Acoustic Mitigation Devices (AMDs) are used to deter marine mammals from construction sites, in order to prevent hearing injury by offshore pile driving noise. To estimate the distance at which two AMDs designed as ‘seal scarers’ (Ace Aquatec and Lofitech) are detected by harbor porpoises, the 50% hearing detection thresholds for playbacks of recordings of the AMD sounds were assessed. Both became audible at a received broadband sound pressure level (SPL) of 55 dB re 1 μPa. The effect of the AMDs on porpoise behavior was quantified at three SPLs determined during a pre-test: one which just did not cause a behavioral change, one which caused a small change in surfacing and swimming pattern, and one which caused the harbor porpoise to swim away from the transducers. The corresponding mean received SPLs in the pool were respectively: Ace Aquatec: 77, 117, and 139 dB re 1 μPa; and: Lofitech: 91, 121, and 151 dB re 1 μPa. As the mean received SPL increased, greater displacement occurred, and higher numbers of surfacings and increased swimming speed occurred in test periods than in associated baseline periods. By combining these results with the source level, the current local propagation conditions and the background noise, the deterring distance at sea for the AMDs can be calculated.