Calculate decibels

Question

I'm recording mic input using the XNA library (I don't think this is really technology specific, but it never hurts). Every time I get a sample I would like to calculate the decibels. I have done many searches on the internet and not found a rock solid example...

Here is my attempt at calculating decibels from a sample:

        double peak = 0;

        for (var i = 0; i < _buffer.Length; i = i + 2)
        {
            var sample = BitConverter.ToInt16(_buffer, i);
            if (sample > peak)
                peak = sample;
            else if (sample < -peak)
                peak = -sample;
        }

        var decibel = (20 * Math.Log10(peak/32768));

If I output the decibel value to the screen I can see the values get higher as I get louder and lower as I speak softer. However, it always hovers around -40 when I'm absolutely quiet...I would assume it would be -90. I must have a calculation wrong in the block above?? from what I have read on some sites -40 is equivalent to "soft talking"...however, it's totally quiet.

Also, If I mute my mic it goes straight to -90.

Am I doing it wrong?

Answer 1

When measuring the level of a sound signal, you should calculate the dB from the RMS value. In your sample you are looking at the absolute peak level. A single (peak) sample value determines your dB value, even when all other samples are exactly 0.

try this:

double sum = 0;
for (var i = 0; i < _buffer.length; i = i + 2)
{
    double sample = BitConverter.ToInt16(_buffer, i) / 32768.0;
    sum += (sample * sample);
}
double rms = Math.Sqrt(sum / (_buffer.length / 2));
var decibel = 20 * Math.Log10(rms);

For 'instantaneous' dB levels you would normally calculate the RMS over a segment of 20-50 ms. Note that the calculated dB value is relative to full-scale. For sound the dB value should be related to 20 uPa, and you will need to calibrate your signal to find the proper conversion from digital values to pressure values.