🚀 go-pugleaf

> In sci.misc Kershner Wyatt writes:
> 
>>I am trying to get any information I can concerning a new method of noise
>>reduction, which I do not know the name of at this time.  It involves
>>sampling the original sound, and playing it back 1/2 wavelength later,
>>in order to create an anti-node at the desired location (no sound). 

The original ANR (Active Noise Reduction) headset was marketed by Bose a
couple years ago. Since then, Telex and David Clark have also come out
with similar headsets. These are widely used in aviation.

Passive noise cancelling headsets were based on using a large amount of
clamping force to get a good seal around the ear. The large clamping force
makes these headsets uncomfortable to wear for long periods of time.

The intent behind the ANR headsets is to use less clamping force, sample
the noise inside the headset with a microphone, and produce an out of
phase signal through a speaker in order to reduce noise. I don't believe
the sound is 1/2 wavelength later, it's syncronized with the wave, but
is exactly out of phase.

In practice, they work very well in situations where low frequency noise
is predominent. I fly helicopters, and frequencies generated by rotors
tend to be a few Hz (13-25 would be typical). The amount of noise
reduction when you turn on the circuit is quite amazing. The headsets are
not very effective on higher frequency sounds, and use the passive
technology to dampen those frequencies.

				Paul Cantrell
-- 
cantrell@sequoia.com