I’ve been thinking a different control mechanism for my game,
When player taps at back side of phone, It’ll call a function.
But i need to understand how can i compare sounds or check if it’s same
You can record the sound from the microphone. See Microphone.Start, there is an example of how to acquire an audio record into a clip.
Once you’ve obtain a clip, you can read and analyse the recorded data, see AudioClip.
So you would need to apply signal processing and/or pattern recognition to detect the sound you are looking for.
This is just a lead; analyzing sound is a broad topic.