The 60‐dB rule for birds: An example of the application of a weighting function in environmental impact mitigation

Over the last decade U.S. Fish and Wildlife Service managers in California have required millions of dollars in added expenditure for NEPA consultation, mitigation barriers, and project delays to reduce the effects of noise from construction activities on endangered passerine birds when the hourly A‐weighted Leq is expected to exceed 60 dB. The rule was originally intended to prevent masking of species‐typical songs of endangered birds such as the Coastal California Gnatcatcher. However, no research is available to demonstrate the effectiveness of the rule for any noise‐related impact. Although A‐weighting is probably a conservative estimator of bird exposure in the range from 125 Hz to 8 kHz, it may underestimate exposure at very low frequencies. Its utility as a weighting function has not been tested against other possible weighting procedures, such as use of the species‐typical auditory threshold function. Additionally, where sources are intense but intermittent, Leq is unlikely to be a useful metric. ...