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White noise generator

  1. Oct 22, 2005 #1
    Hi guys

    I'm currently working on a university experiment trying top create acoustic crystals. We're using a white noise generator to generate sounds at all frequencies.

    However I'm unsure exactly how a white noise generator creates the random signal. Can anyone explain how the an analogue (and/or computer) white noise generator works? In particularly I need to know if the generator really does play all frequencies at once, or rather randomly plays sevral from its range at high speed and changing rapidly?

    Being a physicist this sort of circuitry is a bit beyond me, and I can find nothing on the web expect schematics. Any help would be greatly appreciated.


  2. jcsd
  3. Oct 22, 2005 #2


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    You don't get exactly all frequencies at all times. So if you measured for only say a .05 sec window you would have a subset of the spectrum, but the signal would change and the next .05 sec window you include a different subset of the spectrum. So in the end you do get a signal that averages out to include the full frequency bandwidth over a moderate time window but depending on how its generated it may take a second or two.


  4. Oct 23, 2005 #3
    Okay so does that mean that a 1 to 10 second sample of white noise isn't representative of the entire audio spectrum?

    That might explain why previous students have not aquired particularly good results from using our white noise generator, and even using the noise software.

    Is there anyway to calculate how long it'd take the generator to cycle through all the frequencies in its bandwidth?


  5. Oct 23, 2005 #4


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    Once past maybe a half second, there should be a pretty accurate representation of the entire audio spectrum. And the low-frequency parts are the ones most affected since they have such a long period for their waveform.

    If you have access to a reasonably flat microphone, record in a few seconds and software like Cool Edit (now Adobe Audition) can do a frequency analysis.

    There are also many other RTA (real-time analyser) solutions although most setup for audio are going to be setup for pink noise - so with white noise they will show a gently sloping up line as the frequency gets higher (as above, white noise is equal power per freq, pink is equal power per octave which means less power at high freq to mimic human hearing).

    Cool edit can also generate the white noise and you could analyse it right in the software. Now whether or not the transducer you're using can reproduce it is another matter, low frequencies require very large amounts of air to be moved and are a challenge for all drivers at some SPL. High frequencies require very low mass and the coil of wire used in most speakers has an inductance that will really limit the uppper frequency range they can reproduce.

    Your problem may not be with the generation but the reproduction. You may be better off band-limiting your noise input to avoid causing unnecessary reproduction problems, or re-examine the transducers used.
  6. Oct 23, 2005 #5
    Well I've tried measuring the with the microphone and analysing with a fourier transform on MATLAB, we get a reasonably flat signature mid way (with a very slight slope towards) high frequencies, except we are getting noise at low frequencies high amplitude spike at lower frequencies (my guess is this is mains hum and its harmonics with some DC) noise.

    However previous experiments have hardly detected a bandgap in the crsystals (just zones of small attenutaion) and so we're trying to isolate any problem in the set up before moving to the crystals. It was considered that the generators might not have been producing a reliable frequency pattern. Since a sound sample of over a second was used it was probably something else.

    Any idea where I can find some more exact info on the white noise generator and its sampling method so I can check it out and unclude it as a reference?
  7. Oct 24, 2005 #6


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    Maybe someone else will have more info, but I don't have any more sources other than some interesting ones that came up in a google search on "white noise algorithm" that had a few examples.
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