The range is from -160 dB to 0 dB. You probably want to display it in a meter that goes from -90 dB to 0 dB. Displaying it as decibels is actually more useful than as a linear audio level, because the decibels are a logarithmic scale, which means that it more closely approximates how loud we perceive a sound.
That said, you can use this to convert from decibels to linear:
linear = pow (10, decibels / 20);
and the reverse:
decibels = log10 (linear) * 20;
The range for the above decibels is negative infinity to zero, and for linear is 0.0 to 1.0. When the linear value is 0.0, that is -inf dB; linear at 1.0 is 0 dB.
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