U.S. patent number 3,798,374 [Application Number 05/240,495] was granted by the patent office on 1974-03-19 for sound reproducing system utilizing motional feedback.
This patent grant is currently assigned to Rene Oliveras. Invention is credited to Stanley Thayer Meyers.
United States Patent |
3,798,374 |
Meyers |
March 19, 1974 |
SOUND REPRODUCING SYSTEM UTILIZING MOTIONAL FEEDBACK
Abstract
A sound reproducing system utilizes motional feedback to reduce
loudspeaker distortion and to extend the loudspeaker's frequency
response. The system substantially comprises an amplifier which is
jointly responsive to the input source signal and to a feedback
signal, a moving-coil loudspeaker including a main electromagnetic
structure which is responsive to the amplifier's output signal for
effecting axial speaker-cone motion, motional sensing means for
providing a signal which is functionally related to axial cone
velocity, and an equalizer exhibiting a predetermined nonlinear
attenuation versus frequency characteristic and which is responsive
to the motional signal for providing the feedback signal. The
feedback signal is degeneratively applied to the amplifier which,
in turn, forces the loudspeaker to respond linearly to the input
source signal and thereby provide a uniform sound energy output. It
is a feature of the present invention that the loudspeaker cone
exhibits a substantially constant acceleration at low frequencies
and a substantially constant velocity at higher frequencies.
Inventors: |
Meyers; Stanley Thayer (Red
Bank, NJ) |
Assignee: |
Oliveras; Rene (Madison,
NJ)
|
Family
ID: |
22906755 |
Appl.
No.: |
05/240,495 |
Filed: |
April 3, 1972 |
Current U.S.
Class: |
381/96;
381/103 |
Current CPC
Class: |
H04R
3/002 (20130101) |
Current International
Class: |
H04R
3/00 (20060101); H04r 003/04 () |
Field of
Search: |
;179/1F,1D ;330/109,110
;333/28T |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
283,405 |
|
Jan 1966 |
|
AU |
|
659,066 |
|
Oct 1951 |
|
GB |
|
879,560 |
|
Jul 1949 |
|
DT |
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Leaheey; Jon Bradford
Attorney, Agent or Firm: Oliveras; Rene
Claims
What is claimed is:
1. A sound reproducing system comprising:
first amplifying means jointly responsive to a source signal and to
a first feedback signal;
first sound energy producing means responsive to said first
amplifying means;
first means for sensing the sound energy output of said first
producing means; and
first equalizing means exhibiting a predetermined nonlinear
attenuation versus frequency times a constant characteristic and
being responsive to said first sensing means for providing said
first feedback signal, said characteristic having a positive slope
over the lower portion of a first frequency range and a zero slope
over the rest of said first frequency range;
whereby said first sound energy producing means radiates a
substantially uniform sound energy output over said first frequency
range.
2. The sound reproducing system of claim 1 wherein said first sound
energy producing means is a loudspeaker of the moving-coil type
including a cone and a voice coil, said voice coil responding to
said first amplifying means for axially driving said cone; and said
first sensing means responds to the motion imparted to said cone by
said voice coil.
3. The sound reproducing system of claim 2 wherein said first
sensing means responds to the axial velocity imparted to said cone
by said voice coil.
4. The sound reproducing system of claim 3 wherein said first
sensing means includes a coil which is mechanically coupled to said
cone and which interacts with an associated magnetic field to
produce an electrical signal which is functionally related to the
axial velocity imparted to said cone by said voice coil.
5. The sound reproducing system of claim 2 wherein said cone
exhibits a velocity versus frequency characteristic which is
substantially approximated by a first straight line having a slope
of -6 db/octave and by a second straight line having a slope of 0
db/octave, said lines intersecting where 0.476 Rf is approximately
equal to 1.5, R being the effective radius of said loudspeaker cone
and f the operating frequency thereof.
6. The sound reproducing system of claim 4 further including:
in association with said voice coil, a first permanent magnet, a
first inner pole piece, and a first outer pole piece, which in
combination form the main electromagnetic structure of said
loudspeaker;
in association with said sensing coil, a second permanent magnet, a
second inner pole piece, and a second outer pole piece, which in
combination form said motional sensing means; and
an appropriately shaped non-magnetic electrically conductive
metallic member which is rigidly attached to said second outer pole
piece and which is located between said second outer pole piece and
said main electromagnetic structure.
7. The sound reproducing system of claim 4 further including:
in association with said voice coil, a first permanent magnet, a
first inner pole piece, and a first outer pole piece, which in
combination form the main electromagnetic structure of said
loudspeaker;
in association with said sensing coil, a second permanent magnet, a
second inner pole piece, and a second outer pole piece, which in
combination form said motional sensing means; and
an appropriately shaped non-magnetic, electrically conductive
metallic member mechanically securing said second inner pole piece
to said first inner pole piece.
8. The sound reproducing system of claim 2 wherein said cone
exhibits a substantially constant acceleration over a low frequency
region and a substantially constant velocity over a high frequency
region, said regions meeting where 0.476 Rf is approximately equal
to 1.5, R being the effective radius of said loudspeaker cone and f
the operating frequency thereof.
9. The sound reproducing system of claim 1 wherein said first sound
energy producing means is a loudspeaker of the electrostatic
type.
10. The sound reproducing system of claim 1 also comprising linear
amplifying means serially interposed between said first sensing
means and said first equalizing means.
11. The sound reproducing system of claim 2 wherein said first
equalizing means exhibits an attenuation versus frequency
characteristic which is substantially approximated by a first
straight line having a slope of +6 db/octave and by a second
straight line having a slope of 0 db/octave, said lines
intersecting where 0.476 Rf is approximately equal to 1.5, R being
the effective radius of said loudspeaker cone and f the operating
frequency thereof.
12. The sound reproducing system of claim 2 wherein said first
equalizing means exhibits an attenuation versus frequency
characteristic which is substantially approximated by a curve which
is concave upward at its lower end and is concave downward at its
upper end with an intervening slope approaching 6 db/octave and an
ultimate slope at the upper end approaching 0 db/octave.
13. The sound reproducing system of claim 1 further comprising:
second amplifying means responsive to said source signal; filtering
means responsive to said second amplifying means; and second sound
energy producing means responsive to said filtering means.
14. The sound reproducing system of claim 1 further comprising a
plurality of identical sound energy producing means which are
responsive to said first amplifying means and which are identical
to said first sound energy producing means.
15. The sound reproducing system of claim 1 further comprising:
second amplifying means jointly responsive to said source signal
and to a second feedback signal;
second sound energy producing means responsive to said second
amplifying means;
second means for sensing the sound energy output of said second
sound energy producing means; and
second equalizing means exhibiting a predetermined nonlinear
attenuation versus frequency characteristic and being responsive to
said second sensing means for providing said second feedback
signal,
whereby said second sound energy producing means radiates a
substantially uniform sound energy output over a frequency range
which does not overlap with said first frequency range.
16. A sound reproducing system comprising:
an amplifier jointly responsive to an input source signal and to a
feedback signal;
a loudspeaker of the moving-coil type including a cone and a voice
coil, said voice coil responding to said amplifier for axially
driving said cone;
means for sensing the axial velocity of said cone; and
an equalizer exhibiting a predetermined nonlinear attenuation
versus frequency characteristic and being responsive to said
sensing means for providing said feedback signal,
whereby said cone exhibits a velocity versus frequency times a
constant characteristic which is substantially approximated by a
first straight line having a slope of -6 db/octave and by a second
straight line having a slope of 0 db/octave, said lines
intersecting where 0.476 Rf is approximately equal to 1.5, R being
the effective radius of said loudspeaker cone and f the operating
frequency thereof.
Description
FIELD OF THE INVENTION
This invention relates to sound reproducing systems and, in
particular, to such systems which include the loudspeaker as part
of the feedback path.
BACKGROUND OF THE INVENTION
Several prior art sound reproducing systems have included the
loudspeaker in a feedback path for reducing loudspeaker distortion,
for extending the loudspeaker's frequency response, and for
allowing the utilization of smaller acoustic enclosures. Such prior
art systems, especially those which include means for magnetically
sensing the axial motion of the associated speaker-cone, have
considered neither the detrimental effects due to electrical
interference from the loudspeaker's main electromagnetic structure
nor the proper frequency shaping of the motional signal to cause
the loudspeaker to respond linearly to the input source signal.
Further, none of these prior art motional feedback systems have
effectively compensated for inherent amplifier gain
limitations.
It is therefore an object of the present invention to utilize
motional feedback in a sound reproducing system for reducing
loudspeaker distortion.
It is another object of this invention to utilize motional feedback
in a sound reproducing system for linearly extending the associated
loudspeaker's frequency response.
It is a further object of this invention to utilize motional
feedback in a sound reproducing system having relatively small
loudspeaker enclosures.
It is still a further object of this invention to reduce electrical
interference applied to the motional sensing means by the
loudspeaker's main electromagnetic structure.
It is a still further object of this invention to utilize a
motional sensing means which is substantially free of electrical
interference from the associated loudspeaker's main electromagnetic
structure.
It is a still further object of this invention to properly shape
the motional signal so that the loudspeaker responds linearly to
the input source signal.
It is a still further object of this invention to compensate for
inherent amplifier gain limitations in a sound reproducing system
which utilizes motional feedback.
SUMMARY OF THE INVENTION
According to the present invention, a sound reproducing system
utilizing motional feedback to reduce loudspeaker distortion
substantially comprises an amplifier which is jointly responsive to
the input source signal and to a feedback signal, a moving-coil
loudspeaker including a main electromagnetic structure which is
responsive to the amplifier's output signal for effecting axial
speaker-cone motion, motional sensing means for providing a signal
which is functionally related to axial cone velocity, and an
equalizer exhibiting a predetermined nonlinear attenuation versus
frequency characteristic and which is responsive to the motional
signal for providing the feedback signal. The feedback signal is
degeneratively applied to the amplifier which, in turn, forces the
loudspeaker to respond linearly to the input source signal and
thereby exhibit a uniform sound energy output.
According to the present invention, the loudspeaker cone exhibits a
velocity versus frequency characteristic which is substantially
approximated by a first straight line having a -6 db/octave slope
and by a second straight line having a 0 db/octave slope, these
lines intersecting where 0.476 Rf .apprxeq. 1.5; R being the
effective speaker-cone radius in inches and f being the operating
frequency in KHz.
According to specific embodiments of this invention, the motional
sensing means is either of the type which is integral with the
loudspeaker structure or of the type which is attached as an
applique to a pre-existing conventional loudspeaker structure.
It is an advantage of the present invention that relatively small
loudspeaker enclosures can be utilized.
It is another advantage of this invention that loudspeaker
diaphragm performance is substantially independent of enclosure
characteristics.
It is a further advantage of this invention that the motional
sensing means can be used in conjunction with conventional
loudspeakers as an applique and in specially built speakers which
include the motional sensing means built integrally therewith.
It is a feature of the present invention that the loudspeaker
produces a substantially uniform sound energy output.
It is another feature of this invention that the motional signal is
properly shaped and thereafter degeneratively applied to the
amplifier to force the loudspeaker to respond linearly to the input
source signal.
It is a further feature of this invention that the loudspeaker cone
exhibits a unique axial-velocity characteristic.
It is a still further feature of this invention that the equalizer
characteristic modifies the loudspeaker's response throughout the
frequency domain.
DESCRIPTION OF THE DRAWING
The above and other objects, advantages, and features of the
present invention will be better appreciated by a consideration of
the following detailed description and the drawing in which:
FIG. 1 illustrates a sound reproducing system according to the
present invention;
FIG. 2 shows an ideal velocity characteristic according to the
present invention whereby the loud-speaker produces a substantially
uniform sound-energy output;
FIG. 3 shows an equalizer attenuation characteristic according to
the present invention used to shape the motional signal to provide
the feedback signal;
FIG. 4A shows actual loudspeaker velocity characteristics for the
cases wherein no feedback is used and wherein feedback is used
according to the present invention, while FIG. 4B shows the
corresponding sound energy output versus frequency curves;
FIG. 5 illustrates an embodiment of a motional sensing means
similar to the one shown in FIG. 1;
FIGS. 6A and 6B illustrate two further embodiments of motional
sensing means which are particularly adaptable to applique
techniques; and
FIGS. 7A, 7B, and 7C illustrate various configurations which
include the present invention.
DETAILED DESCRIPTION
In FIG. 1, there is shown sound reproducing system 10 according to
the present invention generally comprising power amplifier circuit
30, loudspeaker 40, and equalizer circuit 70. Loudspeaker 40, which
in this case is advantageously of the moving-coil type, further
comprises cone or diaphragm 41, main electromagnetic structure 50,
and cone motion sensing means 60. It should be noted, however, that
the teaching of the present invention can be applied to any type of
loudspeaker which includes relative moving parts to produce an
acoustic output; for instance, a loudspeaker of the electrostatic
type can be utilized. In system 10, input signal Es from source 20
is applied via lines 91 and 92 to amplifier 30 while output signal
Ea of amplifier 30 is applied via lines 93 and 94 to loudspeaker
40. In conventional manner, main electromagnetic structure 50
responds to amplifier signal Ea to cause axial motion of cone
41.
According to the present invention, sensing means 60 responds to
the motion of cone 41 near its apex to provide motional signal Ev;
this signal is functionally related to the axial velocity of the
cone and therefore simultaneously indicates the distortion
components radiated thereby. In this case, cone 41 is
advantageously chosen to move substantially as a rigid piston so
that motional signal Ev reflects the true sound energy output
thereof. Motional signal Ev is then applied via lines 95 and 96 to
linear gain amplifier 80 to produce amplified motional signal E
which, in turn, is applied via lines 95' and 96' to equalizer
circuit 70 to produce feedback signal Ef. Equalizer circuit 70
exhibits a predetermined nonlinear attenuation versus frequency
characteristic which will be discussed in detail hereinafter.
Finally, feedback signal Ef is degeneratively applied via lines 97
and 98 to amplifier 30 whereby loudspeaker 40 is forced to respond
linearly to input source signal Es.
In light of the above, the application of feedback signal Ef to the
amplifier 30 causes the cone's motion to be under the exclusive
control of input source signal Es. Therefore, by choosing an
appropriate equalizer characteristic, the speaker's sound energy
output, i.e., its frequency response, can easily be controlled.
Generally, the equalizer characteristic is chosen so that cone 41
exhibits a predetermined axial velocity versus frequency
characteristic, to be further discussed hereinafter, which, in
effect, causes loudspeaker 40 to radiate a substantially uniform
sound energy output. This, of course, reduces the harmful effects
caused by loudspeaker distortion.
In FIG. 2, there is shown according to the present invention the
ideal velocity versus frequency characteristic which cone 41 must
exhibit in order to provide a substantially uniform sound energy
output over the frequency band of interest. In the FIG., relative
axial cone velocity in decibels (dB) is plotted as a function of
parameter p, which is defined by 0.476Rf, where R is the effective
cone radius in inches and f is the operating frequency in KHz. In
conventional manner, effective cone radius is defined as the
maximum distance from the axis of the cone to that part of the rim
that moves essentially as a rigid continuation of the cone. This
ideal velocity characteristic is shown by the solid curve and can
be theoretically defined by a characteristic which is the inverse
of the radiation resistance as expressed theoretically by
well-known relationships involving Bessel functions. This, in part,
explains the oscillatory portions of the curve above p = 1.5. As is
apparent from this FIG., the ideal velocity characteristic can be
approximated by a first straight line having a slope of -6
db/octave and by a second straight line having a slope of 0
db/octave, these two lines intersecting at p .apprxeq. 1.5. In
light thereof, it is apparent that at low frequencies the cone must
exhibit a substantially constant acceleration; whereas at higher
frequencies the cone must exhibit a substantially constant
velocity. As noted above, the nominal breakpoint between the low
and high frequency regions occurs at p = 1.5. This point occurs
where the operating frequency is equal to about three times the
reciprocal of the effective cone radius. From the control
viewpoint, therefore, the cone is forced to exhibit a substantially
constant acceleration at relatively low frequencies and a
substantially constant velocity at relatively high frequencies, the
nominal breakpoint dividing these two frequency regions being
located at p = 1.5.
In FIG. 3, there are shown ideal and actual equalizer
characteristics which force speaker 40, when respectively driven by
ideal and practicable amplifiers, to substantially exhibit the
ideal velocity characteristic of FIG. 2. In FIG. 3, attenuation in
dB is plotted as a function of p = 0.476 Rf, as defined before. In
the region where p is less than 1.5, the ideal characteristic is
shown by the dashed line while the actual characteristic is shown
by the solid curve. Above p = 1.5, the actual and ideal
characteristics are substantially coincident; therefore, only the
actual characteristic is shown. It is apparent that the ideal
characteristic is approximated by a first straight line having a +6
dB/octave slope and by a second straight line having a 0 dB/octave
slope, the two lines intersecting where p = 1.5. According to the
present invention, the actual attenuation characteristic deviates
from the ideal characteristic, especially at low frequencies, to
provide a relatively lower attenuation effect. In other words, the
actual characteristic comprises a concave upward portion over the
low frequency region at its left end and a nearly +6 dB/octave
slope at its center; the actual characteristic further comprises a
concave downward portion over the higher frequencies which
asymptotically approaches a 0 dB/octave slope at its right end. The
actual characteristic is necessary in order to compensate for
amplifier feedback-gain limitations at low frequencies and for air
loading of the diaphragm at high frequencies. Otherwise, if
equalizer 70 provided the ideal characteristic at low frequencies,
instead of the actual characteristic, then amplifier 30 would have
to provide exceedingly high gain at these low frequencies. It
appears that the ideal equalizer characteristic is a reflected
image of the ideal velocity characteristic while the actual
equalizer characteristic is a distorted reflected image of the
ideal velocity characteristic.
As is shown in FIG. 1, equalizer circuit 70 includes resistors R71
and R72 and capacitors C73 and C74. Elements R71 and C73 in
combination substantially provide the ideal characteristic of FIG.
3 at low and high frequencies; however, elements R72 and C74 in
combination are added to modify the otherwise ideal characteristic
at the low frequency end thereby yielding the actual characteristic
(solid curve) of FIG. 3. In this particular embodiment, the values
of R71, R72, C73 and C74 are 470 ohms, 33 Kohms, 0.25 uf and 0.1
uf, respectively.
In FIG. 4A, there are shown actual cone velocity curves A and B for
system 10 of FIG. 1, wherein speaker 40 has an effective cone
radius of 3 inches and the associated enclosure has a volume of 1.2
cubic feet. Curve A corresponds to the case without motional
feedback, while curve B corresponds to the case including motional
feedback according to the present invention, the above-described
equalizer circuit being utilized. Again, the intersecting straight
line approximation to the ideal velocity curve is shown. It is
apparent that above p = 1.5 (corresponding to f = 1.1 KHz) there
occurs a substantially constant velocity region; whereas below p =
1.5 there occurs a substantially constant acceleration region, as
explained before. It is apparent that the natural speaker-cone
resonance has been reduced from its initial value of 100 Hz for the
case without feedback to a value of approximately 25 Hz for the
case with feedback. If necessary, more elaborate equalizer
circuitry than that used herein could be utilized to yield a closer
approximation to the ideal velocity curve, as will be apparent to
those skilled in the art.
In FIG. 4B, conventional sound amplitude or energy versus frequency
curves corresponding to the velocity curves of FIG. 4A are shown,
curves A' and B', respectively, corresponding to curves A and B of
FIG. 4A. The present curves are derived from the previous curves by
using well known acousto-mechanical relationships, such as
discussed in standard texts on direct radiator loudspeakers. From
FIG. 4B, it is apparent that the overall speaker frequency response
has been substantially improved, especially at the lower and upper
ends of the applicable frequency spectrum. It is therefore apparent
that a small diameter speaker with feedback can replace a large
diameter speaker without feedback and yet provide similar acoustic
performance. Similar reasoning applies to replacing a large number
of required identical speakers not utilizing feedback with a
smaller number of identical speakers which utilize feedback and
still provide comparable performance.
Reference again to FIG. 1 shows power amplifier circuit 30 further
includes high-gain operational amplifier 31 having its
non-inverting and inverting input terminals, respectively,
connected to line 91 and, via resistor R32, to line 97. The
non-inverting input terminal receives input source signal Es and is
connected via resistor R33 to ground 99. The output terminal of
amplifier 31 is connected via resistor R36 to the base of
transistor T38. Capacitor C37 is connected between the base of T38
and ground 99 to improve stability with feedback. Push-pull output
stage 102, including transistors T103 and T104, is driven by the
emitter of transistor T38. The emitters of this output stage are
jointly connected to output line 93. Diode D105 couples the bases
of transistors T103 and T104 in order to compensate for any
inherent joint base-emitter offset voltage. This minimizes breaks
in signal continuity in amplifier output signal Ea when both these
transistors are off during periods of input signal transitions.
Resistor R106, which connects the base of transistor T104 to
negative voltage supply -V, provides for maximum base current and
thus for peak signal drive in the negative direction.
In FIG. 1, net internal feedback is provided via resistors R107 and
R32. The latter is in series with the output impedance of equalizer
circuit 70, which is relatively small. Feedback via resistor R107
provides dc feedback which minimizes dc offset in the output of
amplifier 30 so that the rest position of voice coil 55 is
displaced as little as possible. The ac part of the feedback via
R107 is combined with the equalized motional feedback component Ef
from equalizer 70 via R32 to yield a net ac negative feedback
component to the inverting input of operational amplifier 31. In
this case, for ac control of speaker 40, the ac motional feedback
component should predominate. Distortion normally inherent to
amplifier circuit 30 is substantially reduced, since the amplifier,
in response to feedback signal Ef on line 97, is forced to exhibit
a gain versus frequency characteristic which causes loudspeaker 40
to respond linearly to input source signal Es. Regardless of the
particular design of amplifier circuit 30, the fact that main
voice-coil 55 is a current sensitive device should always be taken
into account.
As mentioned before, loudspeaker 40 includes cone 41, main
electromagnetic structure 50, and motional sensing means 60, which,
in this embodiment, is built integrally with the speaker. In FIG.
1, the horizontal direction corresponds to the speaker's axial
direction. Main electromagnetic structure 50, which is rigidly
secured to speaker frame or housing or chassis 42, further includes
cup-shaped iron outer pole piece 51, solid cylindrical permanent
magnet 52, solid cylindrical iron inner pole piece 53, and main
voice coil 55 would on the rearward end of substantially rigid,
thin cylindrical bobbin 54. Bobbin 54, in turn, is mechanically
connected to the rearward end or apex portion of cone 41. In this
particular embodiment, the axial length of main voice coil 55 is
advantageously made substantially longer than the axial length of
its associated magnetic air gap in order to assure substantially
uniform coil reaction in the gap field over large excursions. This
field's path is shown by the arrows. In conventional manner,
structure 50 responds to amplifier output signal Ea to cause axial
motion of main voice coil 55, the opposite ends of the coil being
connected to lines 93 and 94. This, in turn, causes axial motion of
cone 41 via bobbin 54. Axial motion of the cone is stabilized and
radial motion thereof is suppressed by flexible centering
suspensions or webs 43 and 44 (compliant annulli). The rearward and
forward portions of cone 41 are rigidly attached to the inner
peripheries of webs 44 and 43, respectively, while the outer
periphery of each web is secured to frame 42. In this embodiment,
line 94 is grounded, thereby making amplifier circuit 30 direct
current coupled to loudspeaker 40.
Reference again to FIG. 1 shows motional sensing means 60 further
includes cup-shaped iron outer pole piece 61, ring 64, solid
cylindrical permanent magnet 62, solid cylindrical iron inner pole
piece 63, solid cylindrical member 66, and feedback coil 65 wound
on the forward end of bobbin 54. Ring 64 and member 66 are
advantageously made of a nonmagnetic metal of good electrical
conductivity such as copper, aluminum, etc., to minimize leakage
interference elements 64 and 66 are shown in detail in FIG. 5. The
electrical conductivity of member 66 causes the production of eddy
currents which tend to counteract and substantially compensate for
the interfering electrical fields produced by main electromagnetic
structure 50. Further electrical interference compensation is
effected by short circuiting ring 64. (It is apparent that member
66 mechanically interconnects inner pole piece 53 of main
electromagnetic structure 50 and inner pole piece 63 of motional
sensing means 60.) The inner diameter of ring 64 is advantageously
made as small as possible without interfering with the motion of
feedback coil 65. In other words, ring 64 and member 66 assure
electrical isolation between structure 50 and sensing means 60. As
before, the axial length of feedback coil 65 is advantageously made
substantially longer than the axial length of its associated gap in
order to assure that the coil always moves within a substantially
uniform magnetic flux. It is apparent, however, that while main
voice coil 55 and feedback coil 65 are mechanically coupled via
common bobbin 54, they are electrically independent of each other.
This is due in part to the presence of ring 64 and member 66.
Therefore, the axial motion imparted to cone 41 by structure 50 is
also imparted to feedback coil 65. In turn, feedback coil 65
interacts with the magnetic field produced by associated magnet 62
to produce motional signal Ev. This signal is produced across the
opposite ends of feedback coil 65 which connect to lines 95 and 96,
line 96 being grounded in this embodiment. In a manner similar to
that discussed above, the flux within the gap is produced by pole
pieces 61 and 63. Again, the path of the associated magnetic flux
is shown by the dashed line. According to the present invention,
elements 61, 62, 63, 64, 66, 53, 52, and 51 can advantageously be
secured by adhesive means.
Motional signal Ev, which is functionally related to the axial
velocity of cone 41 and which indicates the sound energy output of
loudspeaker 40, is applied via input resistor R81 to the
non-inverting input of operational amplifier 81. Amplifier circuit
80 is inserted into the feedback path in order to linearly
compensate for the low level of Ev and the attenuating effect
occuring thereafter at equalizer circuit 70. This, of course,
guarantees the application of reasonable feedback signal levels to
amplifier 30. In addition, this allows the use of finer wire and
fewer turns in the construction of feedback coil 65 and minimizes
the magnitude of the required magnetic flux to be provided by
magnet 62. The inverting input of operational amplifier 81 is
grounded via resistor R82 while negative feedback is applied to the
amplifier via resistor R83. The operation of amplifier circuit 80
is well understood by those skilled in the art.
Amplified signal E, which is linearly proportional to motional
signal Ev, is now applied via lines 95' and 96' to equalizer
circuit 70, line 96' being connected to ground 99. Thereafter,
output signal Ef of equalizer circuit 70 is degeneratively applied
to amplifier circuit 30; in this embodiment, the feedback signal is
applied to the inverting input of operational amplifier 31 via
resistor R32. Thus, amplified motional signal E is shaped by
equalizer 70 to yield feedback signal Ef, which, in turn, forces
loudspeaker 40 to exhibit the ideal velocity characteristic of FIG.
2.
In reference now to FIG. 5, there is shown another embodiment of a
motional sensing means which is built integrally with its
associated speaker. In this case, the permanent magnet is one-half
of a hollow toroid rather than a solid cylinder while the outer
pole piece is a hollow cylinder rather than cup-shaped. Again, ring
64 and member 66 are included to electrically isolate the feedback
coil from the speaker's main electromagnetic structure. Herein, the
magnet's inner and outer poles are circular annulli of
approximately equal cross-sectional area.
In FIGS. 6B and 6A, there are respectively shown motional sensing
means which are similar to those already discussed with reference
to FIGS. 1 and 5 but which are adapted to be attached to a
pre-existing conventional speaker as an applique. In these
structures, the connecting members are advantageously made of two
portions; one portion being initially secured to the pre-existing
inner pole piece of the speaker's main electromagnetic structure by
adhesive means while the second portion and the remaining elements
of the motional sensing means are thereafter attached to the first
portion via an associated threaded member. The whole structure is
ultimately secured via a locknut. In addition to having a
connecting member which includes two connecting sections, the
present structures include a separate bobbin piece which is
centered via its own associated web. The connection of the new
bobbin piece to the pre-existing cone and the mechanical connection
of the elements required in the applique device will not be further
discussed herein.
With reference to multiple enclosure sound reproducing systems, it
is apparent that the main electromagnetic loudspeaker structure's
varying characteristics may result in non-uniform response of their
associated cones. The utilization of motional feedback according to
the present invention reduces these differences in resulting cone
motion. Therefore, by carefully controlling the manufacture of the
motional sensing means and the associated equalizer circuit within
reasonable bounds, performance differences from enclosure to
enclosure, which would otherwise occur, are substantially
eliminated.
The broad advantages and features of a sound reproducing system
according to the present invention have already been discussed. As
may be seen in FIG. 7A, one specific application of the invention
is shown wherein common power amplifier circuit 30 responds to
source 20 to drive a plurality of identical loudspeakers 40, 40',
and 40" having identical frequency amplitude and distortion
characteristics. However, only speaker 40 contains a motional
sensing means 60 which provides a motional signal to its associated
equalizer 70. The equalizer, of course, provides the feedback
signal to amplifier 30. This configuration is inherently economical
since only one feedback loop, providing the necessary compensation,
is required to drive more than one speaker. Another specific
application is shown in FIG. 7B wherein amplifier 30 responds to
source 20 to drive speaker 40. Associated with speaker 40 are
motional sensing means 60 and equalizer 70 which in combination
provide a feedback signal to the amplifier. Also included is
separate and independent amplifier 30' which responds to source 20
to drive speaker 40' via filter 110'. In this case, motional
feedback is utilized to drive speaker 40 over a first frequency
range while speaker 40' is driven over a complementary frequency
range which is determined by filter 110'. Speakers 40 and 40' are
not necessarily identical. Finally, in FIG. 7C, two independent
motional feedback circuits are shown. In the first circuit,
amplifier 30 responds to source 20 via filter 110 to drive speaker
40. Speaker 40 further includes motional sensing means 60. As
before, equalizer 70 provides a feedback signal to amplifier 30. In
the second circuit, amplifier 30' responds to source 20 via filter
110' to drive speaker 40'. Associated with speaker 40' are motional
sensing means 60' and equalizer 70' which, in combination, provide
a feedback signal to the amplifier. In this case, filters 110 and
110' are chosen so that speakers 40 and 40' operate in
complementary or nonoverlapping frequency regions. In the above and
other examples which utilize motional feedback according to the
present invention, speakers capable of large cone excursions, such
as that described by R. T. Bozak in his U.S. Pat. No. 3,436,494 are
recommended.
In a sound reproducing system according to the present invention,
as in any conventional sound reproducing system of the prior art,
the required loudspeakers are chosen to fit the particular
application. In other words, the loudspeaker should be
substantially capable of performing at low or high frequencies, as
the case may be. For instance, the loudspeaker should initially be
chosen of the appropriate cone size so that rigid cone motion
results in the frequency range of interest. This, of course,
results in improved loudspeaker performance, especially at the low
frequencies. The loudspeaker is also chosen so that high frequency
beaming and low frequency Doppler distortion are minimal. Further,
well known enclosure design and multispeaker system design
techniques should be considered. These considerations, of course,
will be apparent to those skilled in the art inasmuch as such
considerations need be taken into account in the design of almost
any sound reproducing system and not just in the design of a sound
reproducing system according to the present invention.
While the arrangement according to the present invention utilizing
motional feedback in a sound reproducing system has been described
in terms of specific embodiments, it will be apparent to those
skilled in the art that many modifications are possible within the
spirit and scope of the disclosed principle.
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