Android How To Find Decibels

Question

I'm trying to get the decibels from the microphone and have looked everywhere how to correctly do it but they don't seem to work.

I get the amplitude like this

public class SoundMeter {
static final private double EMA_FILTER = 0.6;

private MediaRecorder mRecorder = null;
private double mEMA = 0.0;

public void start()  {
    if (mRecorder == null) {
        mRecorder = new MediaRecorder();
        mRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
        mRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
        mRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
        mRecorder.setOutputFile("/dev/null/");

        try {
            mRecorder.prepare();
        } catch (IllegalStateException | IOException e) {
            e.printStackTrace();
        }
        mRecorder.start();
        mEMA = 0.0;
    }
}

public void stop() {
    if (mRecorder != null) {
        mRecorder.stop();
        mRecorder.release();
        mRecorder = null;
    }
}

public double getTheAmplitude(){
    if(mRecorder != null)
        return (mRecorder.getMaxAmplitude());
    else
        return 1;
}
public double getAmplitude() {
    if (mRecorder != null)
        return  (mRecorder.getMaxAmplitude()/2700.0);
    else
        return 0;

}

public double getAmplitudeEMA() {
    double amp = getAmplitude();
    mEMA = EMA_FILTER * amp + (1.0 - EMA_FILTER) * mEMA;
    return mEMA;
}

}

Then In my other activity I call the getAmplitude method an it returns the amplitude.To convert it to decibels I use this:

dB =  20 * Math.log10(soundMeter.getAmplitude() / 32767);

Ive tried many different values in place for the 32767 but none of them seem to give me a realistic decibel answer. It's usually negative and sometimes -infinity. Please help if you know how to find decibels the right way.

Answer 1

getMaxAmplitude returns a number between 0 and 32767. To convert that to dB you need to first scale it to to a value between 0 and -1. 20*log10(1)==0 and 20*log10(0)==-inf.

If you're getting -inf then this can only be because you're passing 0 to the log function. This is most likely because you are doing integer division. Change the denominator to a double to force a floating point division.

double dB = 20*log10(x / 32767.0);