[SWLUG] Noise abatement
dmdrummondx at gmail.com
Mon Aug 31 00:18:42 UTC 2009
Would this be to insulate a house from external noises (such as
neighbours with loud stereos)? The difficulty in cancelling noise for a
large area can be demonstrated by putting two speakers a few metres
apart, playing a constant pitched sound through them, and moving from
being next to one speaker to the other. You would experience the volume
changing, from loud, to soft, to loud to soft, as you moved. This is
because the incoming wave, and the produced wave need to be exactly pi/2
out of phase to completely cancel. When you move, you effectively
change the phase shift of the two sounds from the speakers. At certain
points they cancel out (destructive interference), leading to a lower
volume, and at other points you experience constructive interference,
leading to a louder sound.
The other difficulty is in predicting the incoming waveform. For sounds
that are predictable, such as engine noises, this is easier. In the
example of unwanted noise from neighbours, the generated waveform
necessary to cancel out the noise is dependant on the incoming noise.
One way around this may be to sample the incoming noise near the
perimeter of the house, and then by calculating the time needed for the
sample taken to travel to the desired point of quiet, playing back the
generated waveform closer to the listener.
A practical working system for insulating an entire house from external
noise would make someone millions. I've had bad experiences in the past
(when I used to live in a flat with noisy neighbours on all floors,
drunk by 10am, and playing dance or rap music into the early hours), and
unwanted noise can make life miserable. If you make any progress with
this let me know, I may be your first customer!
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