HEARING AID MICROPHONE WITH MODIFIED HIGH-FREQUENCY RESPONSE

Abstract: A microphone for a hearing aid having a modified high-frequency response is disclosed. The hearing aid comprises a hollow housing having an upper wall, a lower wall and a depending wall disposed betwen the upper wall and the lower wall. The upper, lower and depending walls define a main chamber. The microphone further comprises an inlet tube extending outwardly from the depending wall for receiving sound and a diaphragm in continuous peripheral contact with the depending wall for substantially entirely dividing themain chamber into an input chamber in communicating relationship with the inlet tube and an output chamber. The input chamber presents an effective inertance to sound entering via the inlet tube. A structure is disposed within the input chamber between the diaphragm and the upper wall forming 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.

Description: HEARING AID MICROPHONEWITH MODIFIED HIGH-FREQUENCY RESPONSE DESCRIPTION Technical Field The invention relates to a microphone for a hearing aid having a modified high frequency re¬ sponse to eliminate possible high-frequency oscil¬ lations when coupled to a hearing aid receiver. Background Prior Art A hearing aid typically comprises a microphone and a receiver. The microphone receives sound and converts the received sound to an electrical sig¬ nal. The receiver takes the electrical signal, amplifies it, and converts the amplified electrical signal to sound. As a result of various factors, including the inertance of air within the microphone, conven¬ tional microphones have a response curve having a peak generally around 4.8 - 5.0 kHz. Similarly, conventional receivers also have a response curve having a similar peak. When one of these micro¬ phones is coupled to one of these receivers, the resulting closed loop gain can result in high frequency oscillations, due to sound leaking back from the receiver to the microphone. These oscil¬ lations are quite annoying and result in a rela¬ tively large percentage 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 significantly lower than that of the receiver coupled thereto. While this would reduce the high frequency performance of the hearing aid, hearing aid manufacturers have indicated a willingness to accept this reduction as a tradeoff for reduced high-frequency oscillations. Tibbet Industries sells a Series 125 micro¬ phone comprising a generally cylindrical microphone housing having opposing ends and an inlet port dis¬ posed through one of the ends. Tibbet has recently introduced a microphone having an inlet tube exten¬ ding radially from the end having the inlet port. It is believed that this tube was provided to prevent dirt or other foreign substances from entering the microphone via the inlet port, and thereby damaging the diaphragm. However, the elongated tube also has 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. However, this microphone construction has been found to be diffi¬ cult to assemble within a hearing aid. Knowles Electronics, the assignee of this patent application, has previously developed a microphone having selective inertance, which is disclosed in Madaffari, U.S. Patent No. 4,837,833. This microphone had a housing and a diaphragm extending substantially across the housing and defining an input chamber coupled to an input tube and an output chamber. The diaphragm had a notch cut through the diaphragm at the rear of the micro¬ phone, the notch forming a by-pass port. A C-shaped plate having a central recess was disposed in the input chamber between the diaphragm and the housing. The central recess was directed toward the input port. The C-shaped plate formed gener¬ ally narrow channels leading back to the by-pass port. The narrow channels would generally pass low frequency sound to the back-side of the diaphragm, so that low frequency sound would not cause the diaphragm to vibrate. The narrow channels, on the other hand, would block intermediate and high frequency sound from passing through the by-pass port, so that such frequencies would cause the diaphragm to vibrate. This microphone resulted in a high frequency emphasis microphone which provided a steeply rising frequency response. However, this microphone would not reduce the frequency of the peak response of the microphone. The present invention is provided to solve these and other problems. Summary of the Invention It is an object of the invention to provide a microphone for a hearing aid having a modified high frequency response to reduce or eliminate high frequency oscillation when coupled to a receiver. In accordance with the invention, the micro¬ phone comprises a hollow housing defining a main chamber with an inlet tube extending outwardly from the housing for receiving sound. A diaphragm is disposed within the housing to entirely divide the main chamber into an input chamber in communicating relationship with the inlet tube and an outlet chamber. The input chamber presents an effective inertance to sound entering the front chamber via the inlet tube. Means are disposed within the input chamber for increasing the effective inert¬ ance of the input chamber. According to a first embodiment, the inertance increasing means comprises a generally C-shaped plate parallel to the diaphragm and having a cen¬ tral recess. The radial orientation, as well as the detailed size and shape, of the plate, and hence of the central recess, can be varied at the time of manufacture to vary the inertance, and thus the frequency response, of the microphone. According to a second embodiment, the inert¬ ance increasing means comprises an embossment inwardly formed in the housing to provide a gener¬ ally C-shaped structure parallel to the diaphragm and having a central recess. As with the first embodiment, the radial orientation of the emboss¬ ments, and hence of the central recess, can be varied at the time of manufacture to vary the inertance, and thus the frequency response, of the microphone. An added advantage of this design is that one can view the orientation of the emboss¬ ments from the outside of the housing, thus indi¬ cating the orientation, and thus the frequency response, of the microphone. Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawing. Brief Description of Drawings Figure 1 is an exploded view of a microphone in accordance with a first embodiment of the inven¬ tion; and Figure 2 is an exploded view of a microphone in accordance with a second embodiment of the invention. Detailed Description While this invention is susceptible of embodi¬ ments in many different forms, there is shown in the drawings and will herein be described in de- tail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as exemplifications of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. A first embodiment of a microphone 10 for a hearing aid is illustrated in Figure 1. The micro¬ phone 10 has a modified high frequency response wherein the frequency of the peak response of the microphone is reduced to a frequency lower than the frequency of the peak response of a conventional receiver to which the microphone 10 is ultimately connected. The microphone 10 comprises a generally hollow cup 12 having a base wall 14 and four side walls 18. The base and side walls 14, 18, respectively, define a main chamber 20. An inlet tube 22 extends outwardly from one of the side walls 18 for receiving sound and com uni- eating the received sound into the main chamber 20. A diaphragm, generally designated 24, comprises a conventional ring and film assembly is spaced from the base wall 14 and is placed in continuous peri¬ pheral contact with the side wall 18. The dia¬ phragm entirely acoustically divides the main chamber into an input chamber 26 which is in commu¬ nicating relationship with the inlet tube 22, and an output chamber 28. The diaphragm 24 includes a tiny vent opening, of the order of 0.002", to equa¬ lize pressure between the input and output chambers 26, 28. As is well known, the inlet tube 22 and the input chamber 26 present an effective inertance to sound entering the microphone 10. A charged plate 30 is conventionally disposed in contact with the diaphragm 24. The charged plate 30 is coupled to a circuit board 32 by a wire 34 extending through an opening in a plate 36. A cap 38 is secured to the cup 12 to form a microphone housing. As is well known, sound entering the input chamber 26 via the inlet tube 22 causes the diaphragm 24 to vibrate, which vibration is detected by the charged plate 30, developing an electrical signal which is trans¬ mitted to the circuit board 32 via the wire 34. The circuit board 32 contains electronic circuitry, not specifically shown, as is well known. In accordance with the invention, a structure such as a C-shaped plate 40, is disposed within the input chamber 26 between, and in contact with, the diaphragm 24 and the base wall 14. The C-shaped plate 40 is secured to the base wall 14 by spot welding and abuts the diaphragm 24. The C-shaped plate 40 has parallel arms 42, 44 defining a recess 46. The C-shaped plate 40 has a width W of .101", ± .001, a length L of .091", ± .001, and the recess 46 has a width W of 0.05". The recess 46, as well as channels formed be¬ tween the parallel arms 42, 44, and adjacent ones of the side walls 18 form elongated sound paths through which received sound must pass. The elon- gated sound paths increase the effective inertance of the input chamber, thereby reducing the freque¬ ncy of the peak of the response curve. By horizon¬ tally rotating the C-shaped plate, the effective inertance of the input chamber 26 can be adjusted, thereby adjusting the frequency of the peak of the response curve as desired. For example, the C- shaped plate could be adjusted a full 90 degrees, or more. It has been found that the frequency of the peak of the microphone's response curve has been reduced from 5.5 kHz to 4.5 kHz by the addition of the C-shaped plate 40 with the recess 46 oriented towards the inlet tube 22. A second embodiment of the invention is shown in Figure 2. This embodiment is substantially the same as the first embodiment, except that the C- shaped plate 40 has been replaced by an inwardly directed, C-shaped embossment 50. As with the first embodiment, the horizontal orientation of the embossment 50 can be adjusted, thereby adjusting the frequency of the peak of the response curve as desired. This embodiment has an economic advantage over the first embodiment through a reduced piece count. Additionally, the rotational orientation of the C- shaped embossment 50 will appear on the outer surface 14 as a C-shaped groove. Thus one can recognize the frequency characteristic of a partic¬ ular microphone by simple inspection of the outer surface 14a of the base wall 14. It will be understood that the invention may be embodied in other specific forms without depar¬ ting from the spirit or central characteristics thereof. For example, structures of shapes other than C-shaped can be substituted to provide an elongated sound path, such as U-shaped, W-shaped, E-shaped, M-shaped, N-shaped, T-shaped, or the like. The present examples and embodiments, there¬ fore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.

Claims: C L A I M S 1. A microphone for a hearing aid having a modified high frequency response, the microphone comprising: a hollow housing defining a main chamber; an inlet tube extending outwardly from said housing for receiving sound; a diaphragm disposed to substantially entirely divide said main chamber into an input chamber in communicating relationship with said inlet tube and an output chamber, said input chamber presenting an effective inertance to sound entering via the inlet tube; and means disposed within said input chamber for increasing said effective inertance of said input chamber. 2. The microphone of claim 1 wherein said inertance increasing means comprises a structure providing an elongated sound path. 3. The microphone of claim 2 wherein said structure is disposed generally parallel to said diaphragm. 4. The microphone of claim 3 wherein said structure comprises a generally C-shaped plate having a central recess. 5. The microphone of claim 4 wherein said central recess is directed generally towards said inlet tube. 6. The microphone of claim 4 wherein said central recess is directed generally 90 degrees from said inlet port. 7. The microphone of claim 2 wherein said structure comprises an embossment formed in said housing. 8. The microphone of claim 7 wherein said embossment is generally C-shaped and having a cen¬ tral recess. 9. The microphone of claim 8 wherein said central recess is directed generally towards said inlet tube. 10. The microphone of claim 8 wherein said central recess is directed generally 90 degrees from said inlet port. 11. A microphone for a hearing aid having a modified high frequency response, the microphone comprising: a hollow housing having an upper wall, a lower wall and a depending wall disposed between said upper wall and said lower wall, said upper, lower and depending walls defining a main chamber; an inlet tube extending outwardly from said depending wall for receiving sound; a diaphragm in continuous peripheral contact with said depending wall for substantially entirely dividing said main chamber into an input chamber in communicating relationship with said inlet tube and an output chamber, said input chamber presenting an effective inertance to sound entering via the inlet tube; and a structure having a central recess and dis¬ posed within said input chamber between said dia¬ phragm and said upper wall for increasing said effective inertance of said input chamber. 12. The microphone of claim 11 wherein said structure is generally C-shaped. 13. The microphone of claim 12 wherein said generally C-shaped structure comprises a generally C-shaped plate. 14. The microphone of claim 11 wherein said central recess is directed generally towards said inlet tube. 15. The microphone of claim 11 wherein said central recess is directed generally 90 degrees from said inlet port. 16. The microphone of claim 12 wherein said generally C-shaped structure comprises an emboss¬ ment formed in said upper wall. 17. A microphone for a hearing aid having a modified high frequency response, the microphone comprising: a hollow housing having an upper wall, a lower wall and a depending wall disposed between said upper wall and said lower wall, said upper, lower and depending walls defining a main chamber; an inlet tube extending outwardly from said depending wall for receiving sound; a diaphragm in continuous peripheral contact with said depending wall for substantially entirely dividing said main chamber into an input chamber in communicating relationship with said inlet tube and an output chamber, said input chamber presenting an effective inertance to sound entering via the inlet tube; and a structure disposed within said input chamber between said diaphragm and said upper wall forming an elongated sound path, said elongated sound path for increasing said effective inertance of said input chamber. 18. The microphone of claim 17 wherein said structure forms a plurality of sound paths. 19. The microphone of claim 17 wherein said structure is generally C-shaped having a recess operative as said elongated sound path. 20. A hearing aid comprising the microphone of claim 17, in combination with a receiver, the receiver having a peak response at a frequency substantially greater than the frequency of peak response of said microphone. AMENDED CLAIMS [received by the International Bureau on 17 February 1994 (17.02.94) ; original claim 20 cancelled ; original claims 1, 11, 16 and 17 amended ; other claims unchanged (3 pages)] 1. A microphone having a modified high frequency response, the microphone comprising: a hollow housing defining a main chamber; 5 an inlet tube extending outwardly from said housing for receiving sound; a diaphragm disposed to entirely acoustically divide said main chamber into an input chamber and an output chamber, said input chamber being in 10 communicating relationship with said inlet tube, and said input chamber presenting an effective inertance to sound entering via the inlet tube; and means disposed within said input chamber for increasing said effective inertance of said input 5 chamber. 2. The microphone of claim 1 wherein said inertance increasing means comprises a structure providing an elongated sound path. 3. The microphone of claim 2 wherein said 0 structure is disposed generally parallel to said diaphragm. 4. The microphone of claim 3 wherein said structure comprises a generally C-shaped plate having a central recess. 5 5. The microphone of claim 4 wherein said central recess is directed generally towards said inlet tube. 6. The microphone of claim 4 wherein said central recess is directed generally 90 degrees 0 from said inlet port. 7. The microphone of claim 2 wherein said structure comprises an embossment formed in said housing. 8. The microphone of claim 7 wherein said 5 embossment is generally C-shaped and having a cen¬ tral recess. 9. The microphone of claim 8 wherein said central recess is directed generally towards said inlet tube. 10. The microphone of claim 8 wherein said central recess is directed generally 90 degrees from said inlet port. 11. A microphone having a modified high frequency response, the microphone comprising: a hollow housing having a base wall, a cap end, and a depending wall disposed between said cap end and said base wall, said cap end, base and depending walls defining a main chamber; an inlet tube extending outwardly from said depending wall for receiving sound; a diaphragm in continuous peripheral contact with said depending wall for entirely acoustically dividing said main chamber into an input chamber and an output chamber, said input chamber being in communicating relationship with said inlet tube, and said input chamber presenting an effective inertance to sound entering via the inlet tube; and a structure having a central recess, the structure disposed within said input chamber be¬ tween said diaphragm and said upper wall for in- creasing said effective inertance of said input chamber. 12. The microphone of claim 11 wherein said structure is generally C-shaped. 13. The microphone of claim 12 werein said generally C-shaped structure comprises a generally C-shaped plate. 14. The microphone of claim 11 wherein said central recess is directed generally towards said inlet tube. 15. The microphone of claim 11 wherein said central recess is directed generally 90 degrees from said inlet port. 16. The microphone of claim 12 wherein said generally C-shaped structure comprises an emboss¬ ment formed in said cape end. 17. A microphone having a modified high frequency response, the microphone comprising: a hollow housing having a base wall, a cap end and a depending wall disposed between said cap end and said base wall, said cap end base and depending walls defining a main chamber; an inlet tube extending outwardly from said depending wall for receiving sound; a diaphragm in continuous peripheral contact with said depending wall for entirely acoustically dividing said main chamber into an input chamber and an output chamber, said input chamber being in communicating relationship with said inlet tube, and said input chamber presenting an effective inertance to sound entering via the inlet tube; and a structure disposed within said input chamber between said diaphragm and said cap end forming an elongated sound path, said elongated sound path contributing to increasing said effective inertance of said input chamber. 18. The microphone of claim 17 wherein said structure forms a plurality of sound paths. 19. The microphone of claim 17 wherein said structure is generally C-shaped having a recess operative as said elongated sound path.

Inventors: HOLESHA KENNETH G, HOLESHA, KENNETH, G.
Applicants: KNOWLES ELECTRONICS INC, KNOWLES ELECTRONICS, INC.
Application year: 1993
Main classification: H04R25
IPC codes: H04R1, H04R25