Large numbers of small cetaceans are caught each year as incidental catch in gill-net fisheries around the world. The 1994 EC ASCOBANS agreement stresses the importance of reducing marine mammal bycatch, in particular of the harbour porpoise Phocoena phocoena. With the uncertainty of the severity of the problem in other fisheries it has become necessary to research into the scale of the problem in the different fishery types. Two 3-year EC (AIR DG XIV) projects have been initiated to analyse the scale of the problem in the pelagic trawls and to advise on possible methods of reducing cetacean bycatch in this fishery, one with the remit to analyse the scale of the problem in commercial fisheries (BIOECO), the other to analyse the reasons for cetacean bycatch and suggest methods of reducing it (CETASEL). As part of CETASEL, trials have been taking place both at sea with pelagic trawls and in dolphinaria in Europe. A cetacean rehabilitation centre in Neeltje Jans, Holland, has been used to examine the behaviour of a single wild harbour porpoise to different forms of acoustic disturbance produced by electronic means. The results of such tests provide valuable information as to the animal's tolerance to sound pressure levels at various frequencies and to different signatures. This information can be used further to design effective deterrents which only produce the signals which are known to deter the relevant species. The porpoise was housed in a floating net pen in minimum (tidal) of 4 metres of water providing a shallow, controlled environment in which the animal can be monitored as it re-acclimatises to the open sea. Signals introduced to the animal via four transducers produced the required aversive effect, however patterns of behaviour emerged which could not easily be explained. The porpoise did not move to the furthest point from the source, instead preferring the furthest point in line with the transducers. Mathematical modelling of the propagation of the signal in the water, showed a possible reason for the behaviour. The complicated signal pressure level pattern around the projectors showed the near field effects of the sparse array (a feature confirmed by measurements in the field), and highlighted the end-fire of the array as the single point in the enclosure where a large stable area was present. If the spacings of the projectors in the array were non integer, which was highly likely, a form of null was formed along the line of the transducers, the exact preferred position of the porpoise. This paper gives the details of the tests and results, and shows the situations modelled in post processing.