. 1. As shown in FIG. 5, the angle
between the diaphragm 9 and the regulator disk 12 in this exemplary
embodiment is 60.degree., with the sides of the diaphragm 9 being
30.degree. off its central axis as shown by the angle .phi.. The
angle .theta. between disk 12 and diaphragm surface 21 may be as
small as 45.degree. or as great as 75.degree., the surface itself
in a preferred embodiment is flat, but may be curved hyperbolically
or exponentially in certain embodiments.
The combination of the inverted conic diaphragm 9 with the fixed
regulator disk 12 provides many long sought advantages over the
conventional cone speaker, without sacrificing the throating action
necessary for full sound output. Since the fixed regulator disk 12
constitutes one side of the acoustic throat, a greater audio output
results than with a cone diaphragm utilizing conventional horn
coupling wherein both sides of the throat are in oscillation.
Further, the loudspeaker baffle ordinarily used to capture and
control emission from the outside or convex cone area is
eliminated, together with the spurious resonances and wave fronts
it creates, and with its customary loss of a portion of the output.
In the multidirection audio driver 2 of this invention, the
out-of-phase components of audio output are limited to the concave
area 22 within the diaphragm 9, making them easier to control and
eliminate.
It can also be seen in FIG. 5 that an angle .theta. equal to
60.degree. provides a favorable degree of angular force for
restoring the diaphragm to its original position after each
excursion. It is known that a .theta. equals angle .PSI., the
interior angle of the large end of the diaphragm; and that W, the
outward component of energy from the cone is W = X cos.PSI.. .
Because the cosine of 60.degree. equals 0.500, the forces in the
oscillating conical diaphragm are equalized in the configuration,
and a minimum of restoring force is required after each axial
excursion.
A bass generation and equalization device 26 is used in combination
with the multidirectional driver 2. The bass equalization device 26
is designed to equalize the bass energy from 16 to 256 Hz, to
provide equal response over the full 360.degree. listening area and
to provide a power handling capacity equal to the output of the
audio driver 2 disclosed above. The equalization device 26 also
destroys the out-of-phase audio signals created by the concave
surface 22 of diaphragm 9 of the audio driver 2.
The bass equalizer 26 includes a tube 27 having an entrance 28 for
the acoustic output generated by the concave side 22 of diaphragm
9. The far end 30 of the tube is closed; the length and resistance
to the passage of air of the tube are carefully controlled so that
the high frequency sound waves are fully attenuated during their
travel through the tube; the low frequency waves are delayed only
long enough so that they are in phase with the sound generated by
the convex surface of the main driver 2, thus reinforcing the low
frequency sound output without adding any distortion to the audio
output. The bass equalization device 26 to be used in cooperation
with the main audio driver 2 allows the use of a diaphragm 9 of
reduced mass for driver 2, because the desirability of a diaphragm
with a low resonance point is reduced. Reduction in mass of the
diaphragm 9 also allows a reduction in the flux density of the
magnetic core 16, making the diaphragm 9 of the audio driver 2 more
quickly responsive to changes in frequency and amplitude in the
exciting input signal current.
The multidirectional audio driver 2 is positioned so that the audio
output from the concave surface area 22 is directed into one end 28
of the tube 27 of bass equalizer 26. In the embodiment disclosed
herein, audio driver 2 is directly coupled to one end 28 of bass
equalizer 26. Accurate reproduction of the long wave lengths of low
frequency sounds requires a physically large tone reproduction
chamber. The longer the tube 27, the lower the minimum frequency of
its acoustic output, because the resonant frequency of the tube is
a function of tube length. Thus, the tube 27 of bass equalizer 26
provides the long air path required to amplify bass frequencies
(below 200 Hz).
The resonant frequency of bass equalizer 26 is a function of the
length of tube 27. For a resonant frequency of 55 Hz, which would
provide a broad, full-range response, the length of tube 27 is
approximately one-quarter of the wavelength of a 55 Hz signal. When
a conical shell diaphragm of 6 inches in axial depth is employed,
the optimum tube length if about 4 feet. An ideal height to
diameter ratio for equalization device 26 is 8 to 1; therefore, the
diameter of tube 27 is 6 inches.
To reduce the space occupied by tube 27, it may be folded, for
example, into two or three sections without loss of bass response,
due to the long wavelengths in the frequency spectrum of interest.
FIGS. 6 and 9 show a tube having a double fold; FIGS. 7 and 8 show
a single folded tube. FIGS. 6 and 9 show a tube folded upon itself
in two planes, resulting in an esthetically desirable configuration
which occupies a minimum of space. In a further modification, if
the size of the sound system itself is sought to be reduced, the
length of the tube may be shortened (and thus the diameter as
well), although with a consequent rise in resonant frequency, and
thus a reduction in the range of linear bass response.
The use of tube 27 alone for the bass generation and equalization
results in undesirable peaks in the bass output signal, produced by
the natural resonances of the system, i.e., the frequencies at
which the system vibrates most freely and provides the greates
reinforcement to the acoustic output of audio driver 2. A typical
response curve 30 for the tube of equalization device 26, shown in
FIG. 4, has a major peak 32 at about 70 Hz., the resonant frequency
of an exemplary tube 27 having dimensions as disclosed above and a
less pronounced peak 34 at about 140 Hz.
As described above, the length of the tube 27 provides the desired
extended and amplified bass frequency reproduction; thus the peaks
cannot be reduced by adjusting the tube length without affecting
the acoustic response of the equalization device. Rather in the
sound system of this invention, the resonant peaks have been
"flattened" by introducing acoustic network 42 which reduces the
"Q" of the bass equalization device 26. In an acoustic system "Q"
is a figure of merit representing the ratio of the acoustic
reactance to the acoustic resistance, the two components of the
total acoustic impedance (or resistance to the propagation of
sound) which determines the acoustic response of the system. The
acoustic reactance is frequency dependent, and is at a minimum at
the resonant frequency of a system. Because the operating
frequencies of interest of bass equalization device 26 are at or
near its resonant frequency, the acoustic response of the device is
controlled to a great degree near this frequency by the acoustic
resistance of the system. The formula for the inertance M of the
circular tube of radius R is:
M = PL/.pi.R.sup.2,
where
P = grams per cubic centimeter of atmosphere in tube; and
L = length of tube in centimeters.
The acoustic reactance X - 2.pi.fM. Increasing the acoustic
resistance or inertance by introducing network 42 lowers the "Q" of
the acoustic system and maintains the peak output level at or near
the normal output level of the system, as shown in curve (FIG. 4),
which has reduced resonant output peaks 38 and 40. A further
benefit of increased acoustic resistance is that the frequency
dependence of the total system acoustic response decreases as the
acoustic resistance increases; thus the change in audio output
across the frequency spectrum of interest, i.e., the bass
frequencies from 16-200 Hz., is reduced, as shown in curve 36.
In the bass equalization device 26, the acoustic resistance is
increased and the peaks in frequency response greatly reduced as
described above by the insertion of an acoustic network 42 in the
tube 27 of equalization device 26. A typical acoustic network 42
includes materials of three different porosities arrayed in a
sandwichlike combination. A typical sandwich network 42 includes a
layer 44 of polyurethane 10, a layer 46 of polyurethane 30, and a
layer 48 of polyurethane 100. The numeral following the term
polyurethane indicates the porosity in pores per lineal inch of the
materials of the network, which are commercially available under
the same Scott Renticulated polyurethane. The network 42 is used to
increase the acoustic resistance of tube 27 to the desired level,
thereby reducing the frequency dependence of the audio output of
equalization device 26. The polyurethane also absorbs the midrange
to high frequency sound waves. It should be understood that as with
all other materials and dimensions specifically stated herein,
dimensions and materials for the construction of an exemplary
acoustic network are stated for the purpose of example only and are
not intended to be limiting or restrictive in any way.
The length of tube 27 and thus the resonant frequency of bass
equalizer 26 being established, the acoustic network 42 is inserted
therein to lower the "Q" and thus reduce the response peak at
resonant frequency. A suitable network 42 for a tube 27 4 feet in
length includes a first layer 4 approximately 1 inch in thickness
of a material having a porosity of 10 pores per lineal inch; a
second layer 46 approximately three inches in thickness, having a
porosity of 30 pores per lineal inch; and a third layer 48
approximately 4 inches in thickness having a porosity of 100 pores
per lineal inch. The exact composition of the inserted network 42
to produce the desired changes in acoustic output of the tube 27 of
a specific embodiment of this invention are determined by linear
extrapolation after the other parameters (length, diameter, and
maximum acoustic power intended) of the tube 27 are determined.
Acoustic network 42 is positioned within tube 27 at a point where
the sound wave encounters the maximum acoustical resistance. In a
closed tube (the tube 27 of this system is effectively a closed
tube because of the main driver's diaphragm coupled to one end) the
points of maximum acoustical resistance occur at one-third and
two-thirds of the effective tube length. The plug is ordinarily
placed at the point of maximum acoustical resistance farther from
the diaphragm 9 of main generator 2; the plug might otherwise have
a greater effect than desired upon the tube's resonant frequency.
It is known from experiment than an ideal placement is at a point
between three-fifths and two-thirds of the way to the far, closed
end 30 of the tube.
The sidewalls 50 of the tube 27 are lined with a layer 52 of
acoustic damping material which attenuates the midrange and upper
frequencies which are generated by the concave or inside area 22 of
main generator 2. A typical material is a polyurethane material or
Tuflex, having a known porosity suitable for such usage. The layer
52 is preferably ridged, and cooperates with the straight sidewalls
50 to prevent any horizontal transmission of high frequencies
through the tube walls to the listening environment.
A bass equalization device 26 provides an amplified audio output in
the low frequency ranges of heretofore unatainable clarity, a
result previously impeded by the large mass and low efficiency of
low frequency electroacoustic transducers. Further, as a result of
using the acoustic network 42 with the bass equalizer 26, a linear
output without a peak at the resonant frequency is provided. The
addition of network 42 to equalization device 26 also extends the
lower limits of linear frequency response. The clearly appears in
the acoustic response curves of FIG. 4, wherein curve 30 shows the
frequency response of the bass equalization device 26 without the
acoustic network and curve 36 shows the response with the network
42 inserted in tube 27 of equalization device 26.
A preferred embodiment of the invention also includes a horizontal
radial dispersion system 59, horizontal radial tweeter 60 in
combination with a radial dispersion element 62. The horizontal
radial tweeter 60 is of conventional design having a diaphragm 64
whose outer edges are permanently affixed by means of a connecting
ring 66 to supporting framework 4. The voice coil and magnet
structure 70 shown are of conventional design to accurately
reproduce the applied high frequency signals.
A conventional electrical crossover network 72 supplies the
frequencies of approximately 2,500 Hz. and above to the magnetic
deflection system 70 of tweeter 60. The tweeter 60 faces hyperbolic
dispersion surface 74 of dispersion element 62 to project in a
360.degree. pattern the midrange and high frequencies and harmonics
of instruments which project their sound to the listener in a
horizontal radial pattern. An examplary dispersion surface 74 which
testing has demonstrated provides full 360.degree. radiation of the
audio frequency output with minimum distortion, has a curvature
such that a plane including the vertical axis of the dispersion
element 62 forms an intersection with the dispersion surface 74 in
the shape of an arc of a circle whose center lies on the outer edge
76 of connecting ring 66 (see FIG. 2) which holds in place the
radial horizontal tweeter. In an alternative embodiment shown in
FIG. 2, a shield of sound absorbing material covers one-third of
the output area of horizontal radial dispersion system 59. The
shield 75, shown in cross section in FIG. 1 is of polyurethane type
material of highly absorbent quality, so that the resulting
acoustic output of the horizontal dispersion system 59 more nearly
approximates the conic dispersion pattern described above.
The vertical axis of the dispersion surface 62 is parallel to the
axis of tweeter 60; in a preferred embodiment it is coaxial with
that of the horizontal radial tweeter 60.
In a preferred embodiment, the radial dispersion element 62 is
suspended in coaxial alignment with the horizontal radial tweeter
60 by means of adjustable bolts 77a, b, c, d. In an alternative
embodiment (not shown) the apex 79 of the dispersion surface 74 may
be attached to the permanent magnetic core 70 at the apex of the
horizontal radial tweeter 60.
In a further alternative embodiment, a vertical radial dispersion
system 81 is provided, including a second radial tweeter 80 mounted
with its axis parallel to the axis of horizontal radial tweeter 60.
It is ordinarily mounted above parabolic dispersion element 62 (see
FIGS. 1 and 2) to project the vertical hemispherical sound patterns
generated by certain instruments. The tweeter 80 is of conventional
design having a diaphragm 82 controlled by a magnetic deflection
system 84 and being held in place by a supporting ring 86 within a
framework 88 which may also include the dispersion element 62 as a
part thereof. In this preferred embodiment the dispersion surface
74 is an integral part of framework 88. In a further addition to
this preferred embodiment a dispersion element 92 is mounted
directly on and supported by apex 94 of vertical tweeter 80
utilizing the center section 95 of magnetic deflection system 84
for support, to broadly disperse the vertical radial sound
patterns. The curvature of the surface 96 of a preferred embodiment
of this dispersion element 92 is such that a plane including the
axis of dispersion element 92 forms a curve of a hyperbolic
function at its intersection with dispersion surface 96. An
alternative embodiment is illustrated in FIGS. 6 and 9 wherein the
supporting structure 4 is divided into two sections, the first
containing only main audio driver 2, the second containing the
horizontal and vertical radial dispersion speaker combinations 59
and 81. Both units are placed on and/or supported by the bass
equalization system of the invention.
While a preferred embodiment of this invention is described above,
and illustrated in the attached drawings, it will be understood
that the invention is not limited thereto, since many modifications
may be made. It is contemplated, therefore, by the appended claims,
to cover any such modifications as fall within the true spirit and
scope of this invention.
* * * * *