In-water impact pile driving sound source spectra comparison and the quest for a “Generic Spectrum”

Abstract

Noise generated from in-water impact pile driving for marine and coastal construction can impact marine mammals. Physiological effects to marine mammals include hearing impairments such as temporary and/or permanent threshold shifts. Because marine mammals’ auditory response to sound levels is frequency-dependent, environmental impact analysis from noise exposure needs to consider noise spectrum in addition to broadband received level. In this study, spectral characteristics of measured noise levels from impact driving of 30-inch steel piles at four locations in Alaska and Washington were compared. Results show for all impact pile driving, most acoustic energies were between 100 and 1,000 Hz. Spectral levels below 100 Hz varied among the locations. Above 1,000 Hz, spectral levels decayed at a rate of 15 dB/decade for the three Alaska sites, but at 30 dB/decade in Washington. Despite these general characteristics, however, there does not seem to be a “generic spectrum” for the 30-inch steel pile impact driving. The variations among the spectral characteristics are likely due to differences in substrate, hammer energy, and bathymetry in different locations.