BRUEL & KJAER 4220 User Manual

In addition, three adaptors are supplied with the Pistonphone, enabling the
opening diameter to be reduced without changing the total volume of the
cavity, for use with 1h, 14 and 1h inch diameter microphones. A special
adaptor is available on request for fitting the old B & K Microphone Type
4111 with diameter 36 mm *). The adaptors are easily mounted onto the
pistonphone by pressing them into the coupler opening and into each other.
See Fig. 7.
The coupler opening and the adaptors are fitted with a rubber ring which
ensures an airtight connection to the microphone under test. The coupler
cavity is also fitted with a capillary tube for static pressure equalization
with the pressure outside.
4220 4131/4132 + 2612 4220 4133/4134 + 2614
-:£-ElF - ~ -J - IE - ---- f-~jffi-
a. b.
Fig. 7. Mounting the different B & K microphones on the Pistonphone.
(a) "one inch" Microphone Type 4131 or 4132.
(b) "half inch" Microphone Tupe 41tJ3 or 4134.
The total volume of the cavity is th.e same in both cases.
The pistonphone can be adapted to almost ~my type of microphone, as a
suitable adaptor can easily be made, which ha~ a larger or smaller diameter.
The use of special adaptors does not offF;r any other problems than those of
external leakages.**) However, the dimt.!nsions should remain small in com-
parison with ~ in order to avoid any disturbanc(' due to wave motions in the
cavity, ( ~ in air is approximately 34 em (13") at 250 Hz, and 10 em (4") at
800 Hz), and a ratio of 1 to 3.5 between the total depth and the diameter
of the coupler is recommended in order to obtain the best possible frequency
response. ; I j : I ! jTf j1)
From the pistonphone formula it is seen that the S.P.L. produced by the
Plstonphone is inversely proportional to the volume V of the cavity. The
nominal S.P.L. of the Pistonphone corresponds to the situation where a
B & K microphone, with the normal protecting grid, is inserted. In this case
the
total volume of the cavity is 19.6 cm3, including the equivalent volume
of the microphone: o.15 cm3 for B & K Types 4131-4132. For any change in
the
total volume V, a corresponding correction must be made on the given
S.P.L.
of the Pistonphone. See "Volume Correction" in the following chapter.
*) B & K part number: DB 0386.
**) 'Vhen using the Pistonphone with some types of microphones, a slight and slow vari-
ation
in the reading of the indicating instruments may be noted. This is caused by poor
equalization between
the pressure inside these microphones and the external pressure.
10
f

BRUEL & KJAER 4220 Specifications

General

Brand	BRUEL & KJAER
Model	4220
Category	Measuring Instruments
Language	English