An acoustic device for forming a wall of sound underwater. Walls of sound are useful in water for guiding fish away from areas. Hitherto, systems have used bubbles of water to facilitate the speed of travel of the sound. Those systems have suffered from the ability to maintain a consistent wall of bubbles and have often been inefficient. The present invention seeks to provide an acoustic device for forming a wall of sound underwater and comprises a transducer connected to an acoustic waveguide. The waveguide comprises one or more projections, whereby sound generated by the transducer travels along the projection or projections to form the wall of sound. The waveguide may typically comprise compressible projections formed from neoprene. These projections are arranged in a fan or frond shape. In addition or alternatively, the projections may be laminar. The projections could be of different lengths. The location, arrangement and lengths of the projections are all suited to the underwater location to maximise
