Note: Descriptions are shown in the official language in which they were submitted.
488-158
GL:edm l
INERTIAL MICROPHONE/RECEIVER WITEI EXTENDED
FREQUENCY RESPONSE
Background of the Invention
The present invention relates to sound transducers and
in particular to an inertial microphone/receiver device.
In the conventional microphone air, disturbed by
soundwaves, serves to mové a diaphragm which in turn serves
to disturb an electro-magnetic field to thereby generate an
electrical signal. In the conventional receiver the reverse
occurs. That is, an electric current serves to dlsturb an
electro-magnetic field to in turn drive a diaphragm to
generate sound waves. While such transducers have many
benefits including excellent frequency response, their
principal shortcoming is that they can only be used in an
environment where air is available as a driving (or driven)
medium. Another shortcoming is that they cannot readily
differentiate between sources of moving air and hence,
particularly as a microphone, they are extremely noise
sensitive. As a result they cannot readily be utilized in
certain extreme environments, such as where there is a high
level of ambient noise (e.g. near motorcycles, heavy
equipment, etc.~ or where for one reason or another a
speaker's mouth is masksd (e.g. surgical theaters, fire
fighters with gas masks, etc.).
To overcome the shortcomings of the conventional air
drLven ransducer it has heretofore been s=gqe=ted to
:IL~(~25~ ;
utilize inertial transducers. In an inertial microphone the
vibrations of a sound source are applied to a relatively low
mass connected to a relatively large mass through a spring
diaphragm. The movement of the low mass with respect to the
large mass causes the spring diaphragm to oscillate within
an electro-magnetic field thereby generating an electric
signal. In operation as a speaker the electric signal is
used to vary an electro-magnetic field to thereby drive a
spring diaphragm connecting a large mass to a small mass
thereby causing the one mass to oscillate with respect to
the other to produce sound waves. Since such transducers
need not rely on air movement they can be utilized in bone
conduction microphones and receivers.
Heretofore the principal problem of inertial
transducers has been that they are extremely frequency
limited and hence produce an unnatural sound particularly
¦¦ when used for voice communication. The reason for this is
,l that the spring diaphragm is basically a single frequency
il device so that all signals tend to peak at the natural
1 frequency of the spring. This produces an extremely
degraded and distorted signal for voice communication.
In view of the above, it is the principal object of the
present invention to provide an improved inertial transducer
having a relatively flat response over a relatively wide
~requency ranqe.
-2-
nother object i9 to provide such a transducer in which
the range in which the frequency response is generally flat
corresponds with the principal frequencies of voice
communication.
Still another object is to provide such a transducer in
a si~e and shape that may reaclily be adapted for a
microphone or ear speaker.
A further object is to provide such a transducer in a
form that is realtively simple and economic to assemble.
Still other objects and advantages will be apparent
from the following description of the invention.
SummarY of the Invention
The above and other beneficial objects and advantages
are attained in accordance with the present invention by
providing an inertial microphone in which a spring diaphragm
! acts both as a spring connecting a large mass to a small
, mass and also as a diaphram on a common wall between two
i chambers. As the spring diaphragm moves (in response to
vibrations) an electric signal is generated as a result of
relative movement between a coil and a magnetic Eield.
Alternatively, as a signal is applied to the coil the
spring diaphragm is caused to move to set up vibrations.
The spring and two chambers are each tuned to different
frequencies within the desired frequency range thereby
setting up three frequency peaks and a relatively flat
response betwe-n t peats.
!
o~
The invention will now be described in more detail, by way o~
example only, with reference to the accompanying drawings in
which:-
Fig. 1 is an exploded perspective view of a transducer in
accordance with the present in~ention; and
Fig. 2 is an elevational sectional view of the assembled
transducer.
Reference is now made to the drawings and to Fig. 1 in
particular wherein the transducer :L0 of the present invention
is shown as comprising an inner housing 12 and outer housing
14. Both housings 12, 14 comprises tubular cylinders having
an open end and a closed end. The inner housing 12 is sized
to closely fit in the outer housing 14 and is sealed in
position as shown in Fig. 2.
The transducer 10 further includes an armature 16 in the form
of a disc of a magnetic material (such as Hi Mu 80) bonded by
epoxy or the like to the interior of the closed end of the
inner housing. The armature 16 is spot welded to a spring
diaphragm 18 which in turn sits on a knife edge 20 which
extends from the base.of a brass ring 22. Ring 22 has a
central opening 24 extending through the base and an off-
centered, much smaller opening 26. Opening 26 is sized to
receive a Thuras tube 28 which extends toward the closed end
of outer housing 14. A ring magnet 30 extends through the
central opening 24. Magnet 30 is magnetized so that one pole
is directed toward the closed end of housing 12 and the other
pole is directed toward the closed end of housing 14.
~3~
A magnetic pole piece 32 concentric with the ring magnet 30
also extends through opening 24. One end 34 of pole piece 32
terminates flush to the corresponding pole of magnet 30. The
other end of pole piece 32 terminates in a circular flange 36
one side of which rests against the corresponding pole of
magnet 30. The opposite side of flange 36 rests on a damper
pad 38, foxmed of rubber or the like, which in turn rests
against the closed end of outer housing 14.
A coil 40 is wound about a bobbin 42 which .in turn is
positioned about the central post 43 of pole piece 32. The
leads 44 of coil ~0 are brought to a terminal board 46
mounted to ring 22 and from there through a sealed opening in
outer housing 14 to an exterior terminal board 48 mounted to
the exterior of housing 14.
As can be seen in Fig. 2, a non-perforated spring diaphragm
18 forms a common wall between a first chamber 50 that is
generally de*ined by the volume within the assembled housing
surrounding brass ring 22, and a second chamber 52 that
comprises the volume between the diaphragm and brass ring as
well as that of the Thuras tube 28.
In operation as a microphone, the assembled transducer is
positioned against a source vibrating in response to sounds
being generated, for example, the transducer may be placed
against the head of a speaker behind the ear. As a result of
the vibrations imparted to the transducer, the ring 22,
magnet 30, and pole piece 32 desire to remain stationary due
to the
inertia of their mass while the housings 12 and 14 and the
armature 16 are driven causing deflections in spring
diaphragm 18 thereby changing the gap between pole 32 and
armature 16 and thus altering the lines of flux cut by coil
40. The current induced in coil 40 may then be picked up at
the terminal board 48.
In operation as a receiver a signal i5 applied to coil
40 thereby altering the magnetic circuit and causing the
coil 40, pole 43, and magnet 30 to move toward or away from
the pole piece 32. Due to the inertia of their mass, the
movement is transferrad to the housings 12, 14 and armature
16 through the action of spring 18. The movement may be
detected directly (such as by placing the receiver against
the`s~ull of a listener) or indirectly by placing the
transducer against a sounding board to generate sound waves.
By properly sizing the components the cavities of the
first and second chambers 50,52 can be tuned. Similarly the
resonant frequency of the diaphragm may be tuned by proper
l selection of the diaphragm material. In a preferred
practice of the invention, the resonant frequency of chamber
50 waq 1500 Hz; the resonant frequency of chamber 52 was 600
Hz; and the spring resonance was 2500 ~z. Since the two
tuned cavities and spring havs relatively low q values, the
frequency response between the 600 Hz, 1500 Hz and 2500 Hz
pea~s tends to flatten out thereby providing a relatively
flat frequency response in the voice communication range.
; ~he inertial tr ~=5 cer i5 inherently n~ice cancelling since
'
~ ~3~i~5~i 1
both sides of the transducer housing 12, 14 are moving in
phase to one another and ambient noise is cancelled as a
result.
Thus in accordance with the above the aforementioned
lv~ 'le~t~e~
.
. 1~
--7--
