Microphone with frequency pre-emphasis

A stepped frequency microphone particularly adapted to a hearing aid application provides a stepped frequency response characteristic relative to frequency, and has a low-pass sonic attenuator for providing to the undriven side of the microphone diaphragm a sonic counterpressure which at low frequencies substantially cancels ambient sound pressure delivered to the drive side of the diaphragm, the attenuator reducing this counterpressure at elevated frequencies to provide accentuated high frequency response.

DESCRIPTION 
1. Technical Field 
The technical field of the invention is electrical transducers and in 
particular miniature electrical microphones for hearing aids. 
2. Background Art 
The present invention is an improvement on U.S. Pat. No. 4,450,930 
entitled "Microphone with Stepped Response" issued to Mead C. Killion. The 
Killion patent describes an acoustic network whose function is to provide, 
when incorporated into a microphone, the transduction of sound to an 
electrical output wherein the higher frequencies have a greater signal 
level with respect to the lower frequencies. The benefits of such 
selective adjustment of signal according to frequency for the hearing 
impaired is described therein. 
The Killion patent describes a microphone assembly wherein a housing having 
a cavity is separated into two principal chambers by a main diaphragm, and 
further including a microphone transducer element disposed to be actuated 
by movement of this main diaphragm. Ambient sound is spit at an input port 
so that a fraction of the sound enters one of the chambers without 
significant attenuation. The remainder of the incoming sound is passed 
through a series of relatively short passages and apertures to enter a 
sealed chamber having a secondary diaphragm forming one wall thereof. 
Sound entering this second branch ultimately passes through the flexing of 
this secondary diaphragm to the opposite side of the main diaphragm. 
The compliance and mass of the secondary diaphragm, and the dimensions of 
the passages are chosen so that at relatively low frequency there is 
relatively little acoustical attenuation in this second branch, with the 
result that a significant pressure cancellation occurs at the main 
diaphragm so as to suppress the microphone response at these lower 
frequencies. At higher frequencies the attenuation in this second branch 
becomes substantially greater, resulting in a significant reduction of the 
counterpressure produced by the secondary diaphragm, resulting in 
substantially increased high frequency output. 
The stepped response microphone described in the Killion patent provided 
the necessary frequency variation of a response, but required in the 
smallest embodiment an overall case dimension of approximately 4.0 by 5.6 
by 2.3 millimeters. 
Attempts to further miniaturize microphones of this general design proved 
unsuccessful beyond a certain limit, principally because of the fact that 
the relatively short sound-attenuating passages of the second acoustical 
branch referred to above could not be correspondingly shortened while 
still providing the desired resonance turnover point, namely a point in 
the vicinity of 1 kilohertz. 
Thus, prior to the instant invention, there remained a need for a 
microphone providing the general frequency characteristics of the Killion 
design, while overcoming the above-mentioned disadvantage thereof. 
SUMMARY OF THE INVENTION 
The present invention is an improvement over the above-mentioned frequency 
dependent acoustic attenuating network. In the present design only one 
inlet is required to the microphone case instead of the two necessary in 
this previous design, thus reducing the necessity for a perfect seal 
around the sound inlet. It also allows the use of a reduced dimension 
inlet tube, unlike previous designs wherein the inlet tube diameter and 
tube flange were necessarily of increased size to feed the second inlet. 
The present invention is an improvement over the acoustical network in the 
above-cited patent in that the present design can achieve the same 
frequency response in a physically smaller unit. 
According to a feature of the invention, the secondary diaphragm is 
disposed to confront the transducer main diaphragm, separating the case 
into two principal volumes. Ambient sound is admitted to the chamber 
formed between the two diaphragms, this structure acting as a distributed 
line rather than a lumped element to provide the acoustic inertia required 
for the stepped response shape. The structure used is effectively three 
dimensional rather than two dimensional, and more efficiently uses the 
reduced volume of a smaller transducer. 
According to a related feature of this invention, the principal acoustic 
structure which provides the stepped response shape lies on the side of 
the transducer diaphragm opposite the electrical amplifier and connecting 
circuitry. This placement of the acoustic structure, as opposed to other 
designs which attempted to adapt U.S. Pat. No. 4,450,930 to systems of 
reduced dimensions, allows the step in amplitude to occur at the proper 
frequency of one kilohertz. By means of a unique bypass element around the 
main transducer diaphragm, the present invention achieves additional high 
acoustic inertia, while trapping a majority of the volume between the main 
diaphragm and secondary diaphragm. The placement of the acoustic network 
in an area other than the rear cover allows this surface to be non-planar, 
thus freeing this area for other uses such as a support for terminal pads, 
which further reduces the volume of the microphone. 
According to a further feature of the invention, additional acoustical 
inertia (inertance) is provided in series with the secondary diaphragm to 
further lower the turnover frequency by sealingly interposing a labyrinth 
plate between the two diaphragms, the plate having a suitably dimensioned 
passage coupling sound between the two chambers thus formed. Ambient sound 
is restricted to enter the chamber formed between the labyrinth plate and 
the main diaphragm, to pass across this chamber to pass through the 
labyrinth plate passage, and thereafter to reverse direction to flow 
across the secondary diaphragm. This increased path length thus 
additionally contributes to the necessary total inertance.

DETAILED DESCRIPTION 
While this invention is susceptible of embodiment in many different forms, 
there is shown in the drawings and will herein be described in detail 
preferred embodiments of the invention with the understanding that the 
present disclosure is to be considered as an exemplification of the 
principles of the invention, and is not intended to limit the broad aspect 
of the invention to embodiment illustrated. 
Referring now to the figures, the structure of the microphone assembly 10 
of the present invention comprises a case or housing 12, which, in the 
embodiment shown is square in shape and has depending walls 14. A plate 16 
supports a circuit board 18. An electrical amplifier (not shown) is 
constructed on this board 18, which carries terminals 26 connected to the 
amplifier to protrude to the outside. 
Two of the corners 28 of the main housing 12 are deformed to act as 
supports of predefined height (see FIG. 3). Two corners of a special 
labyrinth plate 30 rest on these supports. The opposite end of this plate 
30 has a protrusion which extends into a case inlet 36, thereby forming a 
three point support. This labyrinth plate 30 generally divides the case 
into two isolated volumes sealed off from each other except for special 
acoustical passages, one of which is a hole 34 through the labyrinth plate 
and disposed generally diametrically opposite the sound inlet 36. An 
annularly disposed ring 33 glued to the right-hand face of the labyrinth 
plate 30 as seen in FIG. 1A acts as a spacer for subsequent assembly. This 
ring 33 has a section removed so as not to impede the flow of sound 
entering the case inlet 36. 
On the left-hand face of the labyrinth plate 30 there is mounted a 
generally circular cup-shaped secondary diaphragm 38 similar in shape to 
those proposed in the previously mentioned Killion patent. The distance 
between the secondary diaphragm 38 and the labyrinth plate 30 is 
restricted so as to play a role in the overall frequency response of the 
microphone assembly. An annular flange portion 40 of the secondary 
diaphragm 38 is glued to the left-hand face of the labyrinth board 30 as 
shown in FIG. 1A. The secondary diaphragm 38 thus stands at a small 
distance from the labyrinth plate 30 to form a generally sealed volume 
therein, except for the acoustical passage. 
A main diaphragm assembly consisting of a compliant conducting main 
diaphragm 42 peripherally attached to mounting ring 44 is affixed to the 
housing interior by glue fillets 46 to be held in a position where the 
main diaphragm 42 confrontingly contacts the spacing ring 33. The glue 
fillets 46 and that portion of the main diaphragm mounting ring 44 in the 
vicinity of the inlet passage 36 effectively seal off the interior 
structure of the microphone assembly to the right of the main diaphragm 
from the inlet passage 36. An electret assembly 49 is mounted (by means 
not shown) to the mounting ring 44 so as to be in contacting engagement at 
peripheral portions with the main diaphragm 42. 
Referring now to FIG. 1A, FIG. 1B and FIG. 2 it will be seen that sound 
(indicated by flow arrows F-F) entering through an inlet tube 48 passes 
through a damping element or filter 50 to provide an inertance and a 
resistance to the incoming sound, the sound thereafter entering the inlet 
port 36. Thereafter the incoming sound travels across the chamber 52 
(excitation chamber) formed by the main diaphragm 42 and the labyrinth 
plate 30, thereby providing energization of the main diaphragm 42. 
Thereafter the sound passes through the small aperture 34 in the labyrinth 
plate 30 to enter the chamber 54 (transfer chamber) formed between the 
secondary diaphragm 38 and the labyrinth plate. Excitation of this 
secondary diaphragm 38 causes sound to be transmitted to the remaining 
volume 56 defined by the interior surface of the case 12, the secondary 
diaphragm 38 and the labyrinth plate 30. 
Sound received in this chamber is then coupled across through a bypass port 
51 (FIG. 2) to enter the volume 58 in the housing lying to the right of 
the main diaphragm 42 so as to impinge on the rear surface of the main 
diaphragm 42. This bypass port 51 is made by cutting away a corner of the 
labyrinth board 30, the diaphragm mounting ring 44 and the spacing ring 33 
in the vicinity of one corner of the housing, as shown FIG. 2. As a 
result, this bypass port 51 transmits sound received from the secondary 
diaphragm 38 around to the rear (right-hand) surface of the main diaphragm 
42. 
The dimensions of the various channels, apertures, and ports, the 
compliances of the two diaphragms 42, 38, the acoustical transmission 
properties of the damping element 50, and the relative volumes of the 
various chambers are arranged so that at low frequencies a substantial 
replication of the pressure excitation delivered to the main diaphragm 42 
from the incoming sound is provided via the bypass port 51 to the rear 
surface of the main diaphragm, thereby materially reducing the excitation 
pressure in such lower frequency ranges. By this means the microphone is 
rendered relatively unresponsive to low frequency sound. At higher 
frequencies, however, significant attenuation of this feed-around occurs 
because of the frequency-dependent acoustical attenuating properties of 
the coupling passages, with the result that at these higher frequencies 
this pressure cancellation effect is largely lost. As a result of this, at 
these higher frequencies the microphone sensitivity is materially 
augmented. 
Considering the various acoustical elements in more detail, at low 
frequencies sound is relatively unimpeded by small clearances, and except 
for the highly complaint secondary diaphragm 38 would be of essentially 
equal magnitude on both sides of the transducer diaphragm 42. The 
secondary diaphragm 38 produces a slight sound pressure imbalance of 
relatively constant magnitude at low frequencies, which results in a low 
level signal output from the transducer. At a well controlled intermediate 
frequency the inertia of the air flowing across the main diaphragm 38 and 
in the remainder of the sound path through the secondary diaphragm causes 
a resonant condition which acoustically seals off this path for all higher 
frequencies. This produces a step in the frequency response pattern 
similar to that proposed by U.S. Pat. No. 4,450,930; however, the present 
invention differs in the design of the structure necessary to achieve the 
same response. 
As shown in FIG. 1B, the main transducer diaphragm 42 and labyrinth plate 
30 form a small cavity 52 of narrow dimension. Unlike the usual 
microphone, this cavity does not act as a lumped capacitive element, since 
the hole 34 in the labyrinth plate 30 allows sound traveling the length of 
the cavity to exit therethrough. As the height of the cavity is small, 
there is restriction to sound flow along the length of the cavity, which 
is also acoustically shunted at each point by a portion of the main 
diaphragm 42. This cavity thus behaves generally as a distributed 
transmission line. Sound then enters the even more restricted cavity 54 
formed between the labyrinth wall 30 and the secondary diaphragm 38, to 
exit therefrom with modest attenuation thereafter to travel to the 
opposite surface of the main diaphragm 42 via the bypass port 51. 
At higher frequencies this feed-around action is greatly attenuated, such 
attenuation arising to a considerable degree because of inertial and 
resistance effects experienced by sound traveling through restricted 
passages. Inertial effects arise in general from the necessary pressure 
differential required to accelerate a column of air confined within an 
acoustical conduit. Quantitatively this phenomenon is referred to as 
inertance. The inertance per unit length of a given conduit is 
proportional to the density of air and inversely proportional to the 
cross-section area of the conduit. Resistance effects are inherently 
dissipative, and arise from viscous drag at the walls of the conduit, such 
drag giving rise to a pressure differential. Clearly, at frequencies 
sufficiently low that inertance effects in a given conduit may be ignored, 
resistance effects may still play a role. In general, the resistance per 
unit length of a given conduit will typically be strongly governed by the 
minimum dimension thereof, e.g., the separation between the main diaphragm 
42 and the labyrinth wall 30, and the separation between the secondary 
diaphragm 38 and the labyrinth wall. 
Although the actual equivalent circuit of the microphone assembly 10 is 
quite complex, certain general observations may nevertheless be made. The 
first is that the turnover frequency, i.e., the frequency at which the 
compensating sound pressure that is fed around to the rear of the main 
diaphragm 42 begins to be severely attenuated, is strongly governed by the 
product of the compliance of the secondary diaphragm 38 and the effective 
inertance of the acoustical passages supplying sound energy to it. To a 
first approximation this inertance may be taken to be the effective 
inertance of the lower half of the input chamber 52, the inertance of the 
labyrinth plate port 34, and the inertance of the lower half of the 
secondary diaphragm cavity 54. The amount of attenuation at frequencies 
well above the turnover point will also be governed by resistances of the 
various relevant conduits and ports, as well as the acoustical damper 50. 
It is clear that additional resistance and inertance effects may be 
provided by similarly adjusting the separation between the interior wall 
of the casing 12 and the secondary diaphragm 38. The labyrinth plate 30 
may be eliminated, and the secondary diaphragm 38 may be moved 
correspondingly closer to the main diaphragm 42; however, the turnover 
frequency rises as a result of this. By using such a labyrinth plate 30 to 
add significantly to the acoustical path length, sufficient inertance is 
provided to achieve the desired stepped frequency response turnover at 
approximately 1 kilohertz in a reduced dimension microphone assembly, in 
accordance with a design objective of the instant invention. In the event, 
that for one reason or another, a significantly higher turnover frequency 
is desired, then the labyrinth plate 30 may, as mentioned above, be 
eliminated. Alternatively, multiple labyrinth plates may be employed to 
increase the labyrinth inertance and/or resistance, if desired. 
The response of the microphone assembly described hereinabove is generally 
stepped, and similar to that of the microphone assembly described in the 
previously mentioned Killion patent. It has a turnover frequency of 
approximately 1 kilohertz, rising thereafter by a factor of approximately 
20 d.b. at a value of 3 kilohertz. This behavior is, however, achieved in 
a structure substantially smaller than the Killion structure, for reasons 
outlined hereinabove. The case dimensions (exclusive of the inlet tube 38) 
of the assembly shown in the figures are approximately 3.6 by 3.6 by 2.3 
millimeters.