13
$\begingroup$

I saw the following graph in a blog post about how shipping noise affects whales. Why does the sound seem to propagate better in deep water than in shallow water? Are there any known marine species that changed their preferred depth due to this effect?

Sound propagation © 2019 Erbe, Marley, Schoeman, Smith, Trigg and Embling

$\endgroup$
5
  • $\begingroup$ Could you please add information on model used (bellhop?) and on frequency? $\endgroup$
    – WMXZ
    Jun 26 at 8:47
  • $\begingroup$ If one of the answers below answers your question, please 'accept' the answer by checking the checkmark under the vote tally. See stackoverflow.com/help/someone-answers $\endgroup$
    – Shannon
    Jul 20 at 22:19
  • $\begingroup$ Could you add a link/credit to the figure? Remember that all content on SE is under the license Creative Common. $\endgroup$
    – Noil
    Aug 6 at 12:49
  • $\begingroup$ @Noil based on reverse image search, I think the original source is frontiersin.org/articles/10.3389/fmars.2019.00606/full $\endgroup$ Aug 15 at 21:03
  • $\begingroup$ I've just added it below the image $\endgroup$
    – Noil
    Aug 16 at 19:16

2 Answers 2

10
$\begingroup$

What you're seeing there is an effect of the ocean thermocline. The speed of sound changes rapidly in the upper few hundred meters of the ocean from a combination of temperature and pressure changes (sound moves faster in warmer water, and in higher-pressure water). This produces a natural waveguide called the SOFAR channel, which can carry sounds for thousands of kilometers.

It's suspected that fin whales use this channel for communication.

$\endgroup$
9
$\begingroup$

Additional to Mark's answer mentioning the sound speed profile and SOFAR channel, there are effects in shallow water that increase transmission loss, especially for lower frequencies:

In shallow water sediment interaction of the sound will often be the dominant source of sound attenuation (for anything but very hard sediments) (see: https://dosits.org/science/advanced-topics/shallow-water-propagation/ & https://dosits.org/science/advanced-topics/how-does-sound-travel-in-very-shallow-waters/).

This attenuation is frequency dependent, but generally the seafloor acts like a highpass filter, causing low frequencies (which dominate ship noise and long range noise) to be absorbed (see: https://www.researchgate.net/publication/343987334_Accounting_for_sea_floor_properties_in_the_assessment_of_underwater_noise_radiated_from_ships_in_shallow_water for good introduction and model).

In general frequencies ~< 200 Hz are absorbed and dissipated in shallow water, and higher frequencies attenuate due to volume scattering and heat loss leaving the dominant noise in shallower waters between several hundred Hz and a few kHz.

In deeper waters, with no mechanism for low frequency attenuation, low frequency noise can travel very far (even in the absence of a SOFAR channel) given the negligible attenuation loss (see: http://resource.npl.co.uk/acoustics/techguides/seaabsorption/).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.