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 outputSpecification:- Operating Voltage: DC 9V/1A, UK Plug- Output Power: 3W(Max)- Frequency Response: 100Hz-12kHz- Desktop Unit Dimensions: 158*87*46mm- Wall Mount Unit Dimension: 70*20mmTrouble 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.