... for prison,jail room,any small room,very secretly Part of customers: Guizhou Provice prison; Dalian city jail ... )Signal Output Extent 2.5Vpp/-25dBMicrophone Electret condenserSpecial Circuit digital de-noiseProtect Circuit Over-loading protection, Lightning proofAudio Preamp ... Low noise, Low echo, clear speech, low price.*Built in digital audio disposal circuit , two step dynamic de-noise disposal.* ...