Bats use inaudible ultrasonic sounds like a sophisticated radar system to avoid obstacles and locate prey. The bat4 detector takes the inaudible high frequency ultrasound and converts it to frequencies between 100hz and 12khz which are in the range of human hearing. The detector can be tuned to receive a range of frequencies from15 to 130khz. The sounds are heard via a built in waterproof loudspeaker and there is a socket to connect standard stereo walkman type headphones (not supplied). A separate constant 'low level' output is provided for connection to a tape recorder or pc sound card. The detector uses the Heterodyne principle. Ultrasound is picked up, amplified, and mixed in a special way with another signal generated in the bat detector. Custom moulded case-compact efficient design. Front facing low distortion Mylar loudspeaker. Specially contoured microphone housing for enhanced high frequency sensitivity. 4AAA Alkaline, NiCd, or NiMh 'environmentally friendly' rechargeable cells, batteries not included. Low power design gives very long battery life. Top illuminated large diameter frequency dial. Conveniently placed output sockets on the bottom edge of the case. Constant level recorder output socket. Headphone socket which automatically mutes loudspeaker. Built in bright LED torch. Edgewise volume and frequency controls for easy operation with one hand. Wrist strap for added security and convenience when taking notes. 2 year manufacturers guarantee.