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FFT and low Frequencies

  1. Jan 31, 2014 #1
    so my fft is working for notes 8b-1G# so from 7902 Hz to 51.91 Hz
    after that it says 46.25 Hz is the same 49.00Hz : notes 1F# and 1G.

    it then says notes 1E and notes 1F are the same (41.20Hz, 43.65Hz)

    I am using this site :
    http://onlinetonegenerator.com/?freq=5000

    to product the sine-wave and my roommate has a really nice surround sound system to play the Frequencies...
    I hook my ipod up to it and was able to play the sine-waves

    so is this normal ?
    I know 49.00Hz was really low...
    also how low of a Frequency can I play on a surround sound system before I misses up the speakers ?
    or my ears ?
     
  2. jcsd
  3. Jan 31, 2014 #2

    jasonRF

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    In order to answer this we need more information. What is your sample rate? How many samples were you using? Note that if you want to resolve frequencies ~2 Hz apart, then you need to use at least 0.5 seconds of data as input to your FFT.

    jason
     
  4. Jan 31, 2014 #3
    44100 sampling rate
    32768 samples at a time
     
  5. Jan 31, 2014 #4

    jasonRF

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    are you applying a taper prior to the fft?
     
  6. Feb 1, 2014 #5
    so I upload that youtube video :


    I have a filter in that i drop any Frequencies with a Magnitude less then 300000
    this is so I can cut out noise
     
    Last edited by a moderator: Sep 25, 2014
  7. Feb 1, 2014 #6

    AlephZero

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    The plots in that video look a complete mess at low frequencies. Even at high frequencies like 440 Hz you are picking up stray noise from a car outside, etc (you can see it on the FFT and hear it on the video).

    I would start by plotting several cycles of the wave in the time domain, not just the few points you have on the screen. At 49 Hz and 44100 samples/sec, there are 900 sample points in one cycle, so you want to plot a graph of 5000 or 10000 samples to see what the data looks like. (Yes I know the resolution of your computer monitor is less than 5000 pixels wide, but don't worry about that, just plot the graph.)

    Putting your microphone as close as possible to one of the loudspeakers might help.

    If you don't have something that actually looks like a sine wave with a reasonably large amplitude, the FFT won't separate the signal and the noise by magic.
     
  8. Feb 3, 2014 #7
    how do you know that At "49 Hz and 44100 samples/sec, there are 900 sample points in one cycle"?
    also If I plot 5000 samples at a time, my program will compress the graph and it would be hard to see anything, right ?
     
  9. Feb 3, 2014 #8

    AlephZero

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    I divided 44100 by 49.
    You should see about 5.5 cycles of a sine wave across the width of the graph. if you see a mess, something is wrong somewhere.
     
  10. Feb 3, 2014 #9
  11. Feb 3, 2014 #10

    AlephZero

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    There must be something wrong with the way you are reading the audio data. The plots on the video look like they are mostly noise.

    I downloaded a 49 Hz file from your tone generator site and played it in Windows Media Player. The Media Player display looks like this - a nice clean sine wave.

    attachment.php?attachmentid=66270&d=1391483481.png

    It might be a bug in your code, or a poor quality microphone and sound card on your PC, or you are working in a noisy environment.
     

    Attached Files:

  12. Feb 3, 2014 #11
    ok so no it is not a noisy environment. at the end of the video one of my roommates came in, but that is it. before that is was not a noisy environment.

    yeah there is a lot of noisy in my input .. I do not know why...
    but I have to do a filter before I output the fft to the graph ...

    I say any frequencies with a Magnitude less then 3000000 .. do not output...
     
  13. Feb 3, 2014 #12
    is that very bad?
     
  14. Feb 4, 2014 #13

    sophiecentaur

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    If you are using electronically sourced tones then why not feed them directly (electrically) into your analyser? That will eliminate all the ambient noises. When you have checked your system out this way then, by all means, use a microphone to listen to 'real instruments'. "One step at a time" is a good Experimenter's motto.
     
  15. Feb 4, 2014 #14

    ok look that this youtube
    sorry the video misses up at times, but you can see all the needed information
    I changed the setting to my Mic...
    and it looks a lot different ...
    but I do not know what is the best setting for the mic
     
    Last edited by a moderator: Sep 25, 2014
  16. Feb 4, 2014 #15

    davenn

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    that doesn't make sense

    3 million what ?


    Dave
     
  17. Feb 4, 2014 #16
    dBA ... or double dBA ... I read in the values from the Mic as doubles and pass them to the FFT
     
  18. Feb 4, 2014 #17

    sophiecentaur

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    If you have 5.5 cycles of the sin wave on your time array, then , without some intelligent windowing, you will get significant components at 10/11 (I think that's the ratio) of your 'real frequency - plus harmonics and sidebands.
     
  19. Feb 4, 2014 #18

    AlephZero

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    Looking at a still from your video
    attachment.php?attachmentid=66303&d=1391559061.png

    It looks like you have two sine waves, with different amplitudes and 90 degrees out of phase with each other, and you are taking data points alternately from each one.

    I think you need to fix that problem first.

    Possibly you are reading stereo audio data but thinking it is mono, so you are taking alternate signals from the left and right channels. That might also explain your earlier problem with your FFT frequencies being wrong by a factor of 2. But that doesn't explain how where the stereo data is coming from. Maybe the Mac audio system is somehow merging the data from the tone generator website on one channel, and the data from microphone on the other channel, and the phase difference is because distance between your loudspeaker and the mike just happens to be 1/4 of a wavelength at that frequency. But I'm not a Mac user, so I'm just guessing here.
     

    Attached Files:

  20. Feb 4, 2014 #19

    AlephZero

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    Calculating the FFT from noisy data, and then trying to ignore some of the FFT frequencies, won't help. You need to fix the reason why the data is noisy before you do the FFT.
     
  21. Feb 4, 2014 #20

    AlephZero

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    Agreed, but I don't think the OP has got as far as hitting that problem yet.
     
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