Loudspeaker And Regulating Microphone - BRUEL & KJAER 4212 Instructions And Applications

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
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