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a)
Supplementary
Measurement
Transducer
Transducer
Analyzer
b)
Generator
Section
Digital memory
Analyzer
Section
2116
Measurement
Transducer
790265
Fig
.
1
.
Constant excitation level can be achieved by a) a live feedback loop or b) a pre-stored
correction curve as in the 2116
shown in Fig.1.a. The system em-
ployed in the 2116 however, avoids
possible instability in the live feed-
back loop by pre-storing a correc-
tion curve, and simplifies the meas-
urement set-up by using the meas-
urement
transducer
itself
.
See
Fig
.
1 .b.
The 2116's versatility means that
it can be used advantageously for a
wide range of applications. It may
be used with its built-in memory for
measurements requiring regulation
of the excitation signal e .g. on hear-
ing aids, microphones, and hearing
protectors, or without it to provide a
Use with Type 4222 Anechoic Test Chamber
Measurements on Hearing Aids
The evaluation of hearing aid per-
formance
is
essential,
both
for
matching the aid to a patient's hear-
ing loss characteristics and for en-
suring that it subsequently main-
tains that performance
.
Tests are
necessary
1)
at the factory as a quality con-
trol measure,
2)
at the time of fitting
3)
at regular subsequent intervals,
4)
after repair, and
5)
whenever the
user feels that
the aid is not performing ade-
quately
.
The
Audio
Test
Station
Type
211 6 is primarily intended for use
with
the Anechoic Test Chamber
Type 4222, essentially a small ane-
choic chamber with a built-in sound
source,
_
to carry out performance
tests
to
IEC
11 8
and
ANSI
S3 .22-1976. It may also be used
over a limited frequency and dy-
namic range with the earlier Type
421 2 Hearing Aid Test Box.
The following can be measured:
Swept frequency response
Swept 2nd and 3rd order harmonic
distortion
Swept 2nd and 3rd order intermodu-
lation distortion
frequency response can be obtained
over a range of test levels with the
Type 211 6 Audio Test Station
.
Swept harmonic and intermodula-
tion
distortion
measurements
are
both necessary to determine the ext-
ent of distortion which may not be
adequately revealed by simple spot
tests at a few fixed frequencies
.
The following ANSI recommended
parameters can be obtained using
the above functions:
Saturation Sound Pressure Le-
vel Frequency Response Curve
(SSPL90)
constant voltage sinusoidal output
for
tests
on
loudspeakers,
ear-
phones, or bone vibrators. More-
over it may also be used to advan-
tage just as an analyzer
/
recorder
e.g. to check production item noise
levels for quality control purposes.
Digital control from the central
logic circuit, automatic recording of
the response, and convenient con-
trol layout make the instrument par-
ticularly easy to operate, enabling
valid measurements to be made af-
ter only a short period of familiarisa-
tion.
High Frequency Average Satu-
rated
Sound
Pressure
Level
(HFAV SSPL90)
High
Frequency
Average
Full
On Gain
Frequency Response Curve at
Harmonic Distortion
Equivalent Input Noise
Before measurements are begun
the instrument must be correctly cal-
ibrated. This is easily accomplished
using Sound Level Calibrator Type
4230 or Pistonphone Type 4220
.
The preamplifier of the Type 211 6
is then adjusted until the pen indi-
cates the calibration level, i .e
.
94
The frequency response is neces-
sary to match a hearing aid to the
patient's hearing
loss. The swept
Fig .2
.
The Type 2116 in use with the Type 4222 Anechoic Test Chamber
3
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