To determine whether sounds from the FaunaGuard Porpoise Module or Acoustic Porpoise Deterrent-01 (APD-01) can deter harbor porpoises (Phocoena phocoena) far enough away from an offshore pile driving site to prevent hearing damage in the form of permanent hearing threshold shift (PTS) due to the first and following strikes, a harbor porpoise in a pool was exposed to the sounds at seven mean received sound pressure levels (SPLs; range: 74 to 110 dB re 1 μPa). The mean received behav-ioral response threshold SPL of a harbor porpoise responding to the sounds and an acoustic dose-behavioral response relationship were established. Two behavioral parameters were recorded during test and control sessions: (1) the harbor porpoise’s respiration rate and (2) its distance from the transducer. Compared to in the control periods, the harbor porpoise’s respiration rate increased significantly in test sessions at mean received SPLs of 104 dB re 1 μPa and above. The harbor porpoise’s distance from the transducer was significantly greater during test sessions than during control sessions when mean received SPLs in test sessions were 86 dB re 1 μPa and above, indicating that it responded to the APD-01 primarily by swimming away from it. Because of the high frequency of the APD-01 sounds, harbor porpoises can determine the location of the sound source relatively easily. To calculate the effective deterring range of the APD-01 for harbor porpoises at sea, information on the behavioral threshold SPL for distance (established in the present study), the source level, and modeled information on the local propagation conditions and ambient noise need to be combined. The distance at which the APD-01 sounds are effective as deterrents is sufficient for their use to prevent PTS in wild harbor porpoises due to sound from the first strike of offshore pile driving.