Three Decibels
a Decade
Sunlight gives out a couple of hundred metres down. Sound doesn't — it carries for hundreds of kilometres, which is why the ocean's animals run on hearing. Over the last seventy years we have been steadily turning up the volume on top of them, one propeller at a time.
That is the long-run rise in low-frequency ambient sound in the open ocean, tracked across the acoustics literature and matched almost step for step to global economic growth. In acoustics the scale is logarithmic: every 3 dB is a doubling of sound energy. The deep ocean's background roar has, roughly speaking, doubled every ten years since the middle of the last century.
Source: Frisk, Scientific Reports (2012).
A container ship is as loud as a blue whale
Underwater source levels, measured one metre from the source. The scale is logarithmic, so the gap between a chemical tanker and a seismic airgun array is not 54 units of anything — it is a factor of roughly 250,000 in sound pressure.
Underwater decibels use a different reference pressure than the decibels on a sound-level meter in air, so the two scales cannot be compared directly. Within this scale, though, the arithmetic holds: +10 dB is ten times the sound energy.
Forty years, ten times the energy
Two research groups went back to Cold War–era hydrophone sites off California and re-measured the same water. Low-frequency background sound — the band shipping dominates — had climbed about 10 dB since the mid-1960s.
Two-thirds of the room, gone
A whale's "communication space" is the patch of ocean across which its call can still be heard above the background. In Stellwagen Bank National Marine Sanctuary, off Boston, researchers modelled how much of that space passing ships take away from critically endangered North Atlantic right whales.
Modelled loss for right whales in and around Stellwagen Bank, from four years of acoustic recording (2007–2010). On the noisiest days the loss was larger still.
Hatch et al., Conservation Biology (2012)
When shipping paused after 11 September, underwater noise below 150 Hz fell by about 6 dB — and right whale faecal stress hormones fell with it. The first direct evidence that ship noise causes chronic stress in whales.
Rolland et al., Proc. R. Soc. B (2012)
The ocean gets quiet fast
Unlike most pollution, noise leaves nothing behind. Twice this century the world accidentally ran the experiment — and hydrophones heard the difference within weeks.
Why it doesn't stay local
Water is a far better conductor of sound than air, and the deep ocean has a sound channel that traps low frequencies and carries them across basins. Noise made off one continent is audible from another.
Hydrophones moored on the Mid-Atlantic Ridge clearly recorded seismic survey airguns firing from vessels 3,000 km or more away.
Nieukirk et al., JASA (2004)
About 4.4 times its speed in air, with far less energy lost along the way — the reason sound, not light, is the ocean's medium of information. Nominal value.
Vessels of 100 gross tons and above at the start of 2024, carrying roughly 2.4 billion tons of cargo capacity. The fleet is still growing.
UNCTAD, Review of Maritime Transport 2024
The cheapest pollution to clean up
Noise is the one marine pollutant with no residue and no half-life. Stop making it and it is gone the same day — which makes it unusually responsive to ordinary engineering and ordinary rules.
- Slow down. Cutting the global fleet's speed by 10% would cut total radiated sound energy by about 40%; a 20% cut would remove about 67%. It also cuts fuel burn and whale-strike risk. (Leaper, Frontiers in Marine Science, 2019)
- Fix the propellers. Most ship noise is cavitation — collapsing bubbles off the propeller blades. Better blade design, wake-conditioning and hull maintenance are retrofit-scale fixes, not new-fleet fixes.
- Route around the whales. Seasonal lane shifts and voluntary slowdown zones already exist off British Columbia, California and New England, and they measurably work.
- Make the guidelines bite. The IMO's revised underwater-noise guidelines took effect in October 2023 — and remain entirely voluntary. (IMO MEPC.1/Circ.906)
Sources & notes
- Frisk, G.V. "Noiseonomics: the relationship between ambient noise levels in the sea and global economic trends." Scientific Reports 2:437 (2012). — ~3.3 dB per decade, 1950–2007.
- Andrew, R.K. et al. "Ocean ambient sound: comparing the 1960s with the 1990s for a receiver off the California coast." Acoustics Research Letters Online 3(2) (2002).
- McDonald, M.A., Hildebrand, J.A. & Wiggins, S.M. "Increases in deep ocean ambient noise in the Northeast Pacific west of San Nicolas Island, California." JASA 120(2) (2006). — 10–12 dB higher at 30–50 Hz in 2003–04 vs 1964–66.
- Hatch, L.T., Clark, C.W., Van Parijs, S.M., Frankel, A.S. & Ponirakis, D.W. "Quantifying loss of acoustic communication space for right whales in and around a U.S. National Marine Sanctuary." Conservation Biology 26(6) (2012). — the 63–67% figure is modelled from measured noise, not directly observed.
- Rolland, R.M. et al. "Evidence that ship noise increases stress in right whales." Proceedings of the Royal Society B 279 (2012).
- Thomson, D.J.M. & Barclay, D.R. "Real-time observations of the impact of COVID-19 on underwater noise." JASA 147(5) (2020).
- Nieukirk, S.L. et al. "Low-frequency whale and seismic airgun sounds recorded in the mid-Atlantic Ocean." JASA 115(4) (2004).
- McKenna, M.F., Ross, D., Wiggins, S.M. & Hildebrand, J.A. "Underwater radiated noise from modern commercial ships." JASA 131(1) (2012). — 188 dB (54 kGT container ship) to 177 dB (26 kGT chemical tanker), broadband source levels.
- Leaper, R. "The role of slower vessel speeds in reducing greenhouse gas emissions, underwater noise and collision risk to whales." Frontiers in Marine Science 6:505 (2019). — the 40% / 67% energy reductions are modelled fleet-wide projections.
- Duarte, C.M. et al. "The soundscape of the Anthropocene ocean." Science 371, eaba4658 (2021).
- UNCTAD, Review of Maritime Transport 2024. — ~109,000 vessels of 100 GT and above at the start of 2024.
- IMO, MEPC.1/Circ.906, "Revised guidelines for the reduction of underwater radiated noise from shipping," in force 1 October 2023 (non-mandatory).
- Discovery of Sound in the Sea (DOSITS), University of Rhode Island. — snapping shrimp peak-to-peak source levels up to 189 dB re 1 µPa at 1 m; blue and fin whale calls up to ~190 dB. The ~231 dB airgun figure is a peak level from an experimental array measurement; airgun arrays vary widely.
- Illustrative: the two-circle communication-space diagram is schematic — the areas are drawn to scale, the geometry is not a map of any real acoustic footprint. Sound speed of 1,500 m/s is a nominal seawater value.