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Ocean Acidification Not Likely to Increase Ambient Background Noise

Ocean, Science, Shipping Add comments

(this item appeared recently on AEI’s lay summary of new research page)
Reeder, Chiu. Ocean acidification and its impact on ocean noise level: An analysis using empirical and physical models of acoustic transmission loss. Presented at ASA October 2009 meeting.

Udovydchenkov, Duda. Ocean noise level change in response to ocean acidification. Presented at ASA October 2009.

These two papers take a closer look at the widely-noted ocean noise implications of increasing ocean acidification (a combination of factors related to global warming is triggering a steady increase in the ocean’s pH, which decreases sound absorption). When the acidification results were first released there was much speculation that shipping noise would propagate farther, resulting in cumulative ambient noise increases throughout the oceans. These two papers, presented at this fall’s Acoustical Society of America meeting, both come to the same conclusion: low frequency noise will not be significantly increased due to ocean acidification; rather, the primary changes will occur at mid-frequencies. This calms concerns about shipping noise, but may (over time) lead to slightly larger areas being impacted by mid-frequency active sonar, some acoustic harassment devices used by fish farms, noise from recreational boating, and other mid-frequency noise.

Rather than simply looking at the effects of sound absorption, both papers assessed the combined effects of many factors in sound propagation: seabed attenuation and boundary effects, waveguide physics, and the distribution of pH and absorption through different depths of water. The first paper looked at shallow water, a surface duct, and deep ocean, and found that the maximum change in noise transmission occurs at about 2kHz, with no significant change below 800Hz. The second paper notes that at low frequency, little sound is absorbed anyway, so pH-related absorption effects are much smaller than all the other effects, and that at high frequencies, sound does not propagate very far, also minimizing effects of decreased absorption; the authors note that effects can be sizable at middle frequencies. (Ed. note: for both papers, pre-conference abstracts do not quantify the mid-frequency effects; details will follow after the presentations and with later publication of results.)

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