AudioKit FFT conversion to dB?

Question

First time posting, thanks for the great community!

I am using AudioKit and trying to add frequency weighting filters to the microphone input and so I am trying to understand the values that are coming out of the AudioKit AKFFTTap.

Currently I am trying to just print the FFT buffer converted into dB values

for i in 0..<self.bufferSize {
    let db = 20 * log10((self.fft?.fftData[Int(i)])!)
    print(db)
}

I was expecting values ranging in the range of about -128 to 0, but I am getting strange values of nearly -200dB and when I blow on the microphone to peg out the readings it only reaches about -60. Am I not approaching this correctly? I was assuming that the values being output from the EZAudioFFT engine would be plain amplitude values and that the normal dB conversion math would work. Anyone have any ideas?

Thanks in advance for any discussion about this issue!

Answer 1

You need to add all of the values from self.fft?.fftData (consider changing negative values to positive before adding) and then change that to decibels

Answer 2

The values in the array correspond to the values of the bins in the FFT. Having a single bin contain a magnitude value close to 1 would mean that a great amount of energy is in that narrow frequency band e.g. a very loud sinusoid (a signal with a single frequency).

Normal sounds, such as the one caused by you blowing on the mic, spread their energy across the entire spectrum, that is, in many bins instead of just one. For this reason, usually the magnitudes get lower as the FFT size increases.

Magnitude of -40dB on a single bin is quite loud. If you try to play a tone, you should see a clear peak in one of the bins.