At the stern aspect, noise levels in all bands remained above background for the entire 30 min period (>13 km), with the 95 Hz band having the highest levels. As the bulk carrier approached CPA, noise levels in the 95 Hz band were elevated above background first, at a distance of 11 km from the receiver. At 95 and 800 Hz, container ship RLs were above background symmetrically at the bow and stern aspects, approximately 8.5 km. As the container ship traveled away from the receiver, noise levels at 40 Hz stayed above background for over 30 min (i.e., > 20 km distance from the receiver).

As the container ship approached CPA, the RLs at 40 Hz rose above background at 16 km from the receiver. The temporal and spatial extents were different depending on the ship-type. 2 is the asymmetry between the bow and stern aspects in the lower frequencies, with more radiated energy at the stern aspects (i.e., positive time from CPA). This phenomenon is known as Lloyd’s Mirror Effect and is dependent on the depth of the source and receiver, distance of the source to the receiver, water column properties, and bottom reflection ( Wilmut et al., 2007 et al.,).Īnother pattern seen in the spectrogram plots of Fig. The U-shaped interference patterns at the higher frequencies (>100 Hz) are caused by constructive and destructive interference from a dipole sound source or in other words, the interactions of the direct and surface reflected propagation paths as the point source moves along the source-receiver path (Albers, 1964 Ross, 1976 Wales and Heitmeyer, 2002). Tonal lines result from propeller blade cavitation and their harmonics ( Ross, 1976). Two dominant features shown are tonal lines below 100 Hz and the “U” shaped pattern centered at CPA (Fig. 2, selected ships are designated as “a” in Table I) revealed distinct differences in radiated noise related to ship-type. One-hour spectrograms of a container ship, bulk carrier and product tanker RLs (illustrated in Fig. Ship information (Lloyd’s Registry of ships) 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.

Stern aspect noise levels are 5 to 10 dB higher than bow aspect noise levels. Furthermore, ship noise was observed to radiate asymmetrically. 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. A 54 kGT container ship had the highest broadband source level at 188 dB re a 26 kGT chemical tanker had the lowest at 177 dB re Bulk carriers had higher source levels near 100 Hz, while container ship and tanker noise was predominantly below 40 Hz. A key result was different acoustic levels and spectral shapes observed from different ship-types. This approach allowed for detailed measurements (i.e., source level, sound exposure level, and transmission range) on ships of opportunity. Calibrated acoustic data (<1000 Hz) from an autonomous seafloor-mounted acoustic recorder were combined with ship passage information from the Automatic Identification System. Underwater radiated noise measurements for seven types of modern commercial ships during normal operating conditions are presented.

Experimental modern warships