Introduction - BRUEL & KJAER 4212 Instructions And Applications

Introduction
According to the International Standard for Testing of Hearing Aids which
was approved by the International Electrotechnical Commission in Stock-
holm, Sweden 19, 5 8, 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:
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