WARNING - Your microphone amplifier puts-out an AC audio signal (it goes positive and negative). The Arduino can be damaged by negative voltages!
Hello guys I suppose somebody here could tell me, what does it actually mean if my microphone has -38dB sensitivity
It's an incomplete spec and I don't know what it means.
Decibels need a reference. SPL (acoustic Sound Pressure Level) is always positive, and the 0dB SPL reference is (approximately) the quietest sound that can be heard.
dBV is one of the electrical measures where 0dBV is one volt. -38dBV is about 12.6 millivolts. But, without knowing what SPL level gives you an electrical signal of -38dBV, it's useless. dBV can be negative or positive, but it's usually negative.
db VU is another (older) electrical-audio dB reference. Most modern "VU meters" are not true VU meters.
Digital levels are measured in dBFS (decibels full scale) where 0dB is the "biggest number" a 16-bit or 24-bit (etc.) integer can hold, or 0dBFS is 1.0 when using floating-point numbers. In integer formats, dBFS can never go positive. With floating-point it can got positive, but it's usually negative.
and if I get some digital values that range 0-1023 (my mic does'nt recognize anything below 400)
Are you saying you're not getting any readings less than 400? That's because the Arduino's input is "floating". A 1K - 10K resistor between the Arduino's analog input and ground will "pull down" the input to zero when there's no signal from the amplifier.
But WAIT - Replace C4 with a wire, or a ~1K resistor, or just "short out" the capacitor. Without the capacitor the amplifier's output (and the Arduino's input) will be "biased" at about 2.5V (half the amplifier's supply voltage). That will prevent the signal from going negative (and you won't need the other resistor to ground)
With the 2.5V bias, silence should read about 512 and you can subtract that out in software.
can I actually calculate decibels change
dB change is calculated as 20 x log(A/Aref) where A is amplitude (voltage or your ADC reading, etc.).
For example, if your reference reads 100 and your new reading is 200, that's a +6dB change. And, if you happen to know that 100* corresponds to 70dB SPL, that means 200 is 76dB SPL And, 50 would be 6dB less than the reference or 64dB SPL.
and what is the referent point for my measuremnts? like decibel point where decibels change their amplitude.... I have this amplifier
If you want to read dB SPL, you'll need a real SPL meter to calibrate the one you're building.
COMPLICATIONS - You are sampling an audio waveform and even a constant tone or sine wave will give you "random looking" readings. You'll need to find the peaks, or find the average or RMS. (Note that the average of a sine wave is zero, so you need to average the absolute values.) Normal audio is even more "random like".
SPL measurements are even more complicated and if that's what you're trying to do, search the forum for "SPL".
- With a biased input, you'll need to subtract the bias before making any calculations.