WO1998035530A1

WO1998035530A1 - Microphone with modified high-frequency response

Info

Publication number: WO1998035530A1
Application number: PCT/US1998/002347
Authority: WO
Inventors: Eugene M. Ring
Original assignee: Knowles Electronics, Inc.
Priority date: 1997-02-07
Filing date: 1998-02-06
Publication date: 1998-08-13
Also published as: AU6320498A

Abstract

A microphone having a reduced high frequency response is disclosed. The microphone comprises a housing (12), a diaphragm defining a front cavity and a back cavity, and an inlet port (20) acoustically communicating to the front cavity. The inlet port and front cavity cooperatively form an input sound path. A slot (18) is disposed in the input sound path for increasing the effective inertance and resistance to sound presented to the inlet port. According to one embodiment, the inlet port (20) includes an inlet tube (16), and the slot (18) is disposed within the inlet tube (16). According to another embodiment, the slot (18) is formed in the front cavity.

Description

MICROPHONE WITH MODIFIED HIGH-FREQUENCY RESPONSE

DESCRIPTION

Technical Field

The present invention relates to a microphone for a hearing aid having a modified high frequency response, such as to eliminate possible high frequency oscillations when coupled to a hearing aid receiver.

Background of the Invention

A hearing aid typically comprises a microphone and a receiver. The microphone receives sound and converts the received sound to an electrical signal. The receiver takes the electrical signal and converts it to sound. An amplifier is typically disposed between the microphone and the receiver.

As a result of various factors, including the inertance of air within the microphone, conventional miniature microphones have a response curve having a peak generally around 5.5 - 7 kHz. In fact, typically the smaller the microphone, the higher the peak frequency. Similarly, conventional receivers also have a response having multiple peaks. When one of these microphones is coupled to one of these receivers, the resulting closed loop gain can result in high frequency oscillations, due to feedback consisting of sound leaking back from the receiver to the microphone. This feedback is quite undesirable, and often results in a significant number of hearing aids being returned. It is known that by increasing the inertance presented to sound entering the microphone, the frequency of the peak response of the microphone can be reduced to a frequency which will eliminate this feedback. While this would reduce the high frequency performance of the hearing aid, hearing aid manufacturers have indicated a willingness to accept the reduction as a tradeoff for reduced high frequency oscillations, or feedback.

Holesha, U.S. Patent No. 5,319,717, discloses a method of adding inertance to lower the peak frequency by disposing a C-shaped shim within the input chamber to form an elongated sound path. The elongated sound path increases the effective inertance of the input chamber to thereby lower the frequency of the peak response of the microphone to a frequency lower than the frequency of the peak response of a conventional receiver coupled thereto to eliminate high frequency oscillations. Holesha, however, was found not to be effective because it caused environmental failures due to diaphragm collapse, as a result of the close proximity of the shim to the diaphragm and because the peak damping produced was difficult to control.

In the past, screens have been placed in the inlet tube of microphones to increase the resistance and, hence, dampen the response peak of the microphones. However, these have tended to facilitate clogging of the tubes.

An elongated inlet tube extending from an inlet port has been found to have the effect of increasing the inertance presented to the air as it travels to the diaphragm, thereby lowering the frequency of the peak response of the microphone. The diameter of a generally cylindrical inlet tube may be modified to adjust the peak frequency. Reducing the diameter of the inlet port will decrease the peak frequency.

Reducing the diameter of the inlet port will also increase the damping, however, the increase in the damping may not be adequate because the damping and the peak frequency are not independently adjustable. As shown by simplified equations below, where 1, represents the length of the tube, r represents the radius of the tube, η₀ represents the viscosity of air, and p₀ represents the density of air, both the damping resistance of the port, R^, and the inertance of the port, L_tube, are controlled by the length of the tube and a power of its radius, as follows:

Rtube = 8- η₀ - π-r⁴

A slot to increase the inertance presented to air entering the

microphone can also be formed by lowering the diaphragm to decrease the height of

the microphone's front cavity. The cross-sectional area and depth of the front cavity control the inertance and the resistance the front cavity, as described by the simplified

equations below. Summary of the Invention

It is an object of the invention to provide a microphone having a

reduced high frequency response without the need for a screen in the inlet tube. In

accordance with the invention, the microphone comprises a housing, a diaphragm

defining a front cavity and a back cavity, and an inlet port acoustically communicating

to the front cavity. The inlet port and front cavity cooperatively form an input sound

path. A slot is disposed in the input sound path for increasing the effective inertance

to sound presented to the inlet port.

According to one embodiment, it is contemplated that the inlet port includes an inlet tube, and the slot is disposed within the inlet tube. According to another embodiment, the slot is formed in the front cavity.

Other advantages and aspects of the present invention will become

apparent upon reading the following description of the drawings and detailed

description of the invention.

Brief Description of the Drawings

FIG. 1 is a perspective view of a microphone in accordance with a

first embodiment of the present invention.

FIG. 2 is a side view of the microphone of FIG. 1;

FIG. 3 is a cross-sectional side view of the microphone of FIG. 1 ;

and,

FIG. 4 is a cross-sectional side view of a microphone in accordance

with a second embodiment of the invention.

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 the embodiments illustrated. Referring to Figures 1-3, the structure of the microphone assembly

10 of the first embodiment of the present invention comprises a case or housing 12 which, in the embodiment shown, is generally square in shape and has depending walls

14. A diaphragm 15 is disposed within the housing 12, defining a front cavity 'f and a back cavity 'b' (FIG. 3).

An inlet tube 16 extends outwardly from one of the depending walls

14 for receiving sound. Sound entering the microphone through the inlet tube 16

passes through a slot 18 in a structure 22, before entering the front cavity. The slot 18

is used to modify the inlet tube 16, and is disposed within the inlet tube 16.

Optionally, an aluminum shim stock, which is encapsulated in epoxy to form the

structure 22, can be placed within the inlet port 20 and the slot 18 formed when the

aluminum is etched out. Preferably, the structure could be formed by injection

molding.

The microphone assembly 10 has a modified high frequency

response wherein the frequency of the peak response of the microphone assembly 10 is reduced to a frequency lower than the frequency of the peak response of a receiver to

which the microphone assembly 10 is ultimately connected. With the slot 18, the peak

frequency and the damping may be adjusted independently as shown by the equations below where w represents the width of the slot, t represents the thickness of the slot,

R_s,_ot represents the damping resistance of the slot, and L_s,_ot represents the inertance of

the port.

w-r ^Lsiot ⁼ Mo ^• —

5-w-t

As shown above, both the inertance and resistance are proportional

to 1, and inversely proportional to w, but the inertance is inversely proportional to t

while the resistance is inversely proportional to t³. Thus, by changing the thickness

while holding the area constant, the damping may be adjusted without changing the

peak frequency, or vice versa.

In the first embodiment, the preferred dimensions are as follows,

t = .004 inches;

w - .059 inches; and

1, = .060 inches. A second embodiment of the present invention is illustrated in

Figure 4. According to the second embodiment, the structure of the microphone

assembly 10' comprises a case or housing 12' which, in the embodiment shown, is also generally square in shape and has depending walls 14'. A diaphragm 15' is disposed

within the housing 12', defining a front cavity 'f and a back cavity 'b'. An inlet tube 16' extends outwardly from one of the depending

walls 14' for receiving sound. Sound enters the microphone 10' via the inlet tube 16' and proceeds into the front cavity. The front cavity has a thickness 't', a width 'w' (not shown) and a depth 'd'. According to this embodiment, the diaphragm has been lowered to reduce the thickness 't', to thereby cause the front cavity to perform

acoustically as a slot 18'.

The microphone assembly 10' accordingly also has a modified high

frequency response wherein the frequency of the peak response of the microphone

assembly 10' is reduced to a frequency lower than the frequency of the peak response

of a receiver to which the microphone assembly 10 is ultimately connected. With the

slot 18, the peak frequency and the damping may be adjusted independently.

The inertance and resistance are described by the equations for L_sl01

and described above. In the preferred embodiment, the thickness t of an

otherwise conventional Knowles EM-4046 microphone, available from Knowles

Electronics, Inc., of Itasca, Illinois, US, the assignee of this patent application, has

been reduced from 0.007" to 0.004". Utilizing these dimensions, the following results

were obtained.

While the specific embodiments have been illustrated and described,

numerous modifications come to mind without significantly departing from the spirit of

the invention and the scope of protection is only limited by the scope of the

accompanying Claims.

Claims

CLAIMSI CLAIM:

1. A microphone having a reduced high frequency response

comprising:

a housing;

a diaphragm defining a front cavity and a back cavity;

an inlet port acoustically communicating to said front cavity, said

inlet port and front cavity cooperatively forming in input sound path; and,

a slot disposed in said input sound path for increasing the effective

inertance and resistance to sound presented to said inlet port.

2. The microphone of claim 1 wherein said inlet port includes an inlet

tube, and said slot is disposed within said inlet tube.

3. The microphone of claim 1 wherein said slot is formed in said front

cavity.