BRUEL & KJAER 1022 Instructions And Applications page 39

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Bru~jcar
• Brooi&K;-
50 25
Meosuri"'l Objoct: _ _ _
db db
Sound
transmission
40 20
recording
Without
Filtering
30 15
20 10
Roc:; No.:
Dolo:
Sign.:
Rod"
z ....
t..."
10 5
L Lim. F"
20
Poo.:
50
Wr. Sp.:
8
Popor
Sp.:
0.1
Multiply freq. Scolo
by"_ 1 - o o-
50 100 200 500 1000 2000 5000
1 0000
QP 1123
100 1000 100 00
Fig.5.18. Sound transmission recording without filtering
An instrument set up for the automatic recording of hearing aid harmo-
nics is shown in Fig.5.19.
The test environment is provided by the Type 4212 Hearing Aid Test
Box, which gives practically free-field conditions over the frequency range
150 Hz to 5 kHz. The receiver of the hearing aid under test is excited by a
loudspeaker inside the anechoic chamber of the test box. Sound pressure at
the position of the receiver is maintained constant by a regulating condenser
microphone which provides feedback to the compressor circuit of the oscil-
lator supplying the loudspeaker signal. The hearing aid output is coupled to
an artificial ear as required by the I EC (Recommendation 118). The artifi-
cial ear contains a pressure response microphone which detects the output
from the earphone. Using the Slave Filters "Output" (L.F .) and "Rejection"
outputs the respective frequency and harmonic analysis spectrograms can be
recorded on the same frequency calibrated chart using the Level Recorder.
39