BRUEL & KJAER 2221 - Page 19

The sound level meter should be pointed directly at the source if a free field condition is
assumed to exist (Fig.3.S). In a diffuse field the sound level meter may be orientated in any
oirection.
Fig. 3.5. Measurements to lEG Standards in a free sound field
ANSI Standards:
When performing measurements requ1rmg the use of a sound level meter to ANSI
standards, the "free field" microphone's characteristics should be adapted to a linear
random incidence response by mounting the Random Incidence Adaptor DZ 9566. This
applies for measurements in all types of sound field.
In a truly diffuse field, the sound level meter can be oriented in any direction. Under ideal
free field conditions optimum response is obtained by orientating the sound level meter
as shown in Fig.3.6.
Fig. 3.6. Measurement to ANSI Standards in a free sound field
Measurements not requiring the use of an IEC or ANSI sound level meter:
Measuring with a free field microphone in a diffuse field, or measuring with a linear
random incidence response microphone in a free field results in small innaccuracies in
the values obtained. More accurate results will be obtained by selecting the appropriate
m~crophone frequency response to suit the type of sound field.
Consequently, when adherence to IEC or ANSI Sound Level Meter Standards is not
required, the Random Incidence Adaptor DZ 9566 should not be mounted on the micro-
phone under free field conditions, and the meter should be pointed directly at the sound
source during measurements. A free field condition can generally be assumed for outdoor
measurements or in absorbent rooms.
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