The Artificial Ear - BRUEL & KJAER 4212 Instructions And Applications

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 esponse 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 pressure
22
em .
[
c~'s
I
I
soo -
-
1
-
-
1250
-
- 1600 -
10db
I
.....-
2000 -
,.,...,
- 2500 -
......-
-=::::
3200
~
,
- - - - 4000
-
/
~
5000
. /
.........
15 8 018
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 stened 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