BRUEL & KJAER 4212 User guide
Hearing Aid
Test
Box
Type
4212
A
chamber
for
acoustical
and
electro-acoustical
testing
of
all
types
of
hearing
aids
and
small
microphones
under
practically
free
field
conditions
in
the
frequency
range
150 c/s
to
5ooo
c/s.
BHUEL
&.
KJN..R
Ni!!rum, Denmark .
~
80
05
00
•
~
BRUKJA,
Copenhagen
. Telex: 5316
BB
4212
Hearing
Aid
Test
Box
Type 4212
Repri
nt
:
October
1964
Contents.
Introduction
Page
3
Descripti011 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The
Anechoic
Chamber
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Loudspeaker
and
Regulating
Microphone
. . . . . . . . . . . . . . . . . . . . . . . . 6
The
Artificial
Ear
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Power
Supply
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
A.
Adjustment
of
the
B.F.O.
Type
1022 . . . . . . . . . . . . . . . . . . . . . . . . . .
12
B.
Adjustment
of
the
A.F.
Spectrometer
Type
2112 . . . . . . . . . . . . . . . .
13
C.
Adjustment
of
the
Level
Recorder
Type
2305 . . . . . . . . . . . . . . . . . .
14
D.
Adjustment
of
the
Microphone
Amplifier
Type
2603 . . . . . . . . . . . . 16
E.
Adjustment
of
the
Desired
Sound
Pressure
Level
in
the
Test
Box
17
F.
Calculation
of
the
Acoustical
Gain
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
G.
Automatic
Recording
of
the
Frequency
Characteristic
of
the
llearing
Aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
Instruction
for
Sandfilling
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
Instructions
for
Replacing
the
2
cc
Coupler
with
the
6
cc
Couplers
. .
19
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
Automatic
Recording
of
Harmonics
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Recording
of
the
Frequency
Response
at
Different
Sound
Levels
. .
24
Visual
Check
on
Frequency
Characteristics
. . . . . . . . . . . . . . . . . . . . . .
24
Appendix 26
Some
Investigations
of
the
Acoustical
Qualities
of
the
Hearing
Aid
Test
Box
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Mutual
Influence
of
the
Sound
·waves
Reflected
from
the
Regulating
Microphone
and
the
Artificial
Ear
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Introduction
According
to
the
International
Standard
for
Testing
of
Hearing
Aids
which
was
approved
by
the
International
Electrotechnical
Commission
in
Stock-
holm,
Sweden
19,58,
the
frequency
characteristics
of
hearing
aids
should
be
measured
under
free-field
conditions,
although
most
hearing
aids
are
used
close
to
the
human
body
where
true
free-field
conditions
are
not
present.
The
decision
is
made
primarily
to
fulfill
the
demand
on
a
measuring
procedure
which
ensures
the
measurements
to
be
carried
out
under
well-
known
physical
conditions,
reproduceable
all
over
the
world.
However,
to
obtain
true
free-field
conditions
as
well
for
frequencies
in
the
higher
as
in
the
lower
end
of
the
audio
range,
very
large
and
expensive
anechoic
chambers
are
required.
To
make
the
measurements
possible
for
any
producer
of
hearing
aids
and
small
microphones
Briiel
&
Kjrer
have
developed
a
small
practical
Test
Box
Type
4212
in
close
co-operation
with
the
key
people
having
set
the
International
Standards.
The
Hearing
Aid
Test
Box
Type
4212
is
designed
basically
for
testing
hearing
aids,
but
may
just
as
well
be
used
for
testing
all
types
of
small
microphones.
Being
of
very
small
physical
dimensions,
the
Test
Box
can
of
course
not
provide
true
free-field
conditions
in
the
lower
audio
range,
but
as
will
be
seen
from
the
following
there
is
no
difference
between
the
frequency
characteristics
of
hearing
aids
and
small
microphones
measured
under
free-field
conditions
and
under
the
pressure
conditions
ruling
in
the
Test
Box
in
the
lower
a.f.
range.
Consequently,
a
hearing
aid
may
show
the
same
frequency
characteristic
when
measured
in
the
Test
Box
as
when
measured
under
true
free-field
conditions
in
a
large
anechoic
chamber.
In
Fig.
1
is
shown
a
hearing
aid
placed
in
a
free
sound
field
of
a
sound
pressure
p1
in
the
undisturbed
areas.
Owing
to
the
reflection
of
the
sound
waves
from
the
surface
of
the
hearing
aid
the
sound
pressure
increases
in
front
of
it.
The
increase
depends
on
the
ratio
between
the
wave
length
of
the
impinging
sound
and
the
physical
dimensions
of
the
hearing
aid.
At
low
frequencies
where
the
wave
length
is
large
compared
with
the
physical
dimensions
the
pressure
increase
is
negligible,
so
that
p1 = p2. At
higher
frequencies
the
difference
between
Pl
and
P2
may
amount
to
as
much
as
10
decibels.
On
the
basis
of
the
two
pressures
Pl
and
p2,
the
free-field
response
and
the
pressure
response
of
the
hearing
aid
are
defined:
3
-----------
P.a
--
----
Hearing
Aid----
Po
Artificial Ear 158!05
Fig.
1.
Illustration
of
the
sound
pressure
increase
on
the
surface
of
a
hearing
aid
when
placed
in
a free
sound
field.
The
free-field
response
of
a
hearing
aid
is
the
ratio
between
the
sound
pressure
pa
in
an
associated
artificial
ear
connected
to
the
earp
hone
of
the
hearing
aid
and
the
sound
pressure
Pl
in
the
undisturbed
free
sound
field
as
a
function
of
frequency,
and
the
pressure
response
of
a
hearing
aid
is
the
ratio
between
the
sound
pressure
pa
and
the
sound
pressure
P2
measured
directly
on
the
surface
of
the
hearing
aid
facing
the
sound
source
as
a
function
of
frequency.
In
Fig.
2 is
seen
a
typical
pressure
characteristic
and
a
typical
free-field
characteristic
of
the
same
hearing
aid.
As
can
be
seen
from
the
figure,
the
two
curves
coincide
in
the
lower
a.f.
range
but
show
a
considerable
difference
in
the
higher
end.
-
ooooooooooo
ooooooooooooooooooo
ooooo
oooooooooooooooo
BrO:!!!,Ia~r
.
~
25
~;;Q~:I!II
Iillll!lllllllilll!ll!lllllllllllilll!llllll!l!l!!llllllll
ll
Free
field
fir~
0
s7~r~
F.
response p p
z-....,:-~S..:-
Ll.A.fl'
.:
_20_t.c.Na.:
_
~
_50_o-,
__
.. '
w•.
s,..:
_
S~gr..:
__
'-"
--
'-"'"•
-
oo,.
50:
100)
-,.,.
......
I C
....
QP
0123
10
100
1000
10000
-,
••
;"
..
,.
I c
....
161092
Fig. 2.
Pressure
characteristic
(curve
P)
and
free-field
characteristic
(curve
F)
of
a
hearing
aid.
4
Description.
The
Hearing
Aid
Test
Box
Type
4212, a
sectional
view
of
which
is
seen
in
Fig. 3 is a
lacquered
mahogany
Test
Box
with
relatively
small
outside
dimensions.
It
consists
basically
of
a
small
anechoic
chamber
with
built-in
loudspeaker,
and
built-in
regulating
precision
microphone
which
enables
very
accurate
measurements
to
be
carried
out
according
to
the
Comparison
Method.
On
the
outside
of
the
Test
Box
is
mounted
an
artificial
ear
designed
in
such
a
way
that
measurements
may
be
made
both
on
hearing
aids
with
insert
types
of
earphones
and
with
built-in
earphones
(e.g.
hearing
aid
spectacles,
hair-clips
Pte.).
In
accordance
with
the
standards
the
coupler
has
a
volume
of
2 cc.
Union
nut
~
Coupler
(2cc)
----------
~
Microphone
4132
~
~
fj
Cathode
Follower
2613
-
~
Spring
arrangement
Holder
for
Perforated
Glasswool
Rockwool
Perforated
protecting
cover
Microphone
cables
Rockwool
Glasswool
springs
attenuator
Loudspeaker
terminals
f
S
tJII~
Fig. 3.
Sectional
view
of
the
Hearing
Aid
Test
Box
Type
4-212.
The
Anechoic
Chamber.
As
can
be
seen
from
Fig.
3,
the
Test
Box
consists
of
an
"inner
box
"
and
an
"outer
box"
with
lid.
The
lid
,
which
is
supplied
with
a
handle
and
a
lock
5
arrangement,
is
easily
opened
being
balanced
by
means
of
two
balancing
springs.
Inside
each
spring
is
placed
a
stick,
which
makes
the
lid
be
kept
open
in
a
suitable
position
while
the
hearing
aid
is
placed
in
the
chamber.
In
order
to
obtain
a
high
insulation
figure
for
impact
sound
and
vibrations
the
Test
Box
is
suspended
,
by
means
af
shock-absorbers,
on
a
wooden-
board
with
rubber
wheels.
By
filling
the
room
between
the
inner
and
the
outer
box
with
sand,
the
chamber
is,
furthermore,
effectively
insulated
against
airborne
noise.
(Due
to
shipment
costs
,
the
sand-filling
is
not
carried
out
at
the
factory,
but
has
to
be
done
at
the
customer's
place.
See
page
18).
The
anechoic
chamber
which
is
placed
in
the
inner
box
is
acoustically
damped
by
means
of
rockwool
covered
by
glasswool.
To
protect
the
glasswool
this
is
covered
with
perforated
protecting
plates
on
the
areas
where
it
might
otherwise
have
been
damaged.
t
70
dbG0~----+----+----~----+----+----~-7~+----+--~
~-
15801'1
Fig. 4.
Curv
es
showing:
a.
Sound
insulation
of
the
Test
Box
Type
4212
against
ail'bome
noise.
b
Frequency
response
of
the
free
sound
field
in
the
test
area
wizen tlze
regulating
Microphone
is
used.
c.
Frequency
response
of
tlze free
sound
field
in
tlze
test
area
the
regulatin!J
microphone
is
not
used.
(Tolerance:
± 7
db)
.
At
the
under-side
of
the
lid
the
glasswool
is,
however,
clad
by
cloth,
because
this
surface
is
pressed
against
the
hearing
aid
under
test.
The
position
of
the
test
object
is
in
this
way
fixed
with
respect
to
the
loudspeaker
and
the
regulating
microphone.
With
the
lid
closed
and
with
proper
sandfilling,
the
sound
insulation
of
the
Test
Box
is
as
shown
in
Fig.
4a.
Loudspeaker and Regulating Microphone.
The
loudspeaker
is
as
shown
in
Fig.
3
placed
in
the
vertical
symmetry-line
of
the
chamber
with
its
diaphragm
horizontal.
This
makes
the
direction
of
the
sound
wave
propagation
vertical
and
ensures
a
symmetrical
sound
field
at
the
test
area.
The
uniformity
of
the
sound
field
in
the
test
area,
which
is
equal
to
the
area
covered
by
cloth,
is
seen
from
Fig.
5,
and
the
best
measuring
conditions
are
seen
to
be
obtained
when
the
microphone
of
the
6
hearing
aid
is
placed
symmetrically
opposite
to
the
regulating
microphone
,
with
the
midpoint
of
the
cloth
as
the
centre
of
symmetry.
The
regulating
microphone
is
the
precision
Condenser
Microphone
Cartridge
Type
4132
with
Cathode
Follower
Type
2613.
Placed
horizontally
i.e.
at
a
right
angle
to
the
sound
propagation
the
Microphone
features
a
free
field
frequency
r
es
ponse
characteristic
which
is
flat
to
within
+1.5 - 0 dB
from
150
Hz
(c/s)
to
5ooo
Hz
(c/s) .
By
proper
use
of
the
regulating
microphone
the
measure-
ments
are
carried
out
at
a
constant
sound
pressure
as
seen
from
Fig.
4b.
It
is
furthermore
seen
that
even
without
regulating
microphone
,
the
sound
pre
s
sure
22
em
. [
c~'s
I I
soo
-
-1 -
-1250
--1600 -
10dbI
.....-
2000-
,.,...,
-2500 -
......-
-=::::
3200
~
,
----
4000
-/
~
5000
./
.........
158
018
Fig.
5.
Curves
showing
the
uniformity
of
the
sound
field
in
the
test
area.
is
fairly
constant.
This
is
obtained
by
placing
the
loudspeaker
in
the
middle
of
the
Test
Box
and
surrounding
it
with
acoustic
absorption
material.
The
lower
part
of
the
frequency
range
is,
furthermore
,
improved
by
connecting
a
capacitor
in
series
with
the
loudspeaker
and
by
using
a
low-ohmic
input
attenuator.
The
Artificial
Ear.
The
artificial
ear
is
mounted
on
the
outer
box.
It
consists
of
a
Condenser
Microphone
Cartridge
Type
4132
with
Cathode
Follower
Type
2613, a 2 cc
coupler,
and
a
mechanical
load
system.
The
Cathode
Follower
and
the
Condenser
Microphone
Cartridge,
the
pressure
characteristic
of
which
is
flat
from
150Hz
(
c/
s)
to
5oooHz
(c/s),
are
placed
in
a
hole
on
the
top
of
.
the
Test
Box.
On
the
Condenser
Microphone
is
screwed
a
coupling
piece,
which
in-
cludes
the
standardized
2
cc
coupler
and
a
removeable
cylinder
with
the
standardized
18
mm
sound
tube
(
see
Fig.
6a
).
The
cylinder
is
fixed
by
means
of
a
rubber
ring
and
a
screw
seal.
The
artificial
ear
is
fa
s
tened
to
the
Test
Box
by
means
of
an
union
nut,
which
,
furthermore,
increases
the
total
mass
of
the
coupler
to
the
standardized
200 g.
When
measurements
are
carried
out
on
insert
types
of
earphones,
the
ear
-
pho
.
ne
may
be
fastened
directly
on
top
of
the
screw
seal.
The
nub
of
the
earphone
is
then
fixed
by
means
of
the
rubber
ring.
The
earphone
may,
7
furthermore,
be
pressed
against
the
screw
seal
of
the
spring
arrangement
(see
Fig.
3}.
When
it
is
desired
to
use
a
special
ear
mould,
the
adaptor
may
be
taken
out
of
the
couplingspiece
and
the
ear
mould
inserted
instead,
Fig. 6b.
airtf9ht
rubber
seal
table
top
mountti?g
OS"mm
.qo~o
"w;re
tol/11
co,oillarq
port:.-o/1~·
pressure
e~voltsotion
coptl/ory
Q6
177m-
qoe4-"'
diameter
or
.E
cm
3
COVI'ty-
/ess
than
.!'lmm-
0,75J!i
11
greater
than
18mm
-Q71''
rnotertol
:
tyoss
f580f.f
Fig. 6a.
Sectional
view
of
the
2 cc
coupler
of
the
artificial ear.
For
hearing
aid
spectacles
where
it
is
not
possible
to
separate
the
earphone
and
the
microphone,
the
couplingspiece
and
Condenser
Microphone
Cartridge
+
Cathode
Follower
are
placed
inside
the
Test
Box
in
the
test
area.
This
is
done
by
unscrewing
the
union
nut
and
pulling
out
the
microphone
assembly,
which
can
then
easily
be
placed
in
the
chamber.
It
may
in
certain
cases
be
useful
to
remove
the
cloth
in
the
test
area,
whereby
the
spectacles
and
most
of
the
artificial
ear
may
be
placed
in
the
sound
field
remote
from
the
measuring
plane.
Se
also
p. 22.
On
page
18 is
described
how
the
Test
Box
is
taken
apart.
Furthermore,
facilities
are
provided
for
replacing
the
standardized
2 cc
coupler
with
the
standardized
6 cc
coupler
(ASA
Type
1, Z 24.9/1949J
or
the
6
cc
coupler
(NBS 9A =
ASA
Z 24.5/1951}. See
further
page
19.
8
To
reduce
the
effect
of
disturbing
signals,
a
great
amount
of
which
is
found
to
consist
of
frequencies
be
low
150Hz
(cis), a
high
pas
s
filter
is
inserted
in
series
with
the
microphone
in
the
artifical
ear.
A
smilar
filter
is
also
used
in
the
regulating
microphone
circuit
to
avoid
faulty
level
regulations
to
take
place.
The
filters
introduce
an
overall
attenuation
of
o.5
db
.
If
it
is
desired
to
use
the
artificial
ear
without
filter,
a
switch
on
the
back
of
the
Test
Box
enables
the
filter in
the
artificial ear
circuit
to
be
switched
out.
Power
Supply.
The
power
necessary
to
drive
the
Cathode
Followers
and
the
Microphone
Cartridges
is
supplied
from
the
associated
Microphone
Amplifier
via
the
multicore
cables.
The
loudspeaker
terminals
are
connected
to
the
socket
on
the
back
of
the
Test
Box.
Flexible connecti
ng
tube mounted
on
adapter.
The
adopter
is
in
this
case
replacing the ear
insert
.
2
cm
J coupler used for
free field measurement
on
spectacle-type hearing
aid
Adaptor provided
with
hole for ear insert
Example of mounting
of
earpiece
in
2cml coupler
Fig. 6b.
Typical
mounting
of
different
ear
insert
arrangements
in
the
2 cc
coupler
of
the
Artificial Ear.
The
special
adaptors
shown
are
not
produced
by
Briiel & Kjrer,
but
should
be
made
by
the
user
to
meet
his
specific
demand.
9
Operation.
In
Fig. 7 is
shown
a
complete
measuring
arrangement
for
automatic
recording
of
frequency
characteristics
of
hearing
aids.
The
loudspeaker
is
fed
from
the
BFO
the
output
of
which
is
regulated
by
means
of
the
amplified
output
voltage
from
the
regulating
microphone.
As
microphone
amplifier
is
employed
the
Microphone
Amplifier
Type
2603. By
varying
the
gain
of
this
and
adjusting
the
input
potentiometer
for
the
compressor
in
the
BFO,
the
sound
level
in
the
Test
Box
may
be
adjusted
to
any
value
between
50
and
90
db
SL.
10
210S
f6(8.t,2
Fig.
7.
Set-up
for '
automatic
recording
of
the
frequency
craracteristic
of
hearing
aids.
The
sound
level is
directly
indicated
in
db
SL, RMS
on
the
instrument.
The
output
from
the
hearing
aid
is
by
means
of
the
artificial
ear
fed
to
the
A.F.
Spectrometer,
which
directly
indicates
the
db
SL
of
the
hearing
aid
output
in
RMS
values.
By
recording
the
output
voltage
of
the
A.F.
Spectro-
meter
on
the
Level
Recorder,
the
frequency
characteristic
is
automatically
drawn
on
pre-printed
frequency
calibrated
paper.
To
adjust
the
measuring
arrangement
the
following
procedure
should
he
employed.
11
A.
Adjustment
of
the
B.F.O.
Type
1022.
1.
Adjust
the
frequency
scale
as
described
under
"operation"
item
"A"
in
the
Instruction
Manual
for
1022.
2.
The
setting
of
the
controls
should
be
made
according
to
the
follow-
ing
table.
Terminal,
Knob,
Switch
I
Position
POWER
"
On"
POWER
FREQUENCY
BEAT
Only
used
for
frequency
calibration
AUTOMATIC SCANNING
"Off"
FREQUENCY
INCREMENT
"0"
MODULATION
FREQUENCY
"Mod.
Off"
OSCILLATOR
STOP
Only
used
for
checking
apparatus
noise
levels
REMOTE
CONTROL
Plug
in
FREQUENCY
DEVIATION
Any
pos.
COMPRESSOR
SPEED
"100"
*)
FREQUENCY
SCALE
Is
adjusted
ALIGNEMENT
Only
used
for
checking
level
1ooo
c/s
REF.
SIGNAL
and
synchronization
with
paper
on
Level
Recorder
2305
OUTPUT
LEVEL
"0"
*)
At
higher
regulating
speeds
the
distortion
increases
due
to a tendency of
the
Compressor
to
regulate
on
the
wave
shape
of
the
output
signal.
This
phenomenon
is
most
pro-
nounced
at
lower
sound
pressure
levels
in
the
lower
frequency range.
12
Terminal,
Knob,
Switch
Position
COMPRESSOR
INPUT
Connected
to
bushing
marked
"Recorder"
on
2603
COMPRESSOR
VOLTAGE
Approximately
"5"
Connected
to
loudspeaker
LOAD
terminals
on
4212
by
means
of
special
cable
IMPEDANCE
SWITCH
"6£2"
r
ATTENUATOR
Any
pos.
ATTENUATOR
OUTPUT
Not
used
FREQUENCY
DIAL
1ooo
Hz
(c/s)
B. Adjustment
of
the
A.F. Spectrometer
Type
2112.
1.
Adjust
the
"Sensitivity
Condenser
Microphone"
according
to
the
sensitivity
of
the
Microphone
in
the
artificial
ear
as
described
under
"Automatic
Recording
of
Sound
Spectrograms"
in
the
Instruction
Manual
for
2112.
2.
The
setting
of
the
controls
should
be
mad
e
according
to
the
following
table.
Terminal
,
Knob,
Switch
I
Position
Connected
to
the
multicore
CONDENSER
MICROPHONE
cable
from
the
"Artificial
Ear"
on
4212
INPUT
POTENTIOMETER
Any
pos.
INPUT
SWITCH
"Condenser
Microphone"
13
Termina.l,
Knob,
Switch
I
Position
AMPLIFIER
INPUT
Not
connected
AUTOMATIC
SWITCHING
"Off"
FUNCTION
SELECTOR
"Lin.
2--40ooo
c/s"
FILTER
OUTPUT
Not
connected
RANGE
MULTIPLIER
"0
dB
(x
1)"
Connected
to
input
potentio-
RECORDER
meter
terminal
of
Level
Recorder
2305
METER
SWITCH
"Fast,
RMS"
POWER
"On"
METER
RANGE "100 dB SL, 0 dB, 10 V"
FILTER
SWITCH
Lin.
C.
Adjustment
of
the
Level
Recorder
Type
2305.
1.
A 50
db
Potentiometer
ZR 0005 is
used
..
For
·
special
purposes
another
Potentiometer
may
be
used.
2.
A
Paper
Roll
QP
1123,
QP
0123,
or
QP
0423
is
mounted
as
described
in
the
Instruction
Manual
2305.
3.
The
setting
of
the
control'i
should
be
made
according
to
the
follow-
ing
table.
Terminal,
Knob,
Switch
Position
RECTIFIER
RESPONSE
"RMS"
14
!
Terminal
, Knob,
Switch
Position
LOWER
LIMITING
FREQUENCY
"20"
POTENTIOMETER
dB RANGE "50".
If
another
Potentiometer
than
ZR 0005 is
used,
the
control
is
set
to
the
position
corresponding
to
the
db
range
of
the
Potentiometer.
WRITING
S
PEED
"100
mm/sec.
-200
mm/sec."
MOTOR "
On"
POWER
"
On"
S
TOP-START
"
Start
"
REVERSE-FORWARD
"
Forw
a
rd
"
PAPER
SPEED
"30/3
mm/sec."
DRIVE
SHAFT
SPEED
Any
Position
10 : 1
gear
ratio
Outer
Position
INPUT
POTENTIOMETER
knob
Appro
x
imately
"5"
INPUT
ATTENUATOR "
0"
INPUT
POTENTIOMETER
Connected
to
RECORDER
terminal
terminal
of
2112
15
D.
Adjustment
of
the
Microphone
Amplifier
Type
2603.
1.
Adjust
the
SENSITIVITY
CONDENSER
MICROPHONE
according
to
the
sensitivity
of
the
regulating
Microphone,
as
described
under
"Operation"
in
the
Instruction
Manual
2603.
2.
The
setting
of
the
controls
should
be
made
according
to
the
follow-
ing
table.
Terminal,
Knob,
Switch
I
Position
CONDENSER
MICROPHONE
Connected
to
the
multicore
cable
from
the
"Regulating
Micro-
phone".
INPUT
POTENTIOMETER
Any
pos.
INPUT
SWITCH
"Condenser
Microphone"
AMPLIFIER
INPUT
Not
connected
FILTER
INPUT
Not
connected
FILTER
OUTPUT
Not
connected
RANGE
MULTIPLIER
"0
dB
(x
1)"
Connected
to
the
COMPRESSOR
RECORDER
INPUT
of
1022
METER
SWITCH
"Standard
RMS"
POWER
"On"
METER
RANGE
"60
dB,
10
mV"
WEIGHTING
NETWORK
"20-20ooo
Curve
C"
16
E.
Adjustment
of
the
Desired
Sound
Pressure
Level
in
the
Test
Box.
l.
Place
the
hearing
aid
to
be
tested
on
the
cloth
in
the
anechoic
chamber
so
that
the
microphone
of
the
hearing
aid
lies
symmetrically
with
the
regulating
microphone
with
the
centre
of
the
cloth
as
the
centre
of
symmetry.
2.
Connect
the
earphone
of
the
hearing
aid
to
the
aritificial
ear
(see
page
8).
3.
Turn
the
OUTPUT
VOLTAGE
knob
of
the
BFO
to
max.
4.
Set
the
attenuator
on
the
loudspeaker
cable
to
"M"
,
5.
Turn
the
COMPRESSOR
VOLTAGE
knob
of
1022
until
the
meter
pointer
of
2603
deflects
to
the
10
db
mark
on
the
scale.
6.
According
to
the
following
table
the
desired
sound
pressure
level
in
db
re
2 · 10-4
,ubar
is
obtained.
Desired
Sound
Level
50 60 70 80 9{) 100
dB
re
2 X 10-4
,ubar
Attenuator
on
I L M M H H H
Loudspeaker
cable
I
60dB
60dB
60 dB 80
dB
80 dB
100dB
Meter
Range
10mV
10mV 10mV
100
mV
100mV
1V
2603
Range
-20
dB-10
dB
0 dB
-10dB
OdB
-10dB
Multiplier
(x o.1) (x o.3) (x
1)
(x o.3) (x
1)
(x o.3)
Note:
If
a
sound
pressure
level
which
lies
between
the
values
given
in
the
table
is
desired,
the
regulation
is
made
by
means
of
the
COMPRESSOR
VOLTAGE
knob
on
the
BFO.
The
setting
of
the
knobs
of
2603
which
gives
the
nearest
sound
level is
used.
The
correction
,
which
is
the
difference
between
this
value
and
the
desired
one
is
calculated
and
subtracted
from
the
meter
reading
of
2603
by
adjustment
of
the
COMPRESSOR
VOLTAGE
knob
of
the
BFO.
Example:
Desired
sound
pressure
level
68
db.
The
"7
0
db"
adjustment
of
2603
is
used.
By
turning
the
COMPRESSOR
VOLTAGE
knob
of
the
BFO,
the
meter
reading
of
2603 is
decreased
by
70 -68 = 2
db.
Important.
If
a
change
in
the
setting
of
the
COMPRESSOR
VOLTAGE
knob
of
the
B.F.O.
has
taken
place,
a
readjustment
according
to
item
E
is
necessary
before
the
figures
in
the
table
are
correct.
F.
Calculation
of
the
Acoustical
Gain.
The
acoustical
gain
is
equal
to
the
difference
between
the
sound
pressure
level
read
on
the
A.F.
Spectrometer
and
the
sound
pressure
level
read
on
the
Microphone
Amplifier
Type
2603.
This
is
calculated
as
follows:
17
1.
Turn
the
METER
RANGE
knob
on
the
AF
Spectrometer
until
the
meter
pointer
of
the
Spectrometer
deflects
to
a
value
between
0
and
20
db.
(If
this
cannot
be
obtained
for
any
setting,
turn
the
RANGE
MULTIPLIER
knob
of
the
Spectrometer,
until
the
condition
is
obtained).
2.
If
the
reading
so
obtained
is
between
0
and
10 dB,
turn
the
RANGE
MULTIPLIER
until
a
reading
between
10
and
20 dB
is
obtained.
With
a
setting
on
the
METER
RANGE
knob
of
the
Spectrometer
of
"a"
dB SL,
on
the
RANGE
MULTIPLIER
of
"b"
dB,
and
a
meter
reading
of
"c"
db
,
the
sound
pressure
level P2 is
P2 = a + b + c
db
re
· 10-4 ,ubar.
The
acoustical
gain
A
at
looo
c/s
is
consequently
A = P2 -P1
db,
where
P1
is
the
sound
pressure
level
in
the
Test
Box.
G.
Automatic
Recording
of
the
Frequency
Characteristic
of
the
Hearing
Aid.
1.
Adjust
the
input
Attenuator
and
the
input
Potentiometer
on
the
Level
Recorder
until
the
writing
arm
indicates
30
+ c
mm,
where
"c"
is
the
instrument
reading
of
the
Spectrometer.
The
zero
level
on
the
recording
paper
is
then
a + b -
30
db
SL.
"a"
and
"b"
are
the
db
SL-value
indicated
on
the
METER
RANGE
and
the
dB-value
indicated
by
the
RANGE
MULTIPLIER,
respectively.
2.
Scan
the
frequency
range
from
150
c/s
to
5ooo
c/s
on
the
B.F.O.
and
check
that
the
stylus
deflection
of
the
Level
Recorder
is
within
full
deflection.
If
this
is
not
the
case
,
readjust
the
knobs
RANGE
MULTIPLIER
and
METER
RANGE
of
the
Spectrometer.
Note:
One
of
the
two
positions
"0
dB (x
1)"
or
"-10
dB (x
o.3)"
on
the
RANGE
MULTIPLIER
must
be
used.
3.
Set
the
frequency
pointer
of
the
BFO
to
looo
c/s. Ref.
Signal.
4.
Adjust
the
Recording
Paper
so
that
the
writing
arm
indicates
10 c/s,
by
turning
the
grooved
finger
wheel
to
the
right
of
the
gearbox.
5.
Set
the
AUTOMATIC SCANNING
on
the
BFO
in
position
"On".
6.
Press
the
SINGLE
CHART
button
on
the
Level
Recorder.
The
frequency
range
is
now
automatically
scanned,
and
the
frequency,
characteristic
of
the
hearing
aid
drawn
on
the
recording
paper.
lnstmction
for
Sandfilling.
In
Fig.
8,
which
is
an
exploded
view
of
the
Hearing
Aid
Test
Box,
is
seen
which
rooms
should
be
filled
with
sand.
Approximately
70
kg
dry
sand
is
required,
the
total
volume
to
be
filled
being
46.3
litres.
To
take
apart
the
Test
Box
apply
the
following
procedure:
1.
Unscrew
the
bolts
marked
A
on
Fig. 8.
2.
Open
the
lid.
18
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