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Natural Science Forum / Physics / Acoustics / May 2006Noise Control How Can Adding Noise Help Reduce Noise?
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. >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 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 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 > On 12 May 2006 21:00:01 -0700, "Shashi" <connect2shashi@hotmail.com> > wrote: [quoted text clipped - 11 lines] > > Bill > Hi Bill, > [quoted text clipped - 5 lines] > 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. > > Hi Bill, > > [quoted text clipped - 12 lines] > 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 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. >> > Hi Bill, >> > [quoted text clipped - 28 lines] > Best regards, > Tom 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. > snip.....snip > [quoted text clipped - 5 lines] > 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. > [quoted text clipped - 17 lines] > 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 snip.....snip >> 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 [quoted text clipped - 13 lines] > 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. > 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 >> 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. [quoted text clipped - 26 lines] > 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 > 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 >> You obviously don't understand the fundamental aspects of the >> problem. Speed/bandwith of the electronic circuitry is not the [quoted text clipped - 16 lines] > 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. > 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 >> Your clarification is certainly noteworthy. However, it is my >> understanding that feed-forward based systems are only effective in [quoted text clipped - 10 lines] > best regards > Peter I'd like to learn/know more about that approach. Do you have a link to that product? >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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