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Noise Cancellation !



The latest crop of turbo-prop airplanes are reported to be using noise
cancellation techniques on the surface of the fuselage that are adjacent
to the propellers.

The story goes that each time a prop tip swings by the cabin wall, it
produces a powerful pulse of acoustic and sub-acoustic (very low
frequency) noise that is easily transferred through the wall and into
the passenger cabin.

By implanting many (they didn't say how many) microphones in the wall
and by using a technique that was not described, an accurately
proportioned, negative image of that pulse is induced into the wall
which nearly perfectly cancels the incoming wave front. The sound
level inside of the new type aircraft is reportedly lower than that of a
conventional jet.

As these pulses must be in the thousands of watts level, I would be
very interested in getting the details about the type of transducer that
is used in the cancellation process and what their frequency response is
at both ends of the spectrum.

Also as this is a wide band, 100% feedback system and the microphones
are apparently submerged in both sound fields, considerable finesse must
be required to prevent system oscillation at certain frequencies.

Any one who has ever ridden near the prop path zone in a turbo prop
plane knows how the ultra low frequency pulses affect your disposition
after just a short time.

For several years there have been noise canceling earphones on the
market. They use this same principle to deliver a "quiet zone" inside
of the headphone. They are however, very expensive in my opinion.

Bill