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How does shipping affect ocean sound levels?
Ships create noise from their propellers, motors and gears. The amount of noise produced depends on their hull shapes and propulsion systems. Most of the noise ships produce is at low frequencies (roughly 20–500 Hz). In the frequency range of roughly 500–100,000 Hz, however, ambient noise is mostly due to breaking waves, rather than shipping. At low frequencies the background sound level in many places in the ocean is dominated by noise from distant ships, even when there is no nearby ship. When a large ship passes close to a receiver, the noise that is generated will temporarily increase the sound levels at that location substantially.
Geographic variabilityThe amount of low-frequency noise is greater in regions with heavy shipping traffic. The following figure shows an estimate of the average number of ships per 1° latitude by 1° longitude square produced from information on vessels’ departure and destination points.
Common shipping routes are clearly evident in the figure above. Up to ten ships per 1° by 1° square are seen in regions with heavy shipping traffic. There tend to be fewer ships in the southern hemisphere, and low-frequency ambient noise levels are substantially lower there than in the northern hemisphere as a result.
However, making accurate predictions of the noise due to shipping by summing the contributions of individual ships is difficult. The noise radiated by each ship is needed, as well as ocean sound speed profiles and the properties of the seafloor between all the ships and the location at which the noise is being computed.
Changes in shipping noise over timePrior to the mid-19th century Industrial Revolution, low-frequency noise in the ocean was due to spray and bubbles associated with breaking waves, sounds generated by marine life, seismic noise, and other natural sounds. The Industrial Revolution marked the beginning of the use of powered (rather than sailing) vessels to transport goods and provide services. Increases in commercial shipping since that time have caused low-frequency noise levels to increase in many oceanic locations.
The following figure shows that the world’s commercial fleet has approximately tripled in number, and the gross tonnage (size) of vessels has increased by more than a factor of six during the past 50 years.
However, because of changes in ship design, it is unclear how increases in number and tonnage of vessels relates to increases in low-frequency ambient noise. Radiated noise depends on multiple factors such as ship size, speed, horsepower, propeller depth, etc. All of these characteristics have changed over time. Consequently, scientists need to take direct measurements of ambient noise to determine long-term trends in shipping noise.
Measurements made over the period 1950–1970 indicated that low-frequency (50 Hz) ship traffic noise in the eastern North Pacific and western North Atlantic Oceans was increasing by about 0.55 dB per year. These early measurements of low-frequency noise included measurements made during 1963–1965 using the hydrophone arrays installed as part of the U.S. Navy Sound Surveillance System (SOSUS).
New equipment was installed and ambient noise data were recorded from 1994 to 2007 for four of the original SOSUS hydrophone arrays in the eastern North Pacific for which data were obtained in 1963–1965. This has allowed an assessment of the changes that have occurred since those early noise data were collected. The approximate locations of these four systems, labeled d, f, g, and h, are shown in the following figure.
Comparisons of data from 1963–1965 and 1994-2007 show that low-frequency ship traffic noise (25–50 Hz) at sites d, f, and h increased by roughly 8–10 dB, i.e., about 0.3 dB per year, from the mid-1960’s to the mid-1990’s, while site g showed a slightly smaller increase. Although the details of how the noise levels changed over the 30-year period from the early 1960’s to the early 1990’s at these sites are unknown, other data obtained in the northeast Pacific from 1978 to 1986 suggest that the 0.55 dB/year increase seen in the early data continued to around 1980, but then slowed to about 0.2 dB/year .
The changes in ship traffic noise over the recent decade-long records do not show a continuation of the 0.3 dB per year trend, but rather show complex changes over time. Shipping noise at site f off southern California shows little change during this period, while the noise at sites g and h off Oregon and Washington are decreasing. The record at site d off central California is shorter than the others, and the data show different trends at different frequencies. The overall conclusion is that there appears to be no significant evidence of a consistent increase in traffic noise over the last decade for these systems in the eastern North Pacific. Long-term measurements at many other locations will be required to make meaningful assessments of global trends in ship traffic noise.