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or
1 24 dB
respectively (re
20
x
1
o-
6
Pa) on the chart. After calibra-
tion, the
microphone assembly is
laid on the test plane of the cham-
ber so that a frequency sweep can
be made to read in the correction
curve, which compensates for the
frequency characteristics of the test
chamber
.
At
each
frequency
during
the
READ IN process the loudspeaker
output is automatically regulated in
0,3 dB steps until a level of 90 dB
is reached
.
The attenuation neces
-
sary to achieve this level at each fre
-
quency is then stored in a digital
memory
.
During this operation the pen is
raised
,
but indicates progress by fol
-
lowing the frequency along the top
of the chart
.
The instrument is now ready to
make measurements on a hearing
aid
,
which is connected to the mic
-
rophone via a standard 2 cm
3
cou-
pler. A suitable coupler for this pur-
pose is the Type DB 0138, which
meets both IEC and ANSI stand
-
ards. All the items are then placed
10/2/7
9
l:f.EA AING
AID
rype
C-2
RE
M
A
R
K$
J.R.H.
on the supporting net in the test
chamber with the hearing aid in the
same position that the measuring
microphone originally occupied dur-
ing
recording
of
the
correction
curve, as shown in Fig .3
.
Fig.3. Hearing aid to be tested, coupler, and
measuring microphone in place in the
test chamber
The required measurement is car
-
ried out automatically at the se
-
lected level, and the response re
-
corded on the chart, each measure
-
ment preceded by indication of level
and
parameter
chosen.
Typical
measurement charts for a hearing
aid worn behind the ear are repro
-
duced in Fig.4
.
The different meas
-
urements are labelled for clarity on
the figure but would normally be dis
-
tinguished by using pens of
differ-
ent colours.
QATE
10/2/79
HEARING
Am r
ype
C-2
REMAR
KS
J
. R.
H.
dB
1
1
!
6
"f
uo ,....-r--~-,---f--+..,....-t-,--,
..
~
::!!!:
I
I
8
s
t3o
f-+--+---+--+----t--+--t-t--H
:1t
75
~
120
/ '
!
I
Measurement of the frequency re-
sponse of microphones
The frequency response of small
microphones, such as those used in
telephone or communications hand-
sets
,
can be measured simply using
the Type 211 6 and a suitable small
anechoic test chamber such as the
Type 4222
.
A correction curve to
ac-
count for the frequency characteris
-
tics of the test chamber is first re
-
corded in the memory of the 211 6
using
a
Measuring
Microphone
Type 4144 (one inch) or 4134 (
1/
2
inch)
.
During the subsequent test
,
the sound level in the measuring
plane of the test chamber is kept
constant by continuous reference to
this stored correction curve
,
as for
the testing of hearing aids
.
The microphone to be tested is
then placed in the same position in
the chamber
,
in place of the mea
-
suring
microphone,
and
its
fre
-
quency response recorded directly
on
the
chart.
The frequency re
-
sponse of a typical monitoring micro
-
phone produced by this method is
reproduced in Fig .6
.
- -
..
,
,,.
1'
14
0
I
I
130
- f -
/ '
I
I
120
/
l\
--1'\
70
I
N
PUT S PL
110
Freq. Resp.
....--
~nd
H
l
I
~
INP
uT •• ,
1
,
0
,
00
_
1
Freq.
~es/
/
A'i'
\ .
\
CALIB
R
ATE
---eo
t
OO
CALIBRATE
r---so-
~
g
4
dB
---..--
~
M
ODE
00
/
Dis:.rm.
\f.-
I
1
\
----so
(
r
!'--"-"'
N\Vi
\11
-
I
;;;
-
••••
f--;s
I
I
Moo•
oo
/2nd
Int.
V
J1
~
I
IJ\i'\\
~ ~
./
Dist
..
/
N
1
1
\\A
~.·,·
.
q :
eo
r
IN
HAR MONIC
{
~
~
HAR MO
N
IC
{
r-;;.;-
70
~\cJtJ
_ /y
_
f-/-;:;T'-/"'
-j~
-
J
-
f-+--+!
+-+!-+--\\'v~ft-+H
~
60
.I-"
Jr~
In
~.
1
Di,st.
-1-\
INTERMODULATION
{
~
:0
0,250
00
Other Applications
Frequency response of Bone Vibra-
tors
Using the set -up of Fig. 5 it is pos
-
sible to make rapid tests on a bone
vibrator, in order to check its charac
-
teristics against a standard curve
for
quality
control
purposes,
or
against a previous measurement in
order to check on its performance in
use. A constant voltage signal is ob
-
tained from the generator section of
4
INTERMODULATION
{
~
0
0
780723
Fig.4. Complete set of hearing aid test curves
the 211 6 to drive the bone vibrator
which is mounted on the Artificial
Mastoid
Type
4930.
The
output
from the transducer built into the
Artificial Mastoid is fed to the anal-
yzer section of the instrument
.
The
resulting response curve is recorded
automatically on the chart as be
-
fore
.
A typica
l
response curve is re
-
produced in Fig .7.
80
l-u/ 0\.
~r
""
\(\
\
70
yv
I
)
"
v
I
v:
1
\
00
r
I
~
Yl
50
•
n
f
J rd
Harm.
Dist.
'I
0,250
Biu
e
l
I
KJ
•
•
N• •um
,
Oe"m•
••
0
0
•
Generator
Output
Audio Test Station
Type 2116
780724
Bone vibrator
under test
Artificial
Mastoid
Type 4930
790050
Fig .5
.
Arrangement using Audio Test Sta-
tion and Artificial Mastoid Type 4930
for testing bone vibrators
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