I was made aware of a paper published by Aarhus university in Denmark dealing with noise from a high speed ferry. I thought it would be interesting to model the noise from the route and compare that to the hearing threshold of the harbour porpoise.
The actual routes of a similar ship can be found here along with a host of other information (e.g. current speed and position). This page has all bigger ships on it, including their names, some of which were never meant to go online...
After getting a porpoise audiogram from this paper, and noise information from the Aarhus university paper, all I had to do was enter the data into dBSea. The image below shows the sound field from the ferry in colours corresponding to dB's RMS over porpoise hearing threshold. I could even add to the model that the seabed is mud, and it takes into account that most of the noise energy from the ship is at frequencies where the porpoise's hearing isn't very sensitive. This map can then be thought of as a map of the amount of masking the animal experiences at a given location in the inner Danish waters.
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| The amount of noise a porpoise would hear when close to the ferry route between Aarhus and Kalundborg. the data is frequency-weighted by comparing the ship's noise-spectrum with the porpoise's hearing. The grainy coastline is due to lack of high-resolution bathymetry data for the area. |
The modelling approach to finding noise levels of a ferry route does not require that the route is established and operating. The scenario can be modeled with no impact on the area in interest, thus minimising potential harm/disturbance done to local fauna (not to mention reducing cost).
The ability to weigh the spectrum of the noise against the audiogram of species of concern, gives a much more valid basis for planning and legislation than does a simple broad band noise energy measurement.
As a little note. This simulation completed in less than five minutes on my laptop. It calculated levels for 16 route points, with over 10,000 points each, over a frequency range from 12.5 Hz to 160 kHz, and compared these to the audiogram for the porpoise - I think that's impressive.
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