Donald Ross

Underwater radiated noise from modern commercial ships.

Underwater radiated noise measurements for seven types of modern commercial ships during normal operating conditions are presented. Calibrated acoustic data (<1000 Hz) from an autonomous seafloor-mounted acoustic recorder were combined with ship passage information from the Automatic Identification System. This approach allowed for detailed measurements (i.e., source level, sound exposure level, and transmission range) on ships of opportunity. A key result was different acoustic levels and spectral shapes observed from different ship-types. A 54 kGT container ship had the highest broadband source level at 188 dB re 1 μPa@1m; a 26 kGT chemical tanker had the lowest at 177 dB re 1 μPa@1m. Bulk carriers had higher source levels near 100 Hz, while container ship and tanker noise was predominantly below 40 Hz. Simple models to predict source levels of modern merchant ships as a group from particular ship characteristics (e.g., length, gross tonnage, and speed) were not possible given individual ship-type differences. Furthermore, ship noise was observed to radiate asymmetrically. Stern aspect noise levels are 5 to 10 dB higher than bow aspect noise levels. Collectively, these results emphasize the importance of including modern ship-types in quantifying shipping noise for predictive models of global, regional, and local marine environments.

A 50 year comparison of ambient ocean noise near San Clemente Island: A bathymetrically complex coastal region off Southern California.

Repeated ocean ambient noise measurements at a shallow water (110 m) site near San Clemente Island reveal little increase in noise levels in the absence of local ships. Navy reports document ambient noise levels at this site in 1958-1959 and 1963-1964 and a seafloor recorder documents noise during 2005-2006. When noise from local ships was excluded from the 2005-2006 recordings, median sound levels were essentially the same as were observed in 1958 and 1963. Local ship noise, however, was present in 31% of the recordings in 1963 but was present in 89% of the recordings in 2005-2006. Median levels including local ships are 6-9 dB higher than median levels chosen from times when local ship noise was absent. Biological sounds and the sound of wind driven waves controlled ambient noise levels in the absence of local ships. The median noise levels at this site are low for an open water site due to the poor acoustic propagation and low average wind speeds. The quiet nature of this site in the absence of local ships allows correlation of wind speed to wave noise across the 10-220 Hz spectral band of this study.

Underwater ambient noise on the Chukchi Sea continental slope from 2006-2009.

From September 2006 to June 2009, an autonomous acoustic recorder measured ambient noise north of Barrow, Alaska on the continental slope at 235 m depth, between the Chukchi and Beaufort Seas. Mean monthly spectrum levels, selected to exclude impulsive events, show that months with open-water had the highest noise levels (80-83 dB re: 1 μPa(2)/Hz at 20-50 Hz), months with ice coverage had lower spectral levels (70 dB at 50 Hz), and months with both ice cover and low wind speeds had the lowest noise levels (65 dB at 50 Hz). During ice covered periods in winter-spring there was significant transient energy between 10 and 100 Hz from ice fracture events. During ice covered periods in late spring there were significantly fewer transient events. Ambient noise increased with wind speed by ~ 1 dB/m/s for relatively open-water (0%-25% ice cover) and by ~ 0.5 dB/m/s for nearly complete ice cover (> 75%). In September and early October for all years, mean noise levels were elevated by 2-8 dB due to the presence of seismic surveys in the Chukchi and Beaufort Seas.

A quieting ocean: Unintended consequence of a fluctuating economy

Simultaneous long-term monitoring of underwater sound and ship traffic provided an opportunity to study how low-frequency noise correlated with ocean-based commercial shipping trends. Between 2007 and 2010 changes in regional shipping off southern California occurred as a consequence of economic and regulatory events. Underwater average noise levels measured before and during these events showed a net reduction of 12 dB. Statistical models revealed that a reduction of 1 ship transit per day resulted in 1 dB decrease in average noise. This synthesis of maritime traffic statistics with ocean noise monitoring provides an important step in understanding the magnitude and potential effects of chronic noise in marine habitats.

Unintended consequences of recent changes in ship traffic

Underwater ambient noise levels measured off the coast of southern California were correlated with regional changes in commercial shipping trade. Between 2007 and 2010, two events occurred that resulted in a decrease in ship traffic in the Santa Barbara Channel: the economic recession and a coastal air-quality improvement rule. From October 2005 to June 2010, monthly low-frequency ambient noise levels at a site 3 km from a major shipping route were compared to regional traffic levels. Two different metrics of ship traffic showed that on average a 1 dB reduction in low-frequency noise levels resulted from a decrease in traffic by one ship passage per day in a coastal basin.

Measurements of radiated underwater noise from modern merchant ships relevant to noise impacts on marine mammals.

There is mounting concern over the effect of ship noise on marine mammals; however, limited empirical data quantifying this noise impede our ability to evaluate impacts. An opportunistic approach for measuring radiated ship noise (20–1000 Hz) was used in this study. Calibrated acoustic data were combined with archived information on seven types of modern merchant ships transiting the coast of southern California. Three metrics for describing ship noise were applied: received sound levels (RLs) during 1 h passages, estimated source levels (SLs), and sound exposure levels (SELs). 1 h passages provided an estimate of the spatial extent of ship noise. At 40 Hz, container ships elevated noise above background up to 7 km forward of the ship and 19 km aft; bulk carriers elevated noise above background up to 5 km at bow and stern aspects. These ship‐types had similar broad band estimated SL, 186 dB re 1 μPa at 1 m. The cumulative exposure to ship noise varied by ship type; we presented equations for estimating SE...

Ambient noise bathymetric domains

For the purposes of describing and understanding ambient sea noise for frequencies below about 300 Hz, most of the worlds seas can be classified as belonging to one of three bathymetric domains. These domains are distinguished by their proximity to shipping lanes and by the degree to which they are exposed to noises originating at long distances. The three domains display different short-term characteristics as well as different historical patterns. In this paper, the three domains are described and typical ambient noise characteristics for each are shown, including changes which are attributable to increased ocean commerce.

World shipping distributions

Ocean noise in the lower frequencies (25–250 Hz) is due principally to surface shipping. In the absence of extremely close sources, this is due to many distant ships scattered throughout the acoustic basin. Merchant and fishing vessel distribution is presented for the North Atlantic and North Pacific Ocean between the equator and 70 °N. The information is presented for 5° squares with method for achieving distribution in 1° squares. The distribution applies to the time period 1972–1973 but may also be used for periods within two to three years of these dates, requiring only scaling of the total numbers. The shipping distribution presented herein is a coordinated product of three independent studies and represents the most current information available.

Hydrodynamical Considerations in the Theory of Wake‐Vortex Sound

Theoretical expressions for the dipole radiation associated with wake‐vortex shedding have been developed by Curle, Phillips, and others. These well‐known relations show that the sound depends on the lift coefficient associated with the oscillating force, the Strouhal number, and the spanwise correlation length of the vortices. Using hydrodynamical relations for vortex flows, interrelationships between these factors can be developed, and the relationship for the radiated sound can be reduced to an expression involving only the geometry of the vortex pattern and the steady‐state drag force. By applying the Karman‐Sears theory for the lift force on oscillating airfoils, an explanation is derived for observed effects of trailing‐edge geometry on the production of wake‐vortex noise. [This research has been partially supported by the U. S. Bureau of Ships.]

Origin of Low‐Frequency Mechanical Noise from Diesel Engines

In his book, Noise of Marine Diesel Engines, V. I. Zinchenko distinguishes between aerodynamic‐noise sources, such as the intake blowers and the exhaust, and mechanical sources associated with the reciprocating mechanism. Zinchenko proposes that the dominant source of mechanical engine noise is the lateral slap of the piston against the cylinder wall which occurs close to the time of peak cylinder pressure, and he develops an expression for the impulse of such an impact. From this expression, using some simplifying assumptions, one can derive an expression for the radiated sound associated with piston impact. The results are of the right order of magnitude; and the resultant dependence on rpm, horsepower, cylinder pressure, number of cylinders, and engine‐design parameters are all in reasonable agreement with experimental trends.