BRUEL & KJAER 2204 - 6 Operating Characteristics; Directional Characteristics

6. Operating Characteristics
Directional Charactcristit·s.
Ideally a sound level meter should have the same sensitivity for sound com-
ing in from all directions . Unfortunately this can not be achieved in practice
except in the case of relatively low frequencies because of the size of the
instrument on which tht! microphone is mounted or the size and shape of
the microphone itself.
ooo oooo oo oooooo ooooooooo oo ooooooooooooooooooooo oo
l rO ~ J c. r ..
~ - ~ - ~ == ~:nl ll ll . lll:li
1000 Hz 100o0 10 100
1' ¢0 ()09
Fig. 6.1 . Frequency response of the Sound Level Meter for diflerent angles of
incidence. The instrument was equipped with microphone Type 4131, which
is normally used.
For higher frequencies, when the dimensions of the sound level meter are
comparable to the wavelength of the sound, the sound field around the
instrument will be disturbed and the pressure on the microphone diaphragm
will depend on the direction from which the sound is coming.
The directional properties of the Sound Level Meter Type 2203 are seen
from Fig. 6.1, which shows the frequency response for sounds coming in
from different angles. As shown the variation in frequency response for
varying angle of incidence is negligible for frequencies below approximately
3 kHz, while at higher frequencies the change in response is considerable .
When taking sound level measurements the operator usually knows the
source of the sound and automatically points the sound level meter in this
direction. Consequently the directional characteristics of the instrument are
not so important. Sometimes however it may be necessary to pay special
attention to these characteristics because:-
a) Noise, even from a point source, measured in a room with hard bound-
aries undergoes many reflections so that the sound field is more or less
diffuse.
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