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 (LpA) at distances other than one meter from the pipeline 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:

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 .