Ideal for the bank, securities, hospital, bus station etc. where guard glass again theft is installed. - Streamline design. Main unit has internal microphone for extension. One-wire connection and simple installation. - High quality speaker for main unit and extension. High sensitive microphone. Longer metal tube microphone for dual-way intercommunication; - Adjustable volume with separate switch. - Auto dual-way intercommunication without pressing any button; - Double-track adjustment with indicating light; - Spare record jack for the bank; - External speaker, slim fit design; - Anti-interference and noise-free function; - Dual-way audio record output Specification: - Power Adapter: Input AC 110V 60Hz US Plug. Output AC 9V 1.0A; - Output Power: 2W/2W(Max); - Frequency Response: 100Hz-12kHz; - Desktop Unit Dimensions: 1588746mm; - Wall Mount Unit Dimension: 7020mm Trouble shooting: Whistle: Installation environment noise performance is poor, or too much other noise. Should make the distance larger between the host and the outside speaker or adjust the volume down.