Trends over minutes to decades in oceanic ambient sound measured off the southern Californian coast

Ambient sound spectra have been collected since 1994 from a decommissioned deep water military acoustic receiver located southwest of San Nicolas Island (southern California). A new spectrum is collected every 6 minutes. The dataset can be used to study level variability on scales from minutes to years. The calibration curves for this system, however, were considered suspect. Recently, a calibrated acoustic recorder was deployed near the receiver (McDonald et al., JASA, 120(2), August 2006). Comparison of spectra for December 2003 provides a long‐sought‐after modern absolute calibration. With this new correction, the dataset can additionally be compared to levels measured in the 1960’s by the same receiver. Although the corrected measurements corroborate (albeit by construction) the 1960–2000 ambient noise increase reported by McDonald et al., there is scant evidence of an increase over the last decade. This study again highlights the need for on‐going long‐term well‐calibrated observation programs. [Work...