Subject: Re: Sound cancellation technology?
Date: Sun, 29 Dec 2002 03:58:44 +0000 (UTC)
NNTP-Posting-Date: Sun, 29 Dec 2002 03:58:44 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.19 (i686))
In sci.electronics.design Lady Liberty wrote:
> John Woodgate said, and I quote:
>> I read in sci.electronics.design that Lady Liberty
>> wrote (in > .124.0.13>) about 'Sound cancellation technology?', on Wed, 25 Dec 2002:
>>>Fine but not my question. Could someone please answer the actual
>> OK, it is almost certainly possible, but would be very costly, not
>> necessarily in terms of equipment but in the custom DSP software
> That's a one time expense and could be made back over time and sales of the
> product. For example, if someone came up with the means to silence a
> bedroom area for sleeping, a system that could be switched on when the
> people went to sleep, and even tied into the burglar and fire alarms, so it
> would switch off if a problem came up.
In that case, assuming you've done the software, the next problem is the
To avoid nasty diffraction patterns, you want speakers all over the outside
of the room (including doors, windows, floor and ceiling) with a spacing
of a small fraction of a wavelength of the highest frequency you want to
Alternatively, you could just surround the bed with them.
Then again, if you've got a box completely surrounding the bed, you might
as well make it out of something sound deadening.
To get a rough idea of how this works, you can start simple.
Plug a speaker into the mains (probably through a transformer), put this
outside the room.
Now, try cancelling the sound using a 50Hz source, of variable phase and
It doesn't work very well, and then only over a small area.
Hmm, it's also trivial to show that for a complete room system,
if it malfunctions and focuses all the power on the person, it may actually
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