Sound Meter app is shows a decibel values by measure the environmental noise, displays measured dB values in various forms.
FS (dbFS, Full Scale) where 0 dB is the maximum power that the microphone can measure, so the decibel values are negative because the measured power is less than the maximum power.
SPL (Sound pressure level) or acoustic pressure level is a logarithmic measure of the effective pressure of a sound relative to a reference value. The lower limit of audibility is defined as SPL of 0 dB, but the upper limit is not as clearly defined.
RMS (Root mean square) - the energy level of a signal is usually characterized by the square root of the mean energy and this quantity is named the RMS value of the signal function
Microphones in most phone devices are aligned to human voice. The maximum values are limited by the device. Very loud sounds(over ~90 dB) may not be recognized in most devices. So please use it as just an auxiliary tools. If you need more accurate dB values, we recommend a actual sound level meter for that.
The value of sound meter is not as precise as an actual Sound Pressure Level Meter (SPL meter) sound meter, this is due to most device's microphone are aligned to the human voice. To fix this:
1) Go to a very quiet place where sound cannot hear.
2) Remember the level of RMS
3) Put this value in "Quiet Place Level (RMS)" field and save.
4). To reset adjustment put value "0".