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Патент USA US3100826

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Aug-13, 1945sv
F. SWINEHART my.
"
3,100,821
HIGH SENSITIVITY PIEZOELECTRIC MICROPHONE
Filved March a, ‘1961
J23
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27
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INKENTORS
?-amé d‘arme?arlf
81,2040,‘ v/dores Mas
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ATTORNEYS
3,10,821
United States Patent 0” 1C6
Patented Aug. 13, 1963"
1
2
3,100,821
.
ing 4 to 6 db’above microphones employing the- two
point suspension, while using a shorter element than previously, and without suffering adverse effects upon the
'
HIGH SENSITIVITY PIEZOELECC
MICROPHONE
,
,
frequency response.
Frank Swinehart and Robert .1ames Dohias, Cedar Rapids,
Iowa, assignors to The Turner Company, Cedar Rapids,
Iowa, a corporation of Iowa
‘
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' Filed Mar. 6, 1961, Ser. No. 93,754
'
1 Claim.
4
A further description of the present invention will be
made in conjunction with the attached sheet of drawings
which illustrates a preferred embodiment thereof.
(Cl. 179-110)
.
In the drawings:
‘
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FIGURE 1 is a plan view of the microphone assem
The present invention relates to improvements in 10. bly, with the diaphragm removed to illustrate the in
piezoelectric ,type microphones for the purpose of im
terior construction thereof;
proving the voltage output of such microphones, and in
FIGURE 2 is- a cross-sectional view of the microphone
assembly with the diaphragm in place;
improving their frequency response.
'
FIGURE 3 is a circuit diagram illustrating the equiva
While the present invention has applicability to all
types of piezoelectric microphone assemblies, it ?nds
lent electrical circuit for piezoelectric type microphones;
particular utility in conjunction with ceramic elements.‘
and
Such elements have desirable characteristics when em
ployed in piezoelectric type microphones, but, in general, .
their voltage output is signi?cantly lower than that ob
tainable from the use of other piezoelectric elements such
as Rochelle salt crystals.
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As shown in the drawings:
In FIGURE 1, reference numeral 10 indicates general
v1y a thin‘ walled housing having an annular ?ange por
tion 11, a cylindrical wall portion 12, and a circular base
-
A typical ceramic element for use in microphone as
semblies consists of an elongated body of ceramicjma
terial having opposed faces covered with silver or other
electrical conductors. The element is‘ actuated by cou
"
FIGURE 4 is a plan view of the support element em
ployed in the structure of FIGURES 1 and 2.
portion 13 as illustrated in FIGURE 2.
A ceramic piezoelectric element 15 is provided, the
element 15 consisting of a ceramic body having its op
pling one end of the element by means of a pin or the like
posed faces coated with silver or other good conductor
to a ?exible diaphragm which responds to the sound waves
to constitute metallic electrodes ‘at these opposed faces.
energizing the microphone. The mechanical vibration
The element 15 is energized in the conventional fashion
of the diaphragm then sets up stresses in the element
by coupling it to a thin metallic diaphragm 16 by means
30
which are converted to electrical ‘signals by the element.
of a pin 17. Thus, vibration of the diaphragm 16 by the
The problem of securing the ceramic element in the
sound waves stresses the ceramic element 15 and produces
micr'ophoneassembly has always been substantial. The
a voltage across its opposed electrode faces.
‘most common method in use today consists in providing
In order to describe more completely the effect which
a two-point suspension in which a substantial amount of
is achieved in the present invention, we have illustrated
35
the element is clamped, by spring pressure or otherwise,
the equivalent electrical circuit of the microphone as
along two spaced areas of the element.
’
sembly in FIGURE 3. The ceramic piezoelectric ele
We have found that this type of suspension seriously
ment 15 constitutes the equivalent of a low impedance
affects the performance of the piezoelectric element. The
generator 18. The capacity of the element 15 is repre
portion of the element clamped between the two points of
sented by a capacitor 19. When the element is included
suspension is practically useless as vfar as generating volt- '
in a circuit, there is an additional capacity represented
‘age is concerned, but adds a signi?cant amount of ca
bythe capacitor 21 placed across the output of the gen~
pacitive reactance to the electrical circuit of the micro
erator 18 from the capacity of the cable and similar
phone. The net result is a generally low' output level
factors. The actual output voltage of the generator 18
25
and impaired frequency response. Since the portion of
which would appear across terminals 22 and 23 there
the element between the points of suspension is practical 115 fore varies inversely as the reactance provided by the
ly useless, and this length may constitute close to one
half the length of the element, the cost of the unit is quite
high in relation to its performance characteristics.
A principal object of the present invention is to pro
vide an improved piezoelectric type microphone assembly I 50
having a higher voltage output while using a smaller
element than heretofore.
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capacitor 19. The reduction of that reactance is accom
plished by the improved suspension means of the present
invention.
As illustrated best in FIGURES 2 and 4, the suspen
sion means of the present invention provide a support
which holds the element along a single region of limited
area along the end of the crystal element in cantilever
A further object of the invention’is to provide an im
fashion. A lower support element, generally indicated
proved piezoelectric type microphone in which the ca
at numeral 24 in the drawings is secured to the base 13
pacitive load on the electrical system of the microphone 55 by means of a bolt 26 and a nut (not shown) at the op
is reduced.
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Another object of the present invention is to provide
a piezoelectric type microphone assembly which is eco
posite side of the base 13 by means of which electrical
connection can be made to the lower support 24.
The support 24 has a raised portion 27 having a recess
28 formed therein to receive the end portion of the crystal
nomical to manufacture.
We have now found that substantially improved re 60 element 15.
sults are obtained if the conventional two-point suspen
The element 15 is clamped within the recess 28 by
sion system for piezoelectric elements is eliminated and
means of an electrically conductive arm 29 which over
replaced by essentially a single area suspension or can
lies the portion of the element 15 which is seated in the
tilever suspension within the microphone housing. With '
recess 28, as illustrated best in FIGURE 2. The arm 29
this type of mounting, there is practically no length of 65 is electrically insulated from the support 24 but is in piezoelectric element which does not contribute to the
good electrical contact with the face of the element 15
opposite to the face which is seated in the recess‘ 28. The
voltage output, and the overall capacitance of the element
arm 29 is supported in the housing 10 by means of a bolt
is reduced in comparison with those assemblies which
employ the two-point suspension system. With the im
> proved structure of the present invention, we are able to
provide a microphone which has an output level averag
31 which extends through an electrically insulating grom
met 32 passing through an aperture 33 of the support -24.
‘ A nut 34 is employed to apply clamping pressure to the
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a.
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3,100,821
3
arm 29, and providea convenient means for attaching a
lug 36 for making the necessary electrical connections.
We claim as our invention:
>
A piezoelectric type microphone comprising a housing,
from the microphone assembly.
a support means in said housing, said support means in
In the event that the input impedance of the associated
cluding a recess on one surface thereof, a diaphragm'
amplifying or electronic equipment makes it necessary
closing one end of said housing, an elongated piezo~
or desirable, it is sometimes desirable to include a hollow
tube 38 in the housing for the purpose of acoustically
resonating the cavity of the microphone behind the dia
electric element having one end portion snugly received
within said recess and its opposite end portion coupled
to said diaphragm, a grommet extending through said
phragm 16 The tube 38 extends throughthe cylindrical
support means and securing said support means within
wall 12 and is secured thereto by means of a suitable 10 said housing, a pressure applying arm engaging the end
portion of said piezoelectric element which is received
With a microphone assembly of the presentinvention,
in said recess, a bolt arranged to apply clamping pressure
tests have indicated that the capacity of the element
to said arm and extending through said grommet, and a
changed from 520 micromicrofarads to 360 micromicroe
nut threaded on said bolt to provide an adjustable pres
adhesive 39.
.
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' '
farads when the conventional two-point suspension sys 15
tem was replaced by the improved suspension system il
sure on said arm.
7
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lustrated in the drawings. This decrease in capacity was
References Cited in the ?le of this patent
accompanied by a gain of 4 to 6 db in voltage output,
UNITED STATES PATENTS
despite the fact that the element employed in the im
‘2,587,684
lBauer _______________ __ Mar. 4, 1952
proved suspension system was only about one-half the 20
length of the element employed previously. ‘
It should be evident, therefore, that the improved ,
suspension means of the present invention provide a
greater voltage output while making it possible to use a
shorter length element.
‘
vIt should also be evident that various modi?cations
can be made to the described embodiment without de
parting from the scope of the present invention.
2-5
‘2,931,865
3,004,177
Dvorsky _____________ __ Apr. 5, 1961'
Sobel et al _____________ .._ Oct. 10, 1961‘
FOREIGN ‘PATENTS
732,634
68,474
.1,029,173
I
Germany ____________ __ Mar.r8, 1943
Denmark ____________ __ Jan. 17, 1949
Germany ____________ __ Apr. 30, 1958 1
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