The use of active high frequency echo sounders for commercial activities and marine research has been increasing in recent years. Compared to other anthropogenic noise sources, high frequency echo sounders have received little attention in terms of their potential impacts on marine life. However, while these devices typically operate at centre frequencies outside the hearing range of most marine species, recent work has demonstrated that they may produce unintended energy at lower frequencies. These lower frequencies may extend into the audible range for several species of marine mammals and have the potential to affect their behaviour (Deng et al. 2014). Given the theoretical detectability of these lower frequencies by marine mammals, both signal types have the potential to elicit behavioural responses towards them. This should be considered in environmental impact assessments of activities using these devices and when planning marine mammal monitoring studies alongside ecosystem studies using active acoustic sonar systems.