In seismic surveys, underwater sounds from airguns are used to detect rocks that may contain gas and oil below the sea floor. Airguns produce broadband high-amplitude impulsive sounds with most energy below 100 Hz. Captive harbor porpoises (Phocoena phocoena) respond strongly to firing down-scaled airguns. To reduce porpoises’ exposure to airgun sounds, a plastic and aluminum screen with encapsulated air bubbles was placed between the airgun and the harbor porpoises in a pool. The bubble screen reduced the energy of the broadband sounds above 250 Hz, but the broadband single-shot sound exposure level (SELss) was reduced by only 3 dB. The bubble screen was very effective in reducing the behavioral responses of the porpoises to the airgun sounds, even when the broadband SELss experienced was 157 dB re 1 μPa2s. This study provides qualitative support for the hypothesis that frequency content matters in the assessment of the responsiveness of harbor porpoises to impulsive broadband sounds. New airgun designs with reduced high-frequency components (> 1 kHz) may be effective in reducing the behavioral responses of harbor porpoises, but a more systematic and quantitative study is required to address the frequency-dependence of their responses to underwater sounds.

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