This new AQUATEC Duo-Alert is very compact in size. Only the best quality component is used in manufacturing of the AQUATEC Duo-Alert. The loud resonating frequency generated by this horn can be heard on the surface over a 1km. range, and underwater approximately 100 metres. Features: Horn works both UNDERWATER and on the SURFACE. Volume: Underwater 115-125 dB / Surface 123-130 dB.