Gas Turbine Handbook : Principles and Practices

154 Gas Turbine Handbook: Principles and Practices
overall A-weighted sound level or sound power level is typically used
for reporting sound levels. The A-weighted level is commonly denoted
simply as the sound level (or power level) given as a decibel level
without the suffix (A) added to dB. All sound levels are now implicitly
understood to be A-weighted unless it is defined as something else.
This simplification is acceptable for generally making a determination
as to the amount of acoustical energy a device has near the
equipment and out to a moderate distance but is limiting in determining
equipment sound levels at large distances. This is because
the atmosphere greatly attenuates the middle and high frequencies
leaving the low frequencies as the main source of noise. So, at a few
hundred meters the overall A-weighted sound level is essentially controlled
by the low frequency noise from the equipment. This is most
apparent when having to meet very low sound level requirements,
say less than 45 dB in a remote area. For this reason, octave band
sound power (preferred) or sound pressure levels are required for any
detailed noise control modeling and analysis.
NOISE CRITERION
Before applying noise control or mitigation attributes to the
turbine system we first need to know how quiet does it have to
be? Environmental sound or noise considerations in the United
States is a collection of state, county, and local laws, ordinances,
and regulations that range from well written, to well meaning, to
woefully inadequate. In many states and communities there are no
environmental noise limits and most only have a general nuisance
ordinance. The federal government (mainly through the EPA) has
provided guidelines and recommendations but there are no federal
laws regarding community noise levels as caused by land based industrial
sites except for the Federal Energy Regulatory Commission
(FERC) which regulates interstate energy transport (mainly oil and
gas pumping stations). Frequently, it is left for the site developer to
negotiate with the local permitting board or a state agency what is
acceptable for noise emissions and, in a cost driven environment, the
minimal is usually done. There are a number of organizations that
promulgate environmental noise limits 6-11 and there are two national
standards that are readily available; they are the American National