Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 157
Silencer Systems
The principal product of choice for silencing is the parallel baffle
(splitter) system where a series of silencer panels are arranged in
parallel in a duct to allow air or exhaust gas to flow between the
panels as illustrated in Figure 10-4. This passive system typically
consists of a sound absorbing material sandwiched between two perforated
facing sheets and is the most widely used method for silencing
turbine inlets and exhausts and the detailed methods of design
and analysis are available in most noise control texts.
Other advanced system designs include reactive or resonator type
chambers that are incorporated into the design of the parallel baffle
system, which offer the ability to tune to a particular frequency or
frequencies. Active noise control has also emerged over the last decade
to offer advanced noise reduction capabilities where a disturbing noise
tone can be cancelled by introducing a second tone that is 180 degrees
out of phase. When the two signals combine the result is a greatly
reduced tone or sound level. Early concepts of active noise control involved
conventional speakers, which required enormous size and power
to effectively cancel a high amplitude tone (and not practical in a hot
exhaust stream). In a basic noise cancellation system great care must
be exercised in how the system is designed, installed and operated.
Performance hinges on the three elements of noise control as shown
in Figure 10-3, any slight change to one control feature can negate the
effectiveness of active noise cancellation. However, advances in noise
cancellation technology and materials are making progress that will
greatly reduce or eliminate these restrictions and improve reliability,
which is the major concern of the end user.
Figure 10-4. Typical Parallel-Baffle Duct Silencer Configuration