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Noise Control How Can Adding Noise Help Reduce Noise?
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Shashi
science forum beginner


Joined: 13 May 2006
Posts: 1

PostPosted: Sat May 13, 2006 4:00 am    Post subject: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Anyone there who can answer this question. I Found This Question Lying
amidst Science fair project topics And want the answer to the question.
I'll take up the science fair project for IISEF. After launching a
coupe of gooogle searches i found this can be achived due to
destructive interference of sound. Provide me with links and useful
info about the topic.
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Ken Plotkin
science forum beginner


Joined: 10 Jun 2005
Posts: 41

PostPosted: Sat May 13, 2006 10:05 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

On 12 May 2006 21:00:01 -0700, "Shashi" <connect2shashi@hotmail.com>
wrote:

Quote:
Anyone there who can answer this question. I Found This Question Lying
amidst Science fair project topics And want the answer to the question.
I'll take up the science fair project for IISEF. After launching a
coupe of gooogle searches i found this can be achived due to
destructive interference of sound. Provide me with links and useful
info about the topic.

There are lots of people who can answer that, and if you continue
googling you should find plenty.

If you're a student planning to do an active control science fair
project, it's inappropriate to ask for startup help on a newsgroup.
Once you've put some work into learning and are confused or have
specific questions, come back.

Ken Plotkin
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Repeating Rifle
science forum Guru Wannabe


Joined: 25 Mar 2005
Posts: 205

PostPosted: Sat May 13, 2006 11:28 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

On 12 May 2006 21:00:01 -0700, "Shashi" <connect2shashi@hotmail.com>
wrote:

Quote:
Anyone there who can answer this question. I Found This Question Lying
amidst Science fair project topics And want the answer to the question.
I'll take up the science fair project for IISEF. After launching a
coupe of gooogle searches i found this can be achived due to
destructive interference of sound. Provide me with links and useful
info about the topic.

The key term is "destructive interference." Follow that! If you do not
understand what that means, you do not yet have sufficient background to
make it worth while to educate you about it.

Bill
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ttonon@peoplepc.com
science forum beginner


Joined: 30 Oct 2005
Posts: 6

PostPosted: Tue May 16, 2006 3:07 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Hi Bill,

I've also wondered about this. Is the electronic circuitry simply fast
enough to send out a pressure pules from a speaker that can quick
enough interfere with the pulse to be cancelled, which was picked up a
couple nanoseconds ago by a nearby microphone?

Best regards,
Tom


Salmon Egg wrote:
Quote:
On 12 May 2006 21:00:01 -0700, "Shashi" <connect2shashi@hotmail.com
wrote:

Anyone there who can answer this question. I Found This Question Lying
amidst Science fair project topics And want the answer to the question.
I'll take up the science fair project for IISEF. After launching a
coupe of gooogle searches i found this can be achived due to
destructive interference of sound. Provide me with links and useful
info about the topic.

The key term is "destructive interference." Follow that! If you do not
understand what that means, you do not yet have sufficient background to
make it worth while to educate you about it.

Bill
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TheGhost
science forum addict


Joined: 09 Aug 2005
Posts: 85

PostPosted: Tue May 16, 2006 3:32 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

"ttonon" <ttonon@peoplepc.com> wrote in
news:1147748873.652796.30150@u72g2000cwu.googlegroups.com:

Quote:
Hi Bill,

I've also wondered about this. Is the electronic circuitry simply fast
enough to send out a pressure pules from a speaker that can quick
enough interfere with the pulse to be cancelled, which was picked up a
couple nanoseconds ago by a nearby microphone?

Best regards,
Tom


You obviously don't understand the fundamental aspects of the problem.
Speed/bandwith of the electronic circuitry is not the problem. The
problem is the finite distince (time delay) between the reference
microphone and source of the cancellation signal. A secondary, and equally
important aspect of the problem is the fact that it is only possible to
achieve complete cancellation a single point in space.
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ttonon@peoplepc.com
science forum beginner


Joined: 30 Oct 2005
Posts: 6

PostPosted: Thu May 18, 2006 1:24 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

The Ghost wrote:
Quote:
"ttonon" <ttonon@peoplepc.com> wrote in
news:1147748873.652796.30150@u72g2000cwu.googlegroups.com:

Hi Bill,

I've also wondered about this. Is the electronic circuitry simply fast
enough to send out a pressure pules from a speaker that can quick
enough interfere with the pulse to be cancelled, which was picked up a
couple nanoseconds ago by a nearby microphone?

Best regards,
Tom


You obviously don't understand the fundamental aspects of the problem.
Speed/bandwith of the electronic circuitry is not the problem. The
problem is the finite distince (time delay) between the reference
microphone and source of the cancellation signal. A secondary, and equally
important aspect of the problem is the fact that it is only possible to
achieve complete cancellation a single point in space.

Okay, thanks for the detail. I didn't mean to imply that speed of the
electronics is a weak point, but rather only to state what I presumed
to be the basic technique, which, from what you say, seems about right.
You're right, I know little about this. Does a given distance between
mic and cancellation source define an upper frequency limit at which
cancellation will be effective? Does the location and distance of the
ear of the listener (compared to mic/c.source location and separation
distance) also influence the effectiveness of the technique? I'm
beginning to suspect that, for noise with high frequencies, this
approach would have a host of challenges. Is there some simple way to
state what kind of noises this technique works well for?

Best regards,
Tom
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Noral Stewart
science forum addict


Joined: 23 May 2005
Posts: 77

PostPosted: Thu May 18, 2006 10:55 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

The frequency limit is more related to the wavelength of the sound that is
inverse to the frequency. As you go up in frequency and the wavelength gets
shorter, it becomes more difficult to align the canceling sound out of phase
with the sound to be canceled. Cancellation is most practical in cases
where the sound is traveling down a duct or pipe or is within a small cavity
such as with headphones.

"ttonon" <ttonon@peoplepc.com> wrote in message
news:1147915463.584083.253740@i39g2000cwa.googlegroups.com...
Quote:

The Ghost wrote:
"ttonon" <ttonon@peoplepc.com> wrote in
news:1147748873.652796.30150@u72g2000cwu.googlegroups.com:

Hi Bill,

I've also wondered about this. Is the electronic circuitry simply fast
enough to send out a pressure pules from a speaker that can quick
enough interfere with the pulse to be cancelled, which was picked up a
couple nanoseconds ago by a nearby microphone?

Best regards,
Tom


You obviously don't understand the fundamental aspects of the problem.
Speed/bandwith of the electronic circuitry is not the problem. The
problem is the finite distince (time delay) between the reference
microphone and source of the cancellation signal. A secondary, and
equally
important aspect of the problem is the fact that it is only possible to
achieve complete cancellation a single point in space.

Okay, thanks for the detail. I didn't mean to imply that speed of the
electronics is a weak point, but rather only to state what I presumed
to be the basic technique, which, from what you say, seems about right.
You're right, I know little about this. Does a given distance between
mic and cancellation source define an upper frequency limit at which
cancellation will be effective? Does the location and distance of the
ear of the listener (compared to mic/c.source location and separation
distance) also influence the effectiveness of the technique? I'm
beginning to suspect that, for noise with high frequencies, this
approach would have a host of challenges. Is there some simple way to
state what kind of noises this technique works well for?

Best regards,
Tom
Back to top
TheGhost
science forum addict


Joined: 09 Aug 2005
Posts: 85

PostPosted: Thu May 18, 2006 8:00 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

"ttonon" <ttonon@peoplepc.com> wrote in
news:1147915463.584083.253740@i39g2000cwa.googlegroups.com:

snip.....snip

Quote:
Does a given distance
between
mic and cancellation source define an upper frequency limit at which
cancellation will be effective?

Yes, but for non-periodic signals it's the time delay between the mic and
the cancellation source, not the physical distance that matters. For
example, in water, sound travels faster than it does in air. Consequently,
for the same distance between the mic (hydrophone) and cancellation source,
the cancellation would work to a higher frequency in water than it would in
air.


Quote:
Does the location and distance of the
ear of the listener (compared to mic/c.source location and separation
distance) also influence the effectiveness of the technique?

There is no general black/white answer. It depends on the specific details
of the situation.


Quote:
I'm
beginning to suspect that, for noise with high frequencies, this
approach would have a host of challenges.

Your suspicion is correct.


Quote:
Is there some simple way to
state what kind of noises this technique works well for?

It works best if the offending noise is low frequency, tonal and stable
with respect to both amplitude and phase.
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Peter Weis
science forum beginner


Joined: 16 May 2005
Posts: 25

PostPosted: Thu May 18, 2006 8:44 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

The Ghost wrote:

Quote:
You obviously don't understand the fundamental aspects of the problem.
Speed/bandwith of the electronic circuitry is not the problem. The
problem is the finite distince (time delay) between the reference
microphone and source of the cancellation signal. A secondary, and equally
important aspect of the problem is the fact that it is only possible to
achieve complete cancellation a single point in space.

Please consider two things:

1. There are both feedback and feed-forward based systems. The time
delay is basically a problem in the feedback-based (microphone inside
the receiver) systems.

2. In some cases, the one point in space can be extended a little bit,
E.g. in ear canals, where the sound field is almost planar, at least for
relevant frequencies.

best regards
Peter
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TheGhost
science forum addict


Joined: 09 Aug 2005
Posts: 85

PostPosted: Thu May 18, 2006 9:06 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Peter Weis <p.weis@email.dk.slet> wrote in
news:446cdd5f$0$67255$157c6196@dreader2.cybercity.dk:

Quote:
The Ghost wrote:

You obviously don't understand the fundamental aspects of the
problem. Speed/bandwith of the electronic circuitry is not the
problem. The problem is the finite distince (time delay) between
the reference microphone and source of the cancellation signal. A
secondary, and equally important aspect of the problem is the fact
that it is only possible to achieve complete cancellation a single
point in space.

Please consider two things:

1. There are both feedback and feed-forward based systems. The time
delay is basically a problem in the feedback-based (microphone inside
the receiver) systems.

2. In some cases, the one point in space can be extended a little bit,
E.g. in ear canals, where the sound field is almost planar, at least
for relevant frequencies.

best regards
Peter


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.
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Peter Weis
science forum beginner


Joined: 16 May 2005
Posts: 25

PostPosted: Fri May 19, 2006 5:17 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

The Ghost 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.

I know of personal hearing protectors using feed-forward cancellation.
They provide something close to one-dimensional systems.
I haven't tested them thoroughly, yet. I will probably have to do so
(jobwise) in a couple of months.

best regards
Peter
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Greg Locock
science forum Guru Wannabe


Joined: 28 May 2005
Posts: 111

PostPosted: Sat May 20, 2006 10:05 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

The Ghost <theghost@hotmail.com> wrote in
news:yn4bg.105637$6g4.58364@fe05.news.easynews.com:

Quote:
"ttonon" <ttonon@peoplepc.com> wrote in
news:1147915463.584083.253740
@i39g2000cwa.googlegroups.com:

snip.....snip

Does a given distance
between
mic and cancellation source define an upper frequency
limit at which
cancellation will be effective?

Yes, but for non-periodic signals it's the time delay
between the mic
and the cancellation source, not the physical distance
that matters.
For example, in water, sound travels faster than it
does in air.
Consequently, for the same distance between the mic
(hydrophone) and
cancellation source, the cancellation would work to a
higher frequency
in water than it would in air.


Does the location and distance of the
ear of the listener (compared to mic/c.source location
and separation
distance) also influence the effectiveness of the
technique?

There is no general black/white answer. It depends on
the specific
details of the situation.


I'm
beginning to suspect that, for noise with high
frequencies, this
approach would have a host of challenges.

Your suspicion is correct.


Is there some simple way to
state what kind of noises this technique works well
for?

It works best if the offending noise is low frequency,
tonal and
stable with respect to both amplitude and phase.


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.

Cheers

Greg Locock
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The Ghost
science forum Guru Wannabe


Joined: 24 Mar 2005
Posts: 115

PostPosted: Sat May 20, 2006 8:35 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Greg Locock <greglocock@yahoo.com.au> wrote in
news:Xns97C9CC74018C577777777777777777777@211.29.133.50:


Quote:
The Ghost <theghost@hotmail.com> wrote in
news:yn4bg.105637$6g4.58364@fe05.news.easynews.com:

snip.....snip

Quote:
It works best if the offending noise is low frequency,
tonal and
stable with respect to both amplitude and phase.




Quote:
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.
Cheers
Greg Locock

That's an interesting example which falls in the grey area between an
offending pure tonal and entirely random signal. Furthermore, it is the
narrowband response (sound in resonant cabin), not the random input
(surface induced road noise), that is the object of the cancellation
scheme.
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TheGhost
science forum addict


Joined: 09 Aug 2005
Posts: 85

PostPosted: Sat May 20, 2006 8:38 pm    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Peter Weis <p.weis@email.dk.slet> wrote in
news:446d56d2$0$60786$157c6196@dreader1.cybercity.dk:

Quote:
The Ghost wrote:

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.

I know of personal hearing protectors using feed-forward cancellation.
They provide something close to one-dimensional systems.
I haven't tested them thoroughly, yet. I will probably have to do so
(jobwise) in a couple of months.

best regards
Peter


I'd like to learn/know more about that approach. Do you have a link to that
product?
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Angelo Campanella
science forum Guru Wannabe


Joined: 08 May 2005
Posts: 226

PostPosted: Sun May 21, 2006 3:36 am    Post subject: Re: Noise Control How Can Adding Noise Help Reduce Noise? Reply with quote

Greg Locock wrote:
Quote:
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.

Angelo Campanella
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