Audio output level in a form that can be converted to decibel

Question

I need to find a way to get the current audio output volume while the phone is making noise on the headphones, this value will be converted to a decibel level. The android API does not appear to have any way of accessing a constant volume level other than a seemingly arbitrary volume setting level, but I dont see a way to convert that to a standard decibel level or "loudness" measurement. I have seen some ways to use the mic for this, but that wont work with headsets very well.

Does anyone know a way to measure either the maximum possible decibel (or some standard) output level to compare against, or possible the voltage being sent to the headset?

Help is welcomed.

Answer 1

Be aware that there are many different meanings of the word 'deciBel'. It is a means of representing some quantity (such as intensity/power/loudness) relative to a reference point. For audio signals inside equipment, or in an audio application, there is a peak level of 0dB. When sound is emitted from a speaker, the perceived loudness is measured as a Sound Pressure Level, often described as 'dB (SPL)' (or weighted variants such as dBA). When you see the tables of values such as rock concerts at 100dB then this is the SPL that is being described. This measurement is itself relative to a reference level.

So what will have available in the API is the buffer of audio data from which you can easily obtain the audio level in terms of the raw signal (which has a maximum of 0dB). You can't however easily convert this to a physical loudness because this will be dependent on the hardware. It will be different between one model of phone and the next, and will depend on the headphones too. The only way of doing this will be to calibrate the phone by measuring with an SPL meter, but then this will give you a result which will only give reasonable results on this particular phone.

Answer 2

I'm doing it like this:

SLmillibel gain_to_attenuation(float volume)
{
    SLmillibel volume_mb;
    if(volume>=1.0f) volume_mb=SL_MILLIBEL_MAX;
    else if(volume<=0.02f) volume_mb=SL_MILLIBEL_MIN;
    else
    {
        volume_mb=M_LN2/log(1.0f/(1.0f-volume))*-1000.0f;
        if(volume_mb>0) volume_mb=SL_MILLIBEL_MIN;
    }
    return volume_mb;
}