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Fig. 5:
Forms of enveloping surfaces
Enveloping surface,
enveloping surface
dimension
The link between the energy
size (sound power level) and the
field size (sound pressure level)
is established by the enveloping
surface. According to DIN
45635 the sound power level is
determined by the enveloping
surface method. To this end
the average sound pressure level is taken, at appropriate
distance, on a measuring surface enveloping the machine
(enveloping surface). As enveloping surfaces are possible
simple geometric forms like balls, cylinders, cuboids.
The mostly common enveloping surfaces in the plant
engineering are cuboids, cylinders and balls or parts of
them. The indication of the position and the distance from
the test object is an essential part when measuring the
sound pressure
The measuring surface S, or better the enveloping
surface, is the surface in [m²] enveloping the test object
and resulting from the measuring distance. The enveloping
surface dimension is defined as the tenfold decade
logarithm of the ratio that the enveloping surface bears to
the reference surface of 1m². Here, it may be noted that
only in case of a hemispherical radiation, where the noise
source is in the middle of the ball, no reflections occur on
surfaces forming the boundary. The reflections intensify
the sound pressure level on the enveloping surface. The
sound power of the machine fixed on that basis would
be incorrect and needs to be corrected. That is why a
measurement to be taken in a dead room or in a free field
would be ideal.
⎛ S
LS = 10×
lg
⎜
⎝ S0
S = 1m²
0
Issue 04 | 2010
⎞
⎟
⎠
L S
[ dB]
S[
m²]
The sound power level results, as a sum, from the
energetically taken mean of the sound pressure level
obtained over all measuring points and the enveloping
surface dimension
TRANSFER OF TECHNOLOGY
Vollkugel Halbkugel Viertelkugel Achtelkugel
L = L + L
L
W
p
n
W = { 10 × lg( ∑ n i=
1
S
1
10
Lpi
10
)} + L
From this equation can be seen at once that the sound
power level for the plant about that is talked here is always
higher than the sound pressure level. The sound power
level only becomes lower than the sound pressure level, if
the quotient resulting from the enveloping surface divided
by the reference surface of 1m² will get smaller than 1,
because the logarithm will be negatively then. But this
goes only for enveloping surfaces that are smaller than
1m², a rare fact in case of equipment used to handle bulk
material.
With the help of a small example shall be made clear the
yet important link between the three sizes sound pressure
level, sound power level and enveloping surface. On the
slewing crane, about that is talked in more detail later
on, there has been taken, at a distance of 18.5m, a sound
pressure level of Lp = 67,6 dB. Basing on a hemispherical
enveloping surface and on a sound pressure level that is
equal over the whole enveloping surface, the sound power
of the slewing crane is calculated, as follows:
From it results, according to definition, an enveloping
surface dimension of
S
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