Noise has always been present in control valves.It is a natural side effect of the turbulence and
energy absorption inherent in control valves. This chapter will address how noise is created, why it can be a problem, and methods to attenuate noise created in control valves.
The major problem with industrial noise is its affect on humans. Companies usually build town border stations on sites remote from residential developments. Isolation, however, is not always possible, and noise prevention is a must. The U.S. Occupational Safety and Health Act
(OSHA) establishes maximum permissible noise levels for all industries whose business affects
interstate commerce. These standards relate allowable noise levels to the permissible exposure
time. Notice in table 6-1 that the maximum permissible levels depend upon the duration of
exposure. For example, the maximum sound level a person should be exposed to for an eight hour day is 90 dBA. These maximum sound levels have become the accepted noise exposure standard for most regulatory agencies. Thus, they have become the standard by which much noise generating equipment has been specified and measured .
Decibels (dB) are a measure to give an indication of loudness. The “A” added to the term indicates the correction accounting for the response of the human ear. The sensitivity of our ears to sound varies at different frequencies. Applying this “A” correction is called weighting, and the corrected noise level is given in dBA.
The A-weighting factor at any frequency is determined by how loud noise sounds to the
human ear at that particular frequency compared to the apparent loudness of sound at 1000 hertz.At 1000 hertz the A-weighting factor is zero, so if the sound pressure level is 105 dB, we say it sounds like 105 dB. On the other hand, if we listen to a sound at 200 hertz with a sound pressure level of 115 dB, it sounds more like 105 dB. Therefore, we say that the A-weighted loudness of the noise with a sound pressure level of 115 dB is 105 dBA. Essentially, if two or more sounds with different sound pressure levels and frequencies sound like the same loudness, they have the same dBA, regardless of what their individual, unweighted
sound pressure levels may be. The effect of A-weighting on control valve noise
depends upon the flowing medium since each develops its own characteristic spectrum. Noise
levels for hydrodynamic noise, or liquid flow noise, have appreciable energy at frequencies below 600 hertz. When the levels are A-weighted, it makes the low frequency terms more meaningful and the government standards somewhat more difficult to meet.
On the other hand, aerodynamic noise levels produced by steam or gas flow are the same in
either dB or dBA. This is because aerodynamic noise occurs primarily in the 1000 to 8000 hertz
frequency range. The human ear has a fairly flat response in the frequency range of 600 to 10,000 hertz, and the A-weighting factor is essentially zero in this range. Thus, there is negligible difference between the dB and dBA ratings .