12.1 Noise cancellation, also known as anti-noise,
is the process whereby mostly low-frequency (below 500 Hz) repetitive
noises such as made by engines and rotating machinery, is cancelled
out by introducing a cancelling anti-noise signal which is equal to
but 180 degrees out of phase with the noise. This anti-noise is introduced
to the environment in a way that it matches the noise in the region
of interest. The two signals then cancel each other out, effectively
removing a significant portion of the noise energy from the environment.
12.2 Several applications for this technology
exist. They include:
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.1 Active mufflers – have been shown in
other modes of transportation to reduce exhaust noise from internal
combustion engines, compressors, and vacuum pumps without the inefficiencies
caused by back pressure.
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.2 Active mounts – these can contain vibration
from rotating machines to improve comfort, decrease wear on moving
parts, and reduce secondary acoustic noise from vibration.
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.3 Noise-cancelled quiet zones – currently
silent seats and (automobile) cabin quieting systems for various modes
of transportation exist. The possibility exists for producing active-quieted
bunks of other spaces for seafarer comfort and recovery.
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.4 Noise-cancelling headsets – these can
extend hearing protection beyond passive ear defenders to include
low frequencies. Active headsets can also allow communication, by
permitting normal conversation, and improve work place safety.
12.3 It is suggested that information concerning
experience from these active noise-reducing systems be provided to
the Organization to better evaluate the performance parameters of
these systems.