U.S. patent number 3,786,202 [Application Number 05/242,501] was granted by the patent office on 1974-01-15 for acoustic transducer including piezoelectric driving element.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Hugo W. Schafft.
United States Patent |
3,786,202 |
Schafft |
January 15, 1974 |
ACOUSTIC TRANSDUCER INCLUDING PIEZOELECTRIC DRIVING ELEMENT
Abstract
A disk shaped piezoelectric element constructed to operate in a
planar mode so as to define a first overtone nodal ring on one of
the major surfaces, a conically shaped diaphragm having a truncated
apex defining a generally circular area affixed to a major surface
of the element concentric with the nodal ring and spaced radially
therefrom so as to reduce the amplitude of the output of the first
overtone to approximately the amplitude of the output of the
fundamental frequency, and a rubber disk affixed to the opposite
major surface of the piezoelectric element to lower the fundamental
resonance frequency and damp the peak output of the fundamental and
first overtone resonance frequencies to provide a flat response
over a desired bandwidth.
Inventors: |
Schafft; Hugo W. (Des Plaines,
IL) |
Assignee: |
Motorola, Inc. (Franklin Park,
IL)
|
Family
ID: |
22915023 |
Appl.
No.: |
05/242,501 |
Filed: |
April 10, 1972 |
Current U.S.
Class: |
310/324; 181/166;
310/330; 381/354; 310/326; 310/334; 381/190 |
Current CPC
Class: |
H04R
17/00 (20130101) |
Current International
Class: |
H04R
17/00 (20060101); H04r 017/00 () |
Field of
Search: |
;179/11A ;181/32R,32A
;310/8.6,8.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cooper; William C.
Assistant Examiner: Olms; Douglas W.
Attorney, Agent or Firm: Parsons; Eugene A.
Claims
I claim:
1. Apparatus providing conversions between electrical and
mechanical stimuli comprising:
a. a piezoelectric element having a generally flat major surface,
electrodes attached to said element for driving said element in a
planar bending mode when said electrodes are properly energized,
and first overtone nodal lines present on said major surface during
planar bending mode operation of said element;
b. a generally conically shaped diaphragm having a truncated apex
defining a generally circular area with a diameter larger than a
point and sufficiently less than the distance between nodes of the
first overtone to reduce the amplitude of the output of the first
overtone to approximately the amplitude of the output of the
fundamental; and
c. means fixedly attaching the truncated apex of said diaphragm to
said piezoelectric element with the circular area defined by said
apex generally encircled by and substantially centered within the
first overtone nodal lines.
2. Apparatus as set forth in claim 1 wherein the piezoelectric
element is generally disk shaped and the nodal line for the first
overtone defines a generally circular area concentric with the
truncated apex of the diaphragm.
3. Apparatus as set forth in claim 1 wherein the piezoelectric
element includes two piezoelectric wafers affixed together in
parallel contiguous relationship with electrodes on each side of
each wafer.
4. Apparatus as set forth in claim 1 having in addition a resilient
damping member affixed to the piezoelectric element on the side
opposite the diaphragm for lowering the fundamental resonance
frequency and damping the resonance peak thereof to extend the
range of the speaker to lower frequencies and to provide a relative
flat response over the entire range.
5. Apparatus as set forth in claim 4 wherein the damping member is
formed from a material including rubber.
6. Apparatus as set forth in claim 5 wherein the rubber has a glass
transition region approximately including the frequency of the
first overtone.
7. Apparatus as set forth in claim 5 wherein the rubber includes
neoprene.
8. Apparatus as set forth in claim 5 wherein the damping member
includes particles of a relatively heavy material intermixed with
the rubber for providing frictional damping at the higher
frequencies of operation.
9. Apparatus as set forth in claim 8 wherein the particles include
lead of approximately 100 mesh size and three parts by weight of
lead to one part by weight of rubber.
10. Apparatus as set forth in claim 8 wherein the damping member
further includes particles of dry lubricant intermixed with the
particles of relatively heavy material for increasing relative
motion between the particles of relatively heavy material and the
rubber at the higher frequencies of operation.
11. Apparatus as set forth in claim 4 wherein the damping member
extends outwardly beyond the edges of the piezoelectric element for
further damping the lower frequencies of operation.
12. Apparatus as set forth in claim 4 wherein the combined mass of
the piezoelectric element and the damping member is substantially
heavier than the mass of the diaphragm.
13. An improved acoustic transducer comprising:
a. a housing defining a cavity therein;
b. a generally disk shaped piezoelectric element having opposed
generally flat major surfaces and defining thereon a nodal ring for
a first overtone frequency, said element having electrodes attached
thereto for driving said element in a planar bending mode when said
electrodes are properly energized;
c. a generally conically shaped diaphragm having a truncated apex
defining a generally circular area with a diameter larger than a
point and sufficiently different from the diameter of the first
overtone nodal ring to reduce the amplitude of the output of the
first overtone to approximately the amplitude of the output of the
fundamental;
d. means fixedly attaching the truncated apex of said diaphragm to
one of said major surfaces of said piezoelectric element with the
circular area defined by said apex generally concentric with the
first overtone nodal ring; and
e. means operatively mounting said diaphragm within the cavity of
said housing.
14. An improved acoustic transducer as set forth in claim 13 having
in addition a generally disk shaped resilient damping member
affixed to the major surface of said piezoelectric element opposite
the major surface having the diaphragm affixed thereto.
15. An improved acoustic transducer as set forth in claim 14 having
in addition acoustic absorbing material positioned in the cavity of
said housing generally between said housing and said diaphragm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Acoustic transducers are utilized to convert electrical energy to
sound or acoustic energy to electrical energy. In the present
embodiment a piezolectric element is utilized for a driver, which
piezoelectric element bends or warps in a particular mode in
response to electrical energy being applied thereacross or produces
electrical energy thereacross in response to bending or warping
thereof. While the present invention might be utilized in acoustic
transducers for converting mechanical energy to electrical energy,
it is especially useful in acoustic transducers, such as speakers
and the like, converting electrical energy to acoustic energy or
sound. In the speaker art, it is highly desirable to provide a
transducer with a flat response over a desired bandwidth, i.e., all
frequencies of sound between two desired frequencies are produced
at approximately equal amplitudes. Because piezoelectric elements
are mechanical vibrating devices, they have specific resonant
frequency points, referred to as the fundamental, first overtone,
second overtone, etc., at which points the amplitude of the output
is substantially increased.
2. Description of the Prior Art
In prior art acoustic transducers, any flattening of the frequency
response is accomplished through design of the electronic circuitry
driving the transducer or, in some instances and for certain
frequencies, may be accomplished to some extent through design of
the housing and size of components. In general, these solutions are
unsatisfactory because they are limited in scope and extent.
Further, for these solutions to have noticeable results the design
becomes extremely complicated and expensive and may apply only to a
specific transducer.
SUMMARY OF THE INVENTION
The present invention pertains to apparatus providing conversion
between electrical and mechanical stimuli including a piezoelectric
element constructed to operate in a planar mode and having first
overtone nodal lines present on a major surface, a generally
conically shaped diaphragm with truncated apex defining a circular
area affixed to the major surface of the element approximately
centrally within the first overtone nodal line so as to be spaced
from the line sufficiently to reduce the amplitude of the output of
the first overtone to approximately the amplitude of the output of
the fundamental, and a resilient damping member affixed to an
opposed major surface of the piezoelectric element to lower the
fundamental frequency of the element and damp the fundamental and
first overtone peaks.
It is an object of the present invention to provide an improved
piezoelectric driven acoustic transducer.
It is a further object of the present invention to provide a
piezoelectric transducer having a substantially flat response over
a desired bandwidth.
These and other objects of this invention will become apparent to
those skilled in the art upon consideration of the accompanying
specification, claims and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings, wherein like characters indicate like
parts throughout the figures:
FIG. 1 is an axial sectional view of an acoustic transducer
constructed in accordance with the present invention;
FIG. 2 is a view (a) in top plan of a piezoelectric driver
illustrating nodal rings, (b) in side elevation of the driver
illustrating the fundamental nodal ring, (c) in side elevation of
the driver illustrating first overtone nodal rings, and (d) the
truncated apex of a diaphragm;
FIG. 3 is a graph illustrating generally the frequency response of
a transducer similar to that illustrated in FIG. 1 constructed in
accordance with prior art techniques and, in dotted lines, the
frequency response of a transducer similar to that illustrated in
FIG. 1 constructed in accordance with the present invention;
FIG. 4 is an enlarged sectional view of a piezoelectric driving
element and a resilient damping member affixed thereto, portions
thereof removed; and
FIG. 5 is a greatly enlarged fragmentary view of the resilient
damping member.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the figures the numeral 10 generally designates a piezoelectric
transducer utilized for providing conversions between electrical
and mechanical stimuli. Applications for transducers of this type
are speakers, sound sensors, etc. The transducer 10 includes a
housing 11 defining a generally cup shaped cavity 12, a generally
conically shaped diaphragm 13 affixed within the cavity 12 by its
outermost edges and having a truncated apex to which is attached a
piezoelectric driver 15 with a piezoelectric element 16 and damping
member 17. The operation of piezoelectric transducers wherein a
piezoelectric driver is connected directly to a diaphragm and
solely supported thereby is described in detail in my U. S. Pat.
No. 3,548,116, entitled "Acoustic Transducer Including
Piezoelectric Wafer Solely Supported By A Diaphragm," and issued
Dec. 15, 1970.
In the prior art structure of the above described patent the apex
of the conically shaped diaphragm is fixedly attached to the center
of a piezoelectric element and the frequency response of the
transducer is approximated by the solid line graph in FIG. 3. The
frequencies specified in FIG. 3 are exemplary and may vary somewhat
with variations in transducers. A first output peak in the response
at approximately 1 KC, designated 20, is produced primarily by a
center supported resonance of the driver at 1 KC (a resonance
caused by mounting the driver at the center with a relatively stiff
diaphragm) which is coupled to the diaphragm by a slight axial
movement of the diaphragm and driver, at the point of connection
therebetween, which axial movement is present because the diaphragm
is not rigid. A second output peak at approximately 5 KC,
designated 21, is produced primarily by the fundamental resonance
of the piezoelectric element 16. A third output peak at
approximately 19 KC, designated 22, is produced primarily by the
first overtone resonance of the piezoelectric element 16.
Referring to FIG. 2, the piezoelectric element 16 is illustrated in
top plan in (a) and in side elevation in (b) and (c). The
piezoelectric element 16, as illustrated in FIG. 2, is disk shaped,
but it should be understood that substantially any flat shape might
be utilized wherein the element operates in a planar bending mode,
i.e., flexing or distorting along more than one axis. For example,
the element 16 might be square or even irregularly shaped. However,
in the present embodiment and to simplify the explanation, a disk
shaped element 16 will be described and the operation thereof
explained.
At the fundamental resonance frequency, the element 16 vibrates
along each diameter thereof as illustrated in FIG. 2b. During the
first half cycle of each vibration the center of the element 16
flexes upwardly while the end portions turn downwardly as indicated
by the dotted line 25 and during the second half cycle of each
vibration the reverse occurs, as illustrated by the dotted line 26.
It will be noted that two nodes are formed along the diameter, at
which points no axial movement of the element 16 occurs. Because
every diameter of the element 16 reacts, at the fundamental
resonance frequency, as illustrated in FIG. 2b, the nodes define a
nodal ring 27 on the upper and lower major surfaces of the element
16, which ring 27 is a locus of nodes or points having no axial
movement at the fundamental resonance frequency.
In a similar fashion FIG. 2c illustrates movement of the element 16
at the first overtone resonance frequency. Because of the first
overtone being a higher frequency, two concentric nodal rings 28
and 29 are defined on the major surfaces of the element 16. The
nodal ring 29 is concentric with the nodal ring 27 defined by the
fundamental frequency and spaced radially inwardly therefrom. It
will be noted from a comparison of FIG. 2b and c that the output,
or amount of axial movement, of the element 16 at the fundamental
resonance frequency is substantially constant throughout the area
encircled by the nodal ring 29. Further, since the amount of
movement of the element 15 determines the amount of movement of the
diaphragm 13, and hence, the output (or conversion between
stimuli), connecting the diaphragm at a point concentric to the
nodal rings 27, 28 and 29, as in the prior art, will provide the
maximum output of all frequencies and result in the frequency
response illustrated in full lines in FIG. 3.
In FIG. 2d a portion of the diaphragm 13 is illustrated with the
apex thereof truncated to define a generally circular area having a
diameter smaller than the diameter of the nodal ring 29. If the
diameter of the truncated apex of diaphragm 13 is equal to the
diameter of the nodal ring 29 the first overtone will be
substantially eliminated, since the output or axial movement of the
element 16 at the nodal ring 29 for the first overtone is zero.
Thus, by forming the diaphragm 13 so that the truncated apex has a
diameter greater than a point and less than the diameter of the
nodal ring 29, the output of the first overtone (peak 22 in FIG. 3)
can be diminished to approximately the amplitude of the fundamental
(peak 21 in FIG. 3). Since the second overtone is substantially
above the first overtone and beyond the desired frequency response
in acoustic transducers, it is not necessary to consider overtones
beyond the first.
Referring more specifically to FIG. 4, the piezoelectric driver 15
is illustrated in enlarged cross section, with a portion thereof
removed. The piezoelectric element 16 includes first and second
piezoelectric wafers 40 and 41 having an electrode 42 sandwiched
therebetween and electrodes 43 and 44 fixedly engaged in overlying
relationship on opposed major surfaces thereof. The operation of
the element 16 is well known to those skilled in the art and, as
previously mentioned, is described in detail in U. S. Pat. No.
3,548,116. It is, therefore, sufficient to state at this time that
the electrodes 42, 43 and 44 drive the element 16 in a planar mode
of operation. The element 16 has resilient damping member 17
affixed to the major surface thereof opposite the major surface
having the diaphragm 13 affixed thereto. In the present embodiment,
since the element 16 is generally disk shaped, the damping member
17 is also disk shaped, and as illustrated, has a slightly larger
diameter than the element 16. The damping member 17 damps or loads
the movement of the element 16 to reduce the fundamental peak 21
and to further reduce the first overtone peak 22 so that the
frequency response of the driver 15 approaches the dotted line
curve 50 in FIG. 3.
The damping member 17 is formed of a resilient material, such as
rubber (the term rubber being understood to include natural and
synthetic materials) or other elastomers. Elastomers generally have
a frequency dependent shear modulus which varies directly with the
frequency of stresses applied thereto, i.e., the shear modulus
increases with the frequency of stresses applied thereto. At static
or slowly occurring stresses the elastomeric material operates in a
rubbery region in which it appears elastic to the forces operating
upon it. However, as higher frequency dynamic stresses are applied
the shear modulus is increased and the elastomer goes through a
glassy transition region into a glassy region where it appears
metallic or hard. At the lower frequencies, around the peaks 20 and
21 of FIG. 3, the damping member 45 is preferably operating in the
glassy transition region and introduces hysteresis losses which
substantially remove the peaks 20 and 21. However, at the upper
frequencies, around the peak 22, the material of the damping member
17 may begin to approach the glassy region and the hysteresis
losses are substantially reduced. Thus, the damping effect of the
member 17 at the peak 22 is greatly reduced.
To compensate for the reduction in hysteresis losses small
particles 46 of a relatively heavy material, such as iron or lead,
are intermixed with the resilient material of the damping member 17
during the formation of the disk. These particles 46 add additional
weight to the member 17 and introduce a coulomb type damping which
is caused by internal friction between the metal particles and
their enclosing rubber walls. This internal friction is caused by a
relative movement due to a difference in inertia of the heavy
particles and the surrounding resilient material of the damping
member 17. This frictional or coulomb type damping increases with
the number of particles and the size of the particles. It has been
found, for example, that lead particles having approximately a 100
mesh size mixed with rubber in a 3 to 1 ratio, by weight, provide a
desired amount of damping for the frequency response illustrated in
FIG. 3. Small amounts of a lubricant, such as graphite, may also be
added to the damping member 17, as illustrated in FIG. 5 by the
numeral 47, to increase the relative movement between the heavy
particles 46 and the elastomeric or rubber material and, therefore,
increase the damping action.
The effect of the reduction in hysteresis losses at the higher
frequencies can also be reduced or eliminated by selecting an
elastomeric material with a glass transition region above the
highest frequency desired for the frequency response of the
transducer 10. It has been found for example that neoprene has a
relatively high glass transition region and may, in many instances,
provide sufficient damping at high frequencies so that the addition
of particles 46 is not required. It should be understood that the
type of materials utilized and the frequency response desired
dictate the ultimate construction of the transducer 10.
In addition to providing the damping function described above, the
damping member 17 increases the mass of the driver 15 and,
therefore, improves the weight ratio of the driver 15 over the
diaphragm 13. This improved weight ratio results in a tighter
coupling between the driver 15 and the diaphragm 13 at the lower
frequencies. Thus, in some instances, although the material
selected for the damping member 17 provides sufficient damping at
the higher frequencies, it may be desirable to add relatively heavy
particles 46 to the member 17 to increase the mass of the driver
15.
The addition of the damping member 17 to the driver 15 (and the
diaphragm 13 to a much smaller extent) lower the fundamental
resonance frequency of the driver 15. Referring to FIG. 3, it can
be seen that the knee 51 of the flattened response curve 50
(illustrated in dotted lines) occurs slightly below the peak 21 for
the fundamental resonance frequency. The diameter and thickness of
the damping member 17 should be adjusted to lower the resonance
frequency of the combined piezoelectric element 16 and damping
member 17 to a point below the fundamental resonance frequency of
the piezoelectric element 16 (peak 21) such that the curve 50 falls
away sharply at the lower frequencies (as illustrated in FIG. 3).
If the resonance frequency of the driver 15 is too high it will add
to the peak 21 and produce too high an output at the lower
frequencies while not extending the frequency response sufficiently
into the lower frequency range. If the resonance frequency of the
driver 15 is lowered too much the curve 50 will not fall away
sharply at the lower frequencies but will rise at a much lower
rate. Thus, through careful selection of the diameter thickness and
mass of the damping member 17 the desired flat response of the
transducer 10 can be extended somewhat into the lower frequency
range. Further, through careful selection of the type and thickness
of material and the amount and particle size of particles 46 the
degree of damping can be adjusted to provide a substantially flat
response over a desired band of frequencies. It should be noted
that the damping member 17 might be constructed with an annular
configuration, in which case high frequency damping is controlled
by adjusting the size of the inside diameter, since high frequency
damping is most effective in the center of the driver 15.
The cavity 12 in the housing 11 has a resonant frequency which, in
many instances appears in the desired frequency response of the
transducer 10. At the cavity resonance there is a tendency to
absorb output power from the transducer 10 and, thus, a notch (not
shown) will appear in the output curve 50 of FIG. 3. To prevent the
power output loss and the resulting distortions, acoustic absorbing
material, in the present embodiment an annularly shaped member 55
of foam rubber or the like, is placed in the cavity 12 between the
housing 11 and the diaphragm 13. The member 55 alters the cavity
resonance, or lowers the Q of the cavity 12, to substantially
eliminate the absorbing of power and consequent notch in the output
curve 50. It should be understood that the acoustic absorbing
material is only utilized when the cavity resonance falls within
the desired frequency response and, in some instances, it may be
possible to eliminate the material through design of the transducer
components.
Thus, an improved piezoelectric transducer is disclosed having an
output which is substantially flat throughout a desired band of
frequencies. The transducer output has been referred to throughout
the specification and it should be understood that this refers to
either mechanical or electrical output in response to either
electrical or mechanical input, respectively. Further, in addition
to a flat response and improved coupling at the lower frequencies,
the critically damped element 16 has an improved transient response
over known electrodynamic drive systems for transducers. While I
have shown and described a specific embodiment of this invention,
further modifications and improvements will occur to those skilled
in the art. I desire it to be understood, therefore, that this
invention is not limited to the particular form shown and I intend
in the appended claims to cover all modifications which do not
depart from the spirit and scope of this invention.
* * * * *