Marine finfish aquaculture fish farm facilities can suffer severe predation from seals and other animals. Underwater transmitting commercial aquaculture acoustic devices (CAADs), intended to provide protection by deterring the close approach of seals are used in many countries. Few reliable acoustic data are available with which to assess the impact of such systems on target and non-target species in the surrounding marine environment. This paper reports an April 2003 study in which 160 kHz bandwidth measurements of source level and power spectra were carried out of three CAAD devices that are currently used in British salmonid fish farm facilities. The three devices tested employed very different signalling methods and whilst the fundamental acoustic frequencies (including harmonics) appear similar, the total energy distribution, delivered into the water column differed considerably.