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146 Gas Turbine Handbook: Principles and Practices
and reducing levels over 60 dB require careful analysis to include
duct breakout noise, flanking noise, and silencer self-flow noise to
be considered.
Noise from combustion turbines can cause a dramatic change
to the environment, particularly in rural settings and communities
if adequate noise control or mitigation is not incorporated. This can
cause negative community responses when future expansions are
needed or new industrial plants are planned. Potential environmental
noise impacts 1 must be considered at the earliest planning
stages of a project. After a project is completed, post installation of
noise control equipment frequently proves to be very expensive and
may also negatively affect turbine performance. Another area that is
sometimes overlooked is low frequency noise or infra-sound 2-4 that
can cause lightweight structures to vibrate from the low frequency
airborne noise coupling with the building or structure. This can have
a dramatic impact upon residential communities, hospitals, offices,
schools, and nearby industrial/commercial enterprises that operate
high precision equipment. The low frequency noises we are addressing
are usually associated with simple cycle exhaust systems (turbine
exhausts directly to the environment) that produce low frequency
tones (less than 40 Hz) that can propagate over large distances; low
frequency noise problems have occurred over one-mile (1.6 km) from
sites.
Recent development of microturbines for local power production
and other applications has gained interests over the last several
years. These units have the same physical properties as the larger industrial
turbines except these units typically operate at much higher
speeds. This has the advantage of producing high frequency sound
which is easier to attenuate; however, care is necessary if the turbine
blade tip speed is supersonic because it creates a sonic shock wave
causing excessively high sound levels and what is frequently termed
as “buzz-saw noise” (and can occur on all turbines). Good packaging
and silencing treatment can have these units in a backyard with
little concern over noise.
When it comes to noise control there are no simple closed form
equations that exist for “simply finding the answer.” In noise control
the process is complex and the variables quite numerous; for
instance, just in designing silencer panels there are upwards of 19
variables to be considered. Then the placement of the silencers in