... microphone.*Low noise, Low echo, clear speech, low price.*Built in digital audio disposal circuit , two step dynamic de-noise disposal ... , 4 core phone line, network twist line, etc.*Built-in over-loading protection and lightning proof circuit.*Pre-amplifier, ... experience, widely working temperature: -25~75°C*According with CE standard.Parameter:Monitoring Range 10~35sq.mtransmiting distance(shield ...