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Forum index » Science and Technology » Physics » Acoustics
Noise Control How Can Adding Noise Help Reduce Noise?
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Ken Plotkin
science forum beginner


Joined: 10 Jun 2005
Posts: 41

PostPosted: Sun May 21, 2006 12:14 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

On Thu, 18 May 2006 21:06:42 GMT, The Ghost <theghost@hotmail.com>
wrote:

Quote:

Your clarification is certainly noteworthy. However, it is my
understanding that feed-forward based systems are only effective in
cancelling periodic signals, and even then primarily with one-dimensional
wave propagation. Please correct me if I am wrong and, if possible,
provide an example in which the feed-forward approach is effective in
cancelling a non-periodic signal.

Take a look at the paper Chris Hobbs presented at Internoise 2000.
More or less 1-D propagation, and results at low frequencies, but it
was jet noise.

With Fourier synthesis, everything is periodic. 1-D matters, so that
the control mic is measuring the relevant signal.

Ken Plotkin
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Greg Locock
science forum Guru Wannabe


Joined: 28 May 2005
Posts: 111

PostPosted: Mon May 22, 2006 7:16 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Angelo Campanella <a.campanella@att.net> wrote in
news:sfRbg.136118$eR6.95823@bgtnsc04-news.ops.worldnet.att.net:

Quote:
Greg Locock wrote:
To give a practical example the group I used to work with built a
system to cancel a particular road surface induced noise in a car.
They used accelerometers on the spindles of the suspension to detect
the excitation, and a 4 microphone 2 speaker cancellation system. The
problem was at 80 Hz, which is the third acoustic cavity resonance of
the cabin. This was a random input, unlike engine noise, which we
could control up to about 400 Hz.

OK.. This is beginning to make sense....

They used to call such systems "FEED FORWARD" control systems.

Today, they give it fancy names like "Control Plant" and other
exotica. "A rose by any other name..."

It is clear that feed forward systems will only work with predictable
signals, which random road noise is not.

Feed foreword works well with periodic signals like motor vibrations,
and possibly tire noise of the tonal variety, etc. The main advantage
of feed forward is that in theory it can lead to perfect cancellation.
In practice, I think, it gets better results than the plain feedback
method where, as I have said many times, the microphone, the
loudspeaker and the ear must all be in the same place.

In feed forward systems, displacement spacings can be compensated as
phase shifts... Quite often, Feed Forward system designers speak of
"settling time" where the control system must find by successive
approximations the proper phase shift parameters, all undoubtedly now
performed by on-board digital computers.

This was a feedforward system, that's what the accelerometers were for.
The excitation was an impact, typically, and not predictable in any
fashion, so the feedforward signal was shaped by preset frequency
dependent gain and phase values to give some initial control, and the
resulting acoustic mess was then modified by the error correction mics.

It can only have worked on the initial peak (if it did) because the
electronics were faster than the mechanical transmission path.

I gather it was pretty impressive once you tuned into it, but the casual
observer would still hear the thump, and so it was not really a viable
solution.

Cheers

Greg Locock
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