Usually drum pads use Piezo or more than often cheap(and ooh the most cheapest and nifty) unidirectional microphone.

Run the signal of the the microphone to a high-pass filter avoid to have 'false trigger' comming from the rumble or low frequency sound generated by the FOH or musician monitor amplifier system.

Most drum-pad system have the ability to adjust the trigger level for the same reason as above.

HTH