BRUEL & KJAER 1022 Instructions And Applications page 40

Measur i ng Ampl if i er
2606
BFO
1022
100 - 120 kH z
120 kHz
le ve l Recorde r
2305
Fig.5.19. Set-up for automatic recording of hearing aid harmonics
The percentage harmonic distortion can now be calculated and is defined
by:
Pr
p-x
100%
Where "Pr" is the RMS harmonic sound pressure level excluding funda-
mental and "P" is the overall RMS sound pressure level.
Alternatively this can be expressed by:
~ 2
2
+
p 3 2
+
p
4
2
+ • - • - - - • -
j
+
Harmonic Distortion= p
12
+
p
22
+
p
32
+
p
42
___ _
X
100%
where P1 =amplitude of fundamental sound pressure
Pn = amplitude of the n th harmonic.
Note: When the "Rejection" output is used for harmonic analysis the
compressor feedback voltage can be taken from the "Output" of the same
instrument, since both "Rejection" and ,;Output" circuits function simul-
taneously.
40