Reference Manual

6−2
hertz, and the A-weighting factor is essentially
zero in this range. Thus, there is negligible
difference between the dB and dBA ratings.
Sound Characteristics
Analyzing noise, in the context of piping and
control valves, requires consideration of its origin.
This indicates how the noise will propagate.
Generally speaking, noise originates from either a
line source or a point source.
A sound level meter is used to determine sound
pressure levels. Readings for line source noise
levels are normally measured one meter from the
pipe’s surface and at a point one meter
downstream from the valve outlet. Measurements
should be made in an unobstructed free field area
with no sound reflecting surfaces nearby.
Line source noise levels are radiated from the
piping in the form of an imaginary cylinder, the
pipe centerline as the axis. As you move away
from the pipeline, the sound pressure level
decreases inversely to the changes in surface
area of the imaginary cylinder. The following
equation defines the sound pressure level (L
p
A) at
distances other than one meter from the pipeline
surface:
L
p
A + F ) 10 log
1 ) r
R ) r
where,
L
p
A = sound pressure level
F = noise level at one meter from the pipe
surface
r = pipe radius in meters based on the
pipe outside diameter
R = distance in meters from the pipe
surface
What this equation tells us is that the sound
pressure level decreases dramatically as the
distance from the pipeline increases. Keep in mind
that this equation determines the noise level
radiated only by the pipeline. Other noise sources
could combine with the pipeline noise source to
produce greater overall sound pressure level.
The other type of noise source needed to be
discussed is point source. Point source noise
measurements are taken at a three meter distance
in the horizontal plane through the source. Vent
applications are typical examples of point source
noise. Point source noise levels are radiated in the
form of an imaginary sphere with the source at the
center of the sphere. As you move away from a
point source, the sound pressure level decreases
inversely in proportion to the changes in the
surface area of the imaginary sphere. The
equation that defines the sound pressure level at
distances other than three meters from the point
source and below a horizontal plane through the
point source is:
L
p
A + F ) 20 log
3
R
where,
L
p
A = the sound pressure level
F = the noise level at three meters from
the source
R = the distance in meters from the
source
This procedure determines the noise level radiated
only by the point source. Other noise sources
could combine with the point source noise to
produce a greater overall sound pressure level.
Combining Noise Sources
The noise level in a certain area is the result of
combining all of the noise generated by every
noise source in the vicinity. The methodology of
combining sources is important to prediction and
actually lies at the root of noise abatement
technology.
To determine the resultant noise level of two noise
sources, it is necessary to combine two sources of
energy. The energy, or power, of two sources
combines directly by addition. The power levels
must be calculated separately and then
logarithmically combined as one overall noise
source. The sources can be line, point, or a
combination of both. Table 6-2 simplifies the
process of combining two known noise levels.