When reality defies common sense

I've previously advocated the use of dBSea as a teaching tool (here and here). It gives the teacher the possibility to set up realistic scenarios and have the students figure it out, and get a feel for the real world without the need for field trips (field trips are lovely in the summer, but can be a real challenge in winter storms here in Ireland).

Case: When a wind farm doesn't behave:

How about estimating noise from a wind farm in a shallow water scenario? Assume six turbines in the Oriel wind farm off  of Clogher Head (Co. Louth, Ireland) that are emitting noise underwater during operation. They emit noise as per the following spectrum:
Adapted from Tougaard et al. 2009 
You then convert these into an approximation of a new point source by summing up source levels across frequency bands and then correct for frequency specific attenuation and spreading loss to get a distance dependent sound field.
Something like this:
Estimation of noise levels at distance (kms) "x" in band "Hz". Assuming cylindrical spreading loss, "att dB/km" is the frequency dependent attenuation.
Soundscape with six wind turbines at Oriel Park in Dundalk bay, Ireland. Noise spectrum from Tougaard et al. 2009. The formula assumes cylindrical spreading because of the shallow setting. Probe location refers to the brown spot.
(That would have been a great task for a set of students)

This is all well and good, and often a good approximation, but it is not what would most likely be the reality.
Due to the sandy nature of the seabed in the area combined with the low frequencies involved, more advanced modelling tells us that there will be heavy attenuation of the lower frequency bands.
Soundscape with six wind turbines at Oriel Park in Dundalk bay, Ireland. Noise spectrum from Tougaard et al. 2009. The model takes into account water layering, bathymetry topography and composition. Probe location refers to the red spot.
This result is markedly different, and should spark some confusion or at least discussion, "How come the low frequencies don't make it further?". This is because the low frequencies can pass into the sediment, and their energy is thus not confined within the water column only.

This is a very real world problem that can be dealt with in school, students can derive the used equation themselves (or with help, depending on level), and when reality is different from our expectations the motivation for having the discussion about the why is so much stronger.


Tougaard et al. (2009)