How porpoises are affected by man-made noise
In 2009, Klaus Lucke, of the Forschungs und Technologie Zentrum in Büsum, Germany, published the results of his studies on how porpoises are affected by low-frequency, high-intensity acoustic impulses. The results show that the sound level required for negatively affecting porpoise hearing is surprisingly low. The studies should be repeated, however, to provide more complete data relating to this situation. The results are important because the sound impulses to which porpoises are exposed in the experiments resemble those generated by the ramming of piles when constructing things like offshore wind turbines, harbour facilities, etc.
Experimental set-up for Klaus Lucke’s experiments at Fjord&Bælt. The airgun is moved closer and closer to the porpoises in the basin to increase the sound level for each exposure. From: Lucke et al. 2009, Journal of the Acoustical Society of America, 125(6): 4060–4070.
The experimental set-up for measuring hearing levels for acoustic brain-stem response is shown on the left. The sounds are emitted through the hydrophone marked TX, and the sound level near the porpoise is monitored using the RX hydrophone. The signals emitted by the porpoise’s brain are recorded using two suction electrodes attached to the porpoise’s back. The porpoise’s hearing threshold at 4 kHz is shown on the right as a function of the sound intensity of the powerful sound impulse emitted by an airgun during the hearing test. The sharp rise in threshold values for sound impulses louder than around 200 dB re 1 µPa pp is due to a temporary hearing loss at frequencies near those which dominate the signal emitted by the airgun. The porpoise’s hearing returns to normal twelve hours after exposure. From: Lucke et al. 2009, Journal of the Acoustical Society of America, 125(6): 4060–4070.