From: "Michael Huffman" <[EMAIL PROTECTED]> Subject: OT Noise Cancelation
<snip> > I read some years ago a little about various different noise cancelation > technologies involving a microphone, a computer with a sound card, and a set > of speakers. The basic idea was to take in a sound wave with the microphone, > produce an opposite sound wave in the computer, and then blast it back > through the speakers aimed at the unwanted noise source to cancel the sound. > I have a bunch of computers around here that I could use, and I would welcome > the opportunity to do a project. Does anyone here know of any links to > homebrew noise cancelation solutions such as this? Speaking from a professional background in electroacoustics, I can say that in principle active noise cancellation works, but only in very specific cases is it useful. Currently Bose and some other high end headphone manufacturers offer active noise cancellation headphones, but they would be uncomfortable for sleeping, as well as needing batteries to run the noice cancellation circuits. You don't set up a microphone here and a speaker there, close a feedback loop, and get silence; you will get howling. The catch is that the wavelength of sound in air at 1 kHz is only a foot, a full 360 degree phase shift. To cancel noise, you have to produce a sound spectrum which is 180 degrees out of phase over a wide range. You cna do this inside an earphone, but not in a room. You will get the cancellation only at one frequency and one point, while may other reflection paths will give positive feedback. There have been cases where a noisy utility transformer was quiteted by active feedback by putting a speaker near the transformer radiating sound out of phase with the transformer. This cna work when the people are at a distance, and the frequency is low. If the mansion has solid walls [stone?] the nolse is coming through the window(s) and leaking through cracks. You could impose several layers of plastic [glass] with air space between over the existing window and seal around. This could reduce noise ransmission through the windows, but also any fresh air flow. Earplugs are cheaper, sorry. Mike Carrell
