In the search for an efficient acoustic method of reducing the bycatch of porpoises in bottom-set gillnets, the effects of presenting sounds at different frequencies and waveforms have been examined using a rehabilitated, ex- stranded, harbour porpoise as the subject in a 20 x 30 m floating net cage experiment. The animal was available for this study during the last phase of a planned programme of re-adjustment prior to its release. Low level sounds (SL<130 do re l µPa at 1m) were presented underwater using a purpose-built digital signal synthesiser with three types of output: narrow band tones; wide band frequency sweeps; and click burst sequences which replicated porpoise echolocation signal characteristics. These signals, derived from pre-programmed data stored in EPROM, could be clocked out at different frequencies by binary division of a master clock. This method produces signals with identical waveforms in a series of octave frequency steps when divided down from the 140 kHz maximum frequency. The signals were introduced into the water using four simple piezo 'bender' transducers spaced apart at 3 metre intervals starting at a corner of the net cage and extending along the longer 30 m side. Surfacing positions were subsequently extracted from the video camera images which recorded an overhead view of the pool. The animal's behaviour was studied in 20 minute periods, i.e. prior to, during and after exposure to a sound signal, and the results obtained demonstrate that certain signals caused marked and rapid movement of the animal to parts of the pool away from the transducers whereas other sounds had little or no obvious effect. Surprisingly, this animal's choice of position did not always favour the furthest corner from the transducers. However, a careful analysis of the acoustic sound pressure field shows that the complex pattern of sound caused by the four interacting sources produced nulls which the animal was apparently well able to sense and exploit. This paper demonstrates the complexity of the sound pressure level variations that must be taken into account when working in shallow water conditions, especially where constructive and destructive interference can occur between multiple sound sources and with reflecting surfaces.