The Ocean Is Never Silent
Even in the deepest, most remote waters, the ocean hums with sound from countless sources. Low-frequency rumbles from distant earthquakes and shipping, mid-frequency hiss from breaking waves and rain, high-frequency crackles from snapping shrimp — together they create an ambient noise field that varies with weather, location, season, and human activity. Understanding this noise floor is essential for designing sonar systems and assessing impacts on marine life.
The Wenz Curves
Gordon Wenz's 1962 compilation of ocean noise measurements remains the standard framework for understanding underwater ambient noise. His curves show that each frequency band is dominated by different physical mechanisms: turbulence and microseisms below 10 Hz, shipping from 10-300 Hz, wind and waves from 300 Hz to 30 kHz, and thermal molecular agitation above 100 kHz. Biological sources and rain add variable contributions that can locally dominate.
Anthropogenic Noise Pollution
Since the advent of motorized shipping, low-frequency ocean noise has increased by approximately 3 dB per decade — a doubling of acoustic power every ten years. This chronic noise pollution reduces the communication range of baleen whales, masks predator detection by fish, and may contribute to cetacean stranding events. Growing awareness has prompted the International Maritime Organization to issue guidelines for reducing underwater noise from commercial ships.
Noise as Signal
Paradoxically, ambient noise itself carries useful information. Acoustic daylight imaging uses ambient noise illumination to form images of underwater objects without active sonar. Seismic noise tomography extracts Earth structure from the cross-correlation of ambient seismic recordings. And the distinctive spectral signature of rain noise enables acoustic rain gauges that can measure precipitation over the open ocean — where conventional rain gauges cannot reach.