FFT which frequencies are in which bins?

Question

I would like to see how certain frequencies, specifically low bass at 20 - 60hz are present in a piece of audio. I have the audio as a byte array, I convert it to array of shorts, then into a complex number by (short[i]/(double)short.MaxValue, 0). Then i pass this to the FFT from Aforge.

The audio is mono and sample rate of 44100. I understand I can only put chucks through the FFT at ^2. So 4096 for example. I don't understand what frequencies be in the output bins.

if I am taking 4096 samples from the audio that is at 44100 sample rate. Does this mean I am taking milliseconds worth of audio? or only getting some of the frequencies that will be present?

I add the output of the FFT to a array, my understanding is that as I am taking 4096 then bin 0 would contain 0*44100/4096 = 0hz, bin 1 would hold 1*44100/4096 = 10.7666015625hz and so on. Is this correct? or im I doing something fundamentally wrong here?

My goal would be to average the frequencies between say 20 - 60 hz, so for a song with very low, heavy bass then this number would be higher than say a soft piano piece with very little bass.

Here is my code.

OpenFileDialog file = new OpenFileDialog();
file.ShowDialog();
WaveFileReader reader = new WaveFileReader(file.FileName);

byte[] data = new byte[reader.Length];
reader.Read(data, 0, data.Length);

samepleRate = reader.WaveFormat.SampleRate;
bitDepth = reader.WaveFormat.BitsPerSample;
channels = reader.WaveFormat.Channels;

Console.WriteLine("audio has " + channels + " channels, a sample rate of " + samepleRate + " and bitdepth of " + bitDepth + ".");


short[] shorts = data.Select(b => (short)b).ToArray();

int size = 4096;
int window = 44100 * 10;
int y = 0;
Complex[] complexData = new Complex[size];
for (int i = window; i < window + size; i++) 
{
    Complex tmp = new Complex(shorts[i]/(double)short.MaxValue, 0);

    complexData[y] = tmp;
    y++;

}




FourierTransform.FFT(complexData, FourierTransform.Direction.Forward);


double[] arr = new double[complexData.Length];
//print out sample of conversion
for (int i = 0; i < complexData.Length; i++)
{
    arr[i] = complexData[i].Magnitude;

}

Console.Write("complete, ");


return arr; 

edit : changed to FFT fro DFT

Answer 1

Here's a modified version of your code. Note the comments starting with "***".

OpenFileDialog file = new OpenFileDialog();
file.ShowDialog();
WaveFileReader reader = new WaveFileReader(file.FileName);

byte[] data = new byte[reader.Length];
reader.Read(data, 0, data.Length);

samepleRate = reader.WaveFormat.SampleRate;
bitDepth = reader.WaveFormat.BitsPerSample;
channels = reader.WaveFormat.Channels;

Console.WriteLine("audio has " + channels + " channels, a sample rate of " + samepleRate + " and bitdepth of " + bitDepth + ".");

// *** NAudio "thinks" in floats
float[] floats = new float[data.Length / sizeof(float)]
Buffer.BlockCopy(data, 0, floats, 0, data.Length);

int size = 4096;
// *** You don't have to fill the FFT buffer to get valid results.  More noisy & smaller "magnitudes", but better freq. res.
int inputSamples = samepleRate / 100; // 10ms... adjust as needed
int offset = samepleRate * 10 * channels;
int y = 0;
Complex[] complexData = new Complex[size];
// *** get a "scaling" curve to make both ends of sample region 0 but still allow full amplitude in the middle of the region.
float[] window = CalcWindowFunction(inputSamples);
for (int i = 0; i < inputSamples; i++)
{
    // *** "floats" is stored as LRLRLR interleaved data for stereo audio
    complexData[y] = new Complex(floats[i * channels + offset] * window[i], 0);
    y++;
}
// make sure the back portion of the buffer is set to all 0's
while (y < size)
{
    complexData[y] = new Complex(0, 0);
    y++;
}


// *** Consider using a DCT here instead...  It returns less "noisy" results
FourierTransform.FFT(complexData, FourierTransform.Direction.Forward);


double[] arr = new double[complexData.Length];
//print out sample of conversion
for (int i = 0; i < complexData.Length; i++)
{
    // *** I assume we don't care about phase???
    arr[i] = complexData[i].Magnitude;
}

Console.Write("complete, ");


return arr;

Once you get the results, and assuming a 44100 Hz sample rate and size = 4096, elements 2 - 4 should be the values you are looking for. There's a way to convert them to dB, but I don't remember it offhand.

Good luck!