Electrical audio signal processing systems and devices – Directive circuits for microphones
Reexamination Certificate
1998-11-16
2003-11-25
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Directive circuits for microphones
C381S104000, C381S313000, C381S097000, C381S103000
Reexamination Certificate
active
06654468
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to devices for matching outputs of a pair of microphones, and in particular to an apparatus and a method that compensates for variations in the sensitivity, low frequency rolloff, and resonance peak of at least one of the microphones.
BACKGROUND OF THE INVENTION
Hearing aids for providing a user selectable directional response have become quite popular in the marketplace. In a noisy environment, the user of such an aid can select the directional pattern and thus eliminate some of the noise coming from the rear. This can increase the signal to noise level enough to improve the intelligibility of speech originating from the forward direction. In a quiet environment, the user would normally switch to the nondirectional pattern in favor of its better performance in quiet.
One way to achieve a directional response in a hearing aid is to use two omnidirectional microphones, and to combine their electrical signals to form the directional beam. Compared to the use of a directional microphone, the Dual Omni approach has some advantages. However, it also carries the requirement that the response of the two microphones be accurately matched in magnitude and phase. The matching must be accurate throughout the frequency band where directionality is needed, and must remain matched throughout the life of the hearing aid. Normal variations in microphone manufacturing do not provide a close enough match for most applications.
Often it has been necessary to specially measure and select the microphones for use in a paired application. The present invention presents an apparatus and method of compensation for the variations in microphone performance. An electrical circuit is used with one or both of the microphones to achieve the necessary match in response for directional processing. The response of the circuit can be “tuned” to each microphone at the final stages of manufacturing, as a part of the fitting porches, automatically, or even at a periodic follow-up visit if the characteristics of the microphone have changed through aging or abuse.
The Microphone Model
A simple model for a microphone is assumed herein. The frequency response shown in
FIGS. 1 and 2
is characteristic of many electret microphone designs used in devices such as hearing aids. Mathematically, the response can generally be represented as:
M
(&ohgr;)=
M
0
L
(&ohgr;)
H
(&ohgr;)
where
L(&ohgr;) models the low frequency rolloff, and
H(&ohgr;) models the mid and high frequency behavior, including the diaphragm resonance.
The assumption that the microphone response can be separated in this way makes the analysis much simpler without introducing a significant error for most actual microphone responses used for directional hearing aids and the like. It works well for any microphone whose low frequency rolloff is separated in frequency from its diaphragm resonance. (The so-called “ski slope” microphone responses are not of this variety and would require a different analysis; but they are not well suited for use in devices such as directional hearing aids.)
The low frequency rolloff is approximated as a single-pole filter:
L
(
ω
)
=
j
ω
ω
l
1
+
j
ω
ω
l
where &ohgr;
l
is the corner frequency for the low frequency rolloff. The higher frequency behavior is approximated by:
H
(
ω
)
=
1
1
-
ω
2
ω
r
2
+
j
ω
Q
ω
r
where &ohgr;
r
is the corner diaphragm resonance frequency and Q is the mechanical quality factor of that resonance.
Variations in production may cause the response of an individual microphone to vary in several ways from this nominal response: 1) The sensitivity level M
0
of the entire curve may shift to higher or lower values due to variations in electret charge or diaphragm stiffness; 2) The corner frequency &ohgr;
l
of the low frequency rolloff may move to a higher or lower frequency due to variation in the size of the barometric relief hole in the diaphragm; and 3) The frequency &ohgr;
r
of the resonance peak may shift to a higher or lower value due to variation in the diaphragm tension or other assembly details. Each of these changes has a different impact on the ability to obtain an adequate match for directional processing.
The phase error caused by differences in &ohgr;
l
and &ohgr;
r
can be seen in FIG.
3
. This shows the phase difference between the two microphone outputs when there is a 10% shift in the low frequency rolloff and a 10% shift in the resonance frequency.
SUMMARY OF THE INVENTION
The present invention provides for matching the response of a pair of microphones.
The structure embodying the present invention is especially suitable for providing directional response. The invention provides for compensating for gain differences between the pair of microphones. Also, the invention compensates for shifts in the low frequency rolloff and resonance frequency of at least one of the microphones.
The circuitry embodying the present invention includes a pair of microphones that generate a first and a second output, respectively, in response to an audible sound. The microphone outputs are subtract from each other to produce a gain control output that operably controls the gain of the first microphone output resulting in a gain compensated microphone output. Also, a phase adjustment circuit responsive to both the gain compensated microphone output and a rolloff control output is provided to produce a matching output. The rolloff control output is generated by a phase difference subtractor circuit responsive to both the matching output and the second microphone output. Moreover, a resonance frequency shifting circuit is provided, response to the output of at least one microphone, to compensate for shifting the resonance frequency of the microphone output.
REFERENCES:
patent: 4420655 (1983-12-01), Suzuki
patent: 4509022 (1985-04-01), Ridel
patent: 4622440 (1986-11-01), Slavin
patent: 5978490 (1999-11-01), Choi et al.
patent: 6389142 (2002-05-01), Hagen et al.
patent: 0 509 742 (1992-10-01), None
patent: 59 064994 (1984-04-01), None
Grier Laura A.
Isen Forester W.
Knowles Electronics LLC
Marshall & Gerstein & Borun LLP
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