Page 687 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 1. 1 Interior noise: Assessment and control

The microphone may be moved manually, or traversed If the source is located on a hard floor, and can be moved,
linearly across the room or mounted on a continuously and is at least a half wavelength from any other reflective
rotating circular boom. The number of measurement surfaces, a version of the substitution method described
locations can be reduced using a rotating diffuser in the earlier may be used to determine sound power. First the
room particularly if the source emits narrow band or tonal spatially averaged sound pressure level L p1 is obtained
noise. An accuracy of measurement of 0.5dB is over a hemispherical surface around the source. Then the
reasonable. source is moved away and replaced with a reference
The simplest method of determining sound power in source of power output L W2 , and the exact procedure is
a reverberant room is to use the substitution method repeated to obtain L p2 . Therefore, the sound power
whereby the spatially averaged sound pressure level L p output L W1 of the source is:
resulting from the source of unknown power output
level L w is compared to the spatially averaged sound L W1 ¼ L W2 þ L p1 L p2 ðdBÞ (21.1.43)
pressure level L 0 p resulting from a source of known
0
power output L . The movement of the source is vital as it allows both
w
Therefore, sets of pressure measurements to be made at the exactly
same locations and therefore with the same mix of direct
0
0
L w ¼ L þðL p L Þ (21.1.41) and reverberant fields.
w
p
If the source cannot be moved, then the substitution
The absolute method may be used as an alternative to method is excluded and hence a knowledge of the
this, whereby the sound power is obtained from the spatial distribution of direct and reverberant fields
spatially averaged sound pressure level L p and the de- must be determined before sound power estimates are
termination of the absorption characteristic of the room made.
(Ver and Holmer, 1971): The sound pressure level L p1 averaged over a portion
of a sphere around a sound source of acoustic power
L w ¼ L p þ 10 log 10 V 10 log 10 T 60 output L W1 is:

Sl D 4
þ 10 log 10 1 þ 13:9dB; re10 12 W L p1 ¼ L W1 þ 10 log þ ðdBÞ (21.1.44)
8V 10 4pr 2 R
(21.1.42)
where the directivity constant is:
where D ¼ 1 for a sphere
3
V ¼ Room volume (m ) D ¼ 2 for a hemisphere
T 60 ¼ Time taken for the sound pressure level to D ¼ 4 for a quarter of a sphere etc.
drop by 60 dB (s) and R is the room constant (Kutruff, 1979); and
2
S ¼ Surface area of the room (m )
l ¼ Wavelength corresponding to the centre fre-
Sa
quency in the analysis band (m) R ¼ (21.1.45)
1 a
21.1.3.5 Measurement of sound power where
2
in a semi-reverberant far field S ¼ total room area (m )
a ¼ average Sabine absorption in the room
Most rooms containing noise sources are neither anechoic The room constant may be taken to be the total ab-
2
nor truly reverberant. When determining the sound sorption of the room measured in units of area (m ).
power of a source in such a space no assumptions are If L p1 is obtained over a hemisphere around a refer-
made about the nature of the field (unlike before, where ence source of known sound power output L W1 , then the
the field was assumed to be either all direct or all dif- contribution to sound pressure level over the test surface
fuse). The only requirements for the room are: made by the direct field is:

1. The sound source should be in its normal position. 2
L pd ¼ L W1 þ 10 log 10 ðdBÞ
2. The room should be large enough to allow measure- 4pr 2
ments to be made in the far field. 2
1
L ¼ L W1 þ 10 log þ 10 log dB
3. The microphone should be kept at least one quarter pd 10 r 2 10 4p
wavelength away from any reflecting surface of the
room. L pd ¼ L W1 20 log r 8 dB (21.1.46)
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