[SWLUG] Noise abatement

[Daniel Drummond]

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!