Thanks. Is there any expression is available to obtain the decibel value

We can't give you an exact formula/expression because your

microphone & preamp are uncalibrated.But,

I'll give you a made-up example - Let's say your real SPL meter reads 80dB when the Arduino reads 270. 270 is your reference analog reading A

_{REF} at 80dB SPL.

Now the volume increases and we get a new analog reading of A = 540 and we can calculate the dB

*difference* or

*change*. dB = 20 log(A/A

_{REF}) = 20 log (540/270) = +6dB.

That's 6dB higher than your reference so we have 86dB SPL. So in this case the formula would be dB SPL = (20log(A/270)) + 80 ...That's the formula, not the C++ expression.

...That assumes that your '270' readings are "true" and proportional to loudness and they are unbiased (so silence reads zero, although you'll never get total silence).