BRUEL & KJAER 4131 - Page 7

4135 • 4136 - INTRODUCTION
constant) of the microphone cartridge circuit. Referring to the schematic
diagram of Fig. 0.2 the cut-off frequency is:
fo = --------------------------
2 :n (C + C + C ) Rt Re
8
1
Rt + Re
Since the sensitivity of the microphone is determined by the relative variation
of capacity Ll C!(C + Cs + Ct), the total parallel capacity should be made
as small as possible. The first amplifier stage is therefore built in the same
housing as the microphone. A cathode follower stage providing a very great
input impedance is employed, because small parallel capacity requires high
Rt and Re in order to obtain a satisfactory low limiting frequency.
Definitions.
Free-field and Pressure Response.
T:he Free-field Response of a microphone is the ratio of the r.m.s. output
voltage to the r.m.s. sound pressure existing in the free field at the micro-
phone location with the microphone removed from the sound field.
The Pressure Response of a microphone is the ratio of the r.m.s. output
voltage to the r.m.s. sound pressure, uniformly applied over the diaphragm.
The two definitions coincide for a microphone having negligible dimensions
with respect to the sound wavelength. In the case of the B & K quarter inch
microphones this is practically fulfilled up to about 5300 Hz, *) where the
wavelength equals ten times the diameter of the microphone (i.e. 63.5 mm).
The difference in response is then less than one decibel.
Free-Field Response= ~o
Pressure Response= _!L
p,
vo~ts ------lrv L__M_icroph-one __f":::tZ:::::::::=--~ -~~ ·
Wavelength
Sound
pressure p
0
____________ ___ _
0° incidence
(62(60
Fig. 0.3. Definitions of Free-field and Pressure response.
*) Hz international
c/s used in United Kingdom and U.S.A.
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