13
3.3.3
Semi-Reverberant Field
This field is most common and exists whenever the sound level being measured
contains a significant amount of both reflected and direct sound waves.
3.4
Microphone Orientation
When making measurements in a free or semi-reverberant field, orient the micro-
phone so that the angle of incidence, formed by incoming sound waves and axis
of the microphone, is equal to the random incidence angle of the microphone. For
the 884-2 this angle is 70° (refer to Figure 3-2).
When making measurements in a reverberant field, orientation of the micro-
phone is not critical. However, it is recommended, where possible, that the opera-
tor use the same orientation (70°) as for the free or semi-reverberant field.
Vertical orientation of the microphone is not critical except where it is in line with
a second sound source or a high-amplitude reflection of a sound source. It usu-
ally is invalidated by a second sound source, or by a nearby reflecting surface.
When evaluating a particular sound source, avoid holding the microphone in
such a way that the reading is invalidated by a second sound source, or by the
proximity of a nearby reflecting surface.
3.5
Effect Of Operator's Presence
When sound is coming mainly from one
direction, the reading might be affected
by the relative positions of the microphone
and operator. For example, if the operator
and microphone are in line and facing
the sound source, there can be a marked
increase in response at high frequencies
because the operator will act as a reflec-
tor and produce errors of several dB at
frequencies above 100 Hz. A more uni-
form frequency response is obtained with
the Instrument extended as far as conve-
niently possible in front of the operator,
and the axis of the microphone oriented
to form an angle of incidence of 70° with
the incoming sound wave. Do not point
the Instrument toward a source of noise
other than the one being measured.
The 884-2 (or its microphone) can be
mounted on a tripod for further reduction
in the effects of the operator's presence.
A practical arrangement is to tilt the mi-
crophone axis at an angle of approxi-
mately 45° to 70° with respect to the hori-
zon. This allows the operator to step back
several feet or completely away and still
be able to make measurements.
P
L
ANE
OF
MICROPHONE
SO
U
ND
SO
U
RCE
A
X
IS
OF
MICROPHONE
INCOMIN
G
SO
U
ND
WAVE
AN
GL
E
OF
INCIDENCE
(70
˚
)
P
L
ANE
OF
MICROPHONE
A
X
IS
OF
MICROPHONE
VERTICA
L
AN
GL
E
(45
˚
–
70
˚
)
Top View
Side View
Figure 3-2. Orientation of
Microphone and Operator with
respect to Incoming and Sound
Waves
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