how can I get the volume in decivels of only a specific sound frequecy?

Question

i need for example get the volume of 20000 hz but not other frequencyes.

or better: get the volume of the frequencies between 19800 and 20200 by the microphone.

Answer 1

You need a pitch detection algorithm. There are many different approaches each with varying accuracy depending on the sound you're analyzing. It's a bit of a deep subject, so I would suggest reading some of the freely available whitepapers on the different kinds of algorithms. Some algorithms are proprietary, so there is no whitepaper to read for these.

Another method would be to plot a histogram in the frequency domain by applying fast Fourier transforms on the waveform in the time domain. This will graphically show the volume around your desired frequency.