66
Chapter 12
Sound power – acoustic environment requirements
67
Norsonic Nor118
User Guide
For the purpose of the survey method (the method
described here), the measurement surface is considered to
lie outside the near field if the measurement distance from
the source under test is equal to or greater than 0.15 m.
Specific requirements
Examples of permitted reflecting planes outdoors include
compacted earth, artificial surfaces such as concrete or
sealed asphalt, while for indoor measurements, the
reflecting plane is usually the floor.
Take care to ensure that the reflecting surface does not
radiate any appreciable sound energy due to vibration.
e reflecting surface must be larger than the projection
of the measurement surface on it.
The sound absorption coefficient (details on this
are given in the ) of the reflecting plane should
preferably be less than
0.1
over the frequency range of
interest. is requirement is usually fulfilled when outdoor
measurements are made over concrete, sealed asphalt or
stone surfaces. For reflecting planes with higher sound
absorption coefficient, e.g. grass- or snow-covered
ground, the measurement distance shall not exceed 1 m.
For indoor measurements, wooden and tile floors are also
permitted.
No reflecting parts that are not part of the source under
test shall be located within the measurement surface.
The K
2A
factor
e environmental correction factor
K
2A
accounts for
the influence of undesired sound reflections from room
boundaries and/or reflecting object near the source under
test.
e magnitude of this factor depends principally on
the ratio of the sound absorption area
A
of the test room
to the area
S
of the measurement surface. e magnitude
does not depend strongly on the location of the source in
the test room.
In the , the environmental correction factor
K
2A
is given by
K
2A
= 10lg[1 + 4(
S/A
)] dB
where
A
is the equivalent sound absorption area in the room at
1 kHz, in m
2
S
is the measurement surface area, in m
2
.
Environmental corrections as a function of
A/S
are illus-
trated on the previous page.
Approximate method
e mean sound absorption coefficient a of the surface of
the room may be estimated using the table shown on the
next page. e value
A
is given, in m
2
, by the formula:
A = a·S
V
Calculating the A:
The value of the
mean acoustic absorption coefficient a is
estimated by using the above table. e value of A
is then given, in m
2
by
A = a
×
S
V
in which
S
V
is the
total area of the surface of the test room (walls, ceil-
ing and floor) in m
2
.
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