Device for presenting acoustical and vibratory stimuli and method of calibration
View Patent ↗A hearing test device is capable of presenting a variety of acoustical or vibratory stimuli and can be easily calibrated. Transfer functions for the device are calculated at two different attenuation levels to derive the characteristic impedance and sensitivity of the acoustic or vibratory transducer. The predicted sound pressure level emitted by the transducer may then be calculated as a function of input signal and attenuation.
1. In a system for presenting an audio stimulus having an input, an amplifier, an attenuator and a transducer, a method of predicting a sound pressure level emitted by the transducer comprising:
applying a broadband audio signal to the input;
inserting a first attenuation between the amplifier and the transducer;
measuring a first output from the transducer;
calculating a first transfer function for a signal path from the input to the measured first output;
inserting a second attenuation between the amplifier and the transducer;
measuring a second output from the transducer, wherein the broadband audio signal applied at the input is measured simultaneously with measuring the first and second outputs from the transducer;
calculating a second transfer function for a signal path from the input to the measured second output, wherein the first and second transfer functions are calculated as respective ratios of the first and second measured outputs to the corresponding measured inputs and wherein each of the first and second transfer functions is expressed as a product of an amplifier transfer function, an attenuator transfer function, an electrical-to-acoustical transfer function and an acoustical-to-electrical transfer function;
combining the first and second transfer functions to solve for a characteristic impedance and sensitivity of the transducer;
calculating a sound pressure level emitted by the transducer as a function of input signal and attenuation
wherein the characteristic impedance Z L (f) of the transducer is calculated as:
Z
L
(
f
)
=
H
BD
(
f
,
R
1
A
,
Z
L
)
×
R
1
-
H
BD
(
f
,
R
2
,
A
,
Z
L
)
×
R
2
H
BD
(
f
,
R
2
,
A
,
Z
L
)
-
H
BD
(
f
,
R
1
,
A
,
Z
L
)
;
where: H BD (f R A , A, Z L )=H AMP (f)×H ATTN (f, R A , Z L )×H E2A (f, A, Z L )×H A2E (f)
H AMP (f) is the electrical transfer function of the amplifier
H ATTN (f, R A , Z L ) is the electrical transfer function of an attenuator
R A =output attenuator resistor value
R 1 =resistor value of first attenuation
R 2 =resistor value of second attenuation
H E2A (f, A, Z L ) is the electrical to acoustic transfer function of the transducer
H A2E (f) is the acoustic to electrical transfer function of a reference measuring microphone and preamp
A
(
f
)
=
p
0
×
10
L
SEN
(
f
)
20
p 0 =20 μPascal
L sen (f)=transducer sensitivity.
2. The method of claim 1 wherein the sensitivity of the transducer is calculated as:
A
(
f
)
=
H
BD
(
f
,
R
1
,
A
,
Z
L
)
×
[
Z
L
(
f
)
+
R
1
]
H
AMP
(
f
)
H
A2E
(
f
)
Z
L
(
f
)
.
3. The method of claim 2 wherein the sound pressure level is calculated as:
20
log
10
(
∑
f
Y
X
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
,
A
,
Z
L
)
·
H
ATTN
(
f
,
R
A
,
Z
L
)
H
ATTN
(
f
,
R
1
,
Z
L
)
2
M
×
1
H
A2E
(
f
)
·
p
0
)
where: H D2E (f) is the frequency response of a D/A converter
Y x (f)=digital spectrum of signal presented to A/D converter during normal operation.
4. The method of claim 2 wherein the sound pressure level is calculated as:
10
log
10
(
∑
f
Y
X
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
,
A
,
Z
L
)
·
H
ATTN
(
f
,
R
A
,
Z
L
)
H
ATTN
(
f
,
R
1
,
Z
L
)
∑
f
Y
XREF
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
A
,
Z
L
)
2
)
+
N
where: Y X (f)=digital spectrum of signal presented to A/D converter during normal operation
Y XREF (f)=digital spectrum of signal presented to A/D converter during calibration;
N=measured sound pressure level in dB SPL
H D2E (f) is the frequency response of a D/A converter.
5. The method of claim 2 wherein the sound pressure level is calculated as:
20
log
10
(
∑
f
Y
X
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
,
A
,
Z
L
)
·
H
ATTN
(
f
,
R
A
,
Z
L
)
H
ATTN
(
f
,
R
1
,
Z
L
)
·
H
A
-
W
(
f
)
2
M
×
1
H
A2E
(
f
)
·
p
0
)
where: H A-W (f) is the electrical transfer function of A-weighting function
M=number of frequency points in spectra
H D2E (f) is the frequency response of a D/A converter;
Y x (f)=digital spectrum of signal presented to A/D converter during normal operation.
6. The method of claim 2 wherein the sound pressure level is calculated as:
10
log
10
(
∑
f
Y
x
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
,
A
,
Z
L
)
·
H
ATTN
(
f
,
R
A
,
Z
L
)
H
ATTN
(
f
,
R
1
,
Z
L
)
·
H
A
-
W
(
f
)
2
∑
f
Y
XREF
(
f
)
·
H
D2E
(
f
)
·
H
BD
(
f
,
R
1
,
A
,
Z
l
)
2
)
+
N
H D2E (f) is the frequency response of a D/A converter;
Y x (f)=digital spectrum of signal presented to A/D converter during normal operation;
H A-W (f) is the electrical transfer function of A-weighting function;
Y xREF (f)=digital spectrum of signal presented to A/D converter during calibration.
7. The method of claim 1 wherein the transducer is an acoustic transducer.
8. The method of claim 1 wherein the transducer is a vibratory transducer.