Estimates of Source Spectra of Ships from Long Term Recordings in the Baltic Sea

Abstract

Estimates of the noise source spectra of ships based on long term measurements in the Baltic sea are presented. The measurement data were obtained by a hydrophone deployed near a major shipping lane south of the island Öland. Data from over 2,000 close-by passages were recorded during a 3 month period from October to December 2014. For each passage, ship-to-hydrophone transmission loss (TL) spectra were computed by sound propagation modeling using 1. bathymetry data from the Baltic Sea Bathymetry Database (BSBD), 2. sound speed profiles from the HIROMB oceanographic model, 3. seabed parameters obtained by acoustic inversion of data from a calibrated source, and 4. AIS data providing information on each ship's position.These TL spectra were then subtracted from the received noise spectra to estimate the free field source level (SL) spectra for each passage. The SL were compared to predictions by some existing models of noise emission from ships. Input parameters to the models, including e.g., ship length, width, speed, displacement, and engine mass, were obtained from AIS (Automatic Identification System) data and the STEAM database of the Finnish Metereological Institute (FMI).

Modelling of ships as a source of underwater noise

“…The transmission loss of the measured noise signature was modelled using XFEM code (Karasalo, 1994) to obtain the noise source at a reference distance. In their paper, Karasalo et al (2017) observed a good fit between the Wittekind predictions and the observed signals for cargo ships, tankers and tugboats, but larger differences were observed for passenger and RoRo vessels, with the Wittekind model overestimating the noise source levels. It is very likely that this is because the Wittekind model was mainly intended for large ocean-going vessels with a single fixed pitch propeller or a single controllable pitch propeller that are operated close to their design pitch (Dietrich Wittekind, personal communication, October 2017).…”

“…For examples of propagation modelling from multiple ships, which facilitates the evaluation of the sound pressure level at an arbitrary point in the water column, the reader is referred to e.g. Karasalo et al (2017) and Gaggero et al (2015). Presenting sound energy as a geographically distributed quantity will help to visualize noisy areas, as has also been investigated by Audoly et al (2015).…”

Modeling of ships as a source of underwater noise

“…The transmission loss of the measured noise signature was modeled using XFEM code (Karasalo, 1994) to obtain the noise source at reference distance. In their paper, Karasalo et al (2017) observed a good fit between the Wittekind predictions and observed signals for cargo ships and tankers and tugboats, but larger differences were observed with passenger and RoRo vessels for which the Wittekind model overestimated the noise source levels. It is very likely that this is because the Wittekind model was mainly intended for large ocean-going vessels with a single fixed pitch propeller or a single controllable pitch propeller when they are operated close to their design pitch (Wittekind D, Oct 2017, personal communication).…”

“…where Pk,m(t) is the sound power (in J s -1 ) emitted by ship m. This quantity is additive and facilitates the summation of ship specific noise energy over a specific time period (in Joules). The sound power map is more of a visual aid than a direct input dataset for noise propagation modeling, which may prefer point source description of noise sources (Karasalo et al, 2017).…”

Occupational Noise on Floating Storage and Offloading Vessels (FSO)

“…This last is set at 87 dB(A) by the European Directive while if the second action level of 85 dB(A) is exceeded, the employer have to impose the use of hearing protection devices. The international community recognizes that noise from commercial ships may have both short and long-term negative consequences for both human (worker’s) life and marine life—especially marine mammals susceptible to underwater-radiated noise from ship’s sonars, propellers, and thrusters [ 8 , 9 ].…”