Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Power spectrum density level (PSL) of background noise in the range 50–500 Hz from 2 days before (25 and 29 August 2011) and 2 days after 11 September 2001 (12 and 13 September 2011) with identical sea-state conditions. Overall noise levels (PSL in dB re 1 µPa2 Hz−1 are lower and the peak frequency (Hz) of the noise shifted to a higher frequency post-9/11.
