BRUEL & KJAER 4111 - Page 7

4
pressure of the free travelling sound wave, on account of the scat-
tering effect of the microphone at higher frequencies, dependent on
the wave length of the sound and the dimensions of the microphone.
This pressure increase for normally impinging sound waves is given
in db in fig. 4.
In order to obtain a flat response in a free sound
field, i.e. a constant electrical output of the microphone versus fre-
quency, for a constant free field sound pressure prior to the intro-
duction of the microphone in the field, the frequency characteristic
of the microphone measured under constant pressure conditions, where
a constant sound pressure is applied to the diaphragm, must decrease
for frequencies higher than Z.ooo c/s.
d6
1D
s
0
-s
-10
0
50 100
200 1000 ell 2k
5k
IOk
Fig. 4. Pressure increase on microphone diaphragm due to diffraction
around a Condenser Microphone type 4111 mounted in a free sound
field {with protective gricJ removed and with the sound wa v es i mpinging
normally).
The actuator measurement as described on page 24 for the determi-
nation of the frequency characteristic simulates such constant pres-
sure conditions.
Therefore, if that measuring procedure should be
employed for checking the microphone, fig.4 should be added alge-
braically to the resulting recorded curve in order to obtain the free