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I've used the FFT data from the Analyser node using the getByteFrequencyData method in the Web Audio API to create a spectrum visualizer as shown below:

Spectrum Visualizer

In this instance I have 256 bins of data. What exactly do the numbers in this correspond to? Is it the decibel level of each frequency component. If so how do I know what the value of the frequency of each bin corresponds to?

I would like to know so I can experiment in building a graphic eq and so would like to know at which points to indicate the filter bands. Ideally I'd like to represent frequencies from 20Hz to 20kHz and plot intervals between those accordingly.

Thanks for any help.

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2 Answers 2

up vote 12 down vote accepted

yes,getByteFrequencyData results in a normalized array of values between 0 and 255. (it copies the data to the array it gets passed-in).

the frequency bands are split equally, so each element N of your array corresponds to:

N * samplerate/fftSize

so, the first bin is 0.
and, assuming a samplerate of 44100 and a <analyzerNode>.fftSize of 512 the second would be: 86.13 Hz, and so on...

you will find these two questions and answers useful, on dsp and on SO:

Note that the length of your sampledata is half the <analyzerNode>.fftSize, effectively limiting the frequency-range to half the samplerate.

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getByteFrequencyData doesn't return anything; rather, it copies the frequency data values (0-255) into the unsigned array (Uint8Array() in javascript) that you pass into the method as a parameter. W3C Draft – Web Audio API – getByteFrequencyData. Just in case anyone gets caught trying to use some form of returned value from this method call... –  fettereddingoskidney Mar 28 '13 at 18:31
@fettereddingoskidney, thanks, updated. –  kr1 Mar 28 '13 at 18:47
rock on! Not trying to be nit-picky :] –  fettereddingoskidney Mar 28 '13 at 19:00
BTW default sample rate may be different. E.g. for me it is 48000. One should check audioContext.sampleRate for certain value. –  kirilloid Aug 25 '13 at 3:07
For anyone looking for more information, half of the sample rate is called the Nyquist Frequency. –  Ian Hunter Jul 8 at 14:05

With 256 bins, each one will be ~86 Hz apart (44100 kHz sample rate / fftSize, where fftSize is twice the number of bins). So you start at zero and go up in 86 Hz increments from there.

The actual values in the bins are just a representation of how much of each frequency is present in the signal (i.e. how "loud" the frequency is).

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the web audio api returns a number of bins which is half the fftSize. so, IMHO you should divide by bins.length * 2 or better using fftSize directly, see also my answer. –  kr1 Feb 9 '13 at 16:17

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