"Quieting the Global Growl" aka noise pollution -- in the ocean.
"Globally, shipping noise in the ocean has doubled every decade from 1960 to 2010. Piercing sonar, thudding seismic air guns for geological imaging, bangs from pile drivers, buzzing motorboats, and shipping's broadband growl can also disrupt natural soundscapes. Such human sounds are not universally problematic, but they become noise when they're unwanted. And while noise can cause acute injury and even death in marine mammals -- for example, by sending animals fleeing to the surface from great depths too quickly -- it also impacts communication, mating, fighting, migrating, and bonding in subtle and wide-ranging ways. Underwater, acoustic space is valuable, and noise is a trespass."
"In 1980 the world's merchant shipping fleet (meaning all ships, not just containers) numbered just under 700,000. In 2020, it's more than two million. Globally, container ships -- with their colorful, Lego-like boxes -- make up only about 10 percent of commercial ships. The rest are vehicle carriers, cruise ships, oil tankers, or bulk carriers toting ore, coal, grain, or other commodities."
"Invisible below the water, Busan's propeller is several meters across. The largest container-ship props can be nine meters. These propellers shed bubbles that flash-boil seawater and oscillate, the vibration becoming a sound. Bubbles vary in size, so collectively they make many frequencies, most in the low hundreds of hertz. Ship noise, then, is broadband, with most of the energy in lower tones."
