BRUEL & KJAER 2204 - Page 33

free sound field for normal incidence of the sound waves. The calibration is
valid only when the operator has a negligible influence on the sound field at
the position of the microphone, i.e. the instrument must be held as far in
front of the operator as possible, or preferably a microphone extension cable
should be used.
120
dB SPL
110
100
90
so
70
60
50
40
30
2~0
./
.Jt..
•
20
h .
.../
.__
""""'
~
_.J
~
/"
'
I
v
"\
-~
~,.
t
)11
'\u
2203:
I: Sound Pressure Level measured
with microphone Type 4134
n: Dummy microphone
1I
l.A ...
d.
... ~_1
....
I
I I
50 100 200
l.....a.
Ia ...A .d
u&IU
1.- :r
...
:"• I
~,., .
..
IN
WY,
500 1 000 2 000 5 000 10 000
Frequency Hz
1'67082
Fig. 5.7. The Sound Level Meter exposed to a sound field of approximately
120 dB. Curve I shows the sound pressure level, and curve II shows the output
of the Sound Level Meter when the microphone was replaced with an equi-
valent impedance.
31