Low-noise directional microphone system
View Patent ↗A low-noise directional microphone system includes a front microphone, a rear microphone, a low-noise phase-shifting circuit and a summation circuit. The front microphone generates a front microphone signal, and the rear microphone generates a rear microphone signal. The low-noise phase-shifting circuit implements a frequency-dependent phase difference between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and to maintain a maximum level of noise amplification over a pre-determined frequency band. The summation circuit combines the front and rear microphone signals to generate a directional microphone signal.
1. A hearing instrument, comprising:
a front microphone that generates a front microphone signal;
a rear microphone that generates a rear microphone signal;
a phase-shifting circuit that implements a frequency-dependent phase shift between the front microphone signal and the rear microphone signal to create a controlled loss in directional gain and maintain a predetermined maximum level of microphone self noise amplification (G N ) over a pre-determined frequency band,
the frequency-dependent phase shift being configured to provide a maximum amount of directional gain given the predetermined maximum level of microphone self noise amplification (G N );
the frequency-dependent phase shift (θ LN ) including an inter-microphone phase shift (Δθ) in addition to a time-of-flight delay (θ C ), such that Δθ=θ LN −θ C ; and
wherein the frequency-dependent phase shift (θ LN ) satisfies the following equation:
θ
LN
=
-
ω
d
v
-
2
tan
-
1
[
x
sin
(
ω
d
/
v
)
/
ρ
1
-
x
ρ
cos
(
ω
d
/
v
)
]
,
where ω is the radian frequency (2πf), d is the spacing between the front and rear microphones, v is the speed of sound,
ρ
=
sin
(
ω
d
/
v
)
(
ω
d
/
v
)
,
and x is a function of the predetermined maximum level of microphone self noise amplification (G N ).
2. The hearing instrument of claim 1 , wherein x satisfies the following set of equations:
T
=
1
G
N
;
a
=
(
2
-
T
)
;
b
=
(
2
T
-
4
)
ρ
cos
(
ω
d
/
v
)
;
c
=
ρ
2
(
2
cos
2
(
ω
d
/
v
)
-
T
)
;
and
x
=
-
b
+
b
2
-
4
ac
2
a
.
3. The hearing instrument of claim 1 , wherein the time-of-flight delay (θ C ) satisfies the following equation:
θ
C
=
-
ω
d
v
-
2
tan
-
1
[
(
sin
(
ω
d
/
v
)
/
ρ
1
-
1
ρ
cos
(
ω
d
/
v
)
]
.