Getting everyone on the same page

Underwater Acoustical Terminology – Working draft

Back in 2008 I had a hunch that marine animal auditory thresholds would correlate well with natural ambient noise levels in the sea, and what were defined by the “Wenz Curves.” We examined and reviewed over 150 papers on marine mammal auditory thresholds, and an equal number of papers on auditory thresholds of fishes, and lined them up along the Wenz Curves. (thanks to Libby Horn on marine mammals, and Gail Hurley on fishes).

What we found was correct – at least for marine mammals. Marine mammal thresholds conformed to the natural noise levels of the sea – answering the hypothetical question of “why would you want your hearing sensitivity to be submerged in omnipresent habitat noise?” The fishes also correlated with the curves, but a in a bit more of a complicated manner (stay tuned…).

I presented our work at the 2008 Acoustical Society conference New Orleans. And while the presentation was about the results of our literature review, the gist of the presentation was more about our “metrics” findings.

“Metrics” are how physical quantities are expressed. Meters (m), kilograms (kg), and seconds” (s) are mono-dimensional metrics based on immutable physical properties, and thus remain the same regardless of whatever planet you’re on. Newtons (force), Pascals (pressure), and Joules (net energy) are also metrics that are expressed relative to set reference physical properties (e.g. kg∙m∙s^-2, kg∙m^-1∙s^- 2, and kg∙m^2∙s^- 2, respectively), and thus can also be used on any planet.

But when you want to express the impacts or interactions of these physical properties on other things – particularly on living things, it gets complicated really fast.

In the ~300 papers we reviewed, the problem we consistently encountered was how woolly the metrics were in expressing the effects of acoustical energy on marine animals. We had to dump over 20% of all papers because the metrics didn’t make sense – or could not be reconciled to indisputable quantities.

Our objective was reconciling the published hearing sensitivity “audiograms” of the subject animals when exposed to sounds or noise levels at a given frequency or frequency range, and determine if there was a correlation between hearing those thresholds and natural ambient noise.

We found in the published, peer-reviewed papers we synthesized, there were no less than nine different formulas expressing exposure sensitivity. Some were simple two-factor expressions (sinusoidal frequency at “n sound pressure level”). Others were more nuanced (1/3 8va “pink noise” x Pa∙m2∙s-2).

We were able to titrate a lot of the papers out to some basic form of equivalence because they used metrics that could be calibrated and converted. But we tossed out a number of the papers for sometimes really embarrassing reasons.

So while we proved our point on the correlation between animal sound perception and ambient ocean noise levels, I realized that if we were going to come up with clear regulatory guidelines (why we did this exercise), we would all need to be fishing in the same metrics pond.

It was at this point that, as a member of “Acoustical Society of America” (ASA), I signed up to the “American National Standards Institute,” (ANSI), and through this, signed up as an “ANSI delegate” to the “International Standards Organization” (ISO).

I mention this to shed light on our collective work of clarifying international metric expressions, but also to complain a bit about having to wake up at 4am last week, trying to be smart with an international organization of scientists – biologists and physicists, attending an ISO conference with members who reach from Berlin (UTC +2) to Tokyo (UTC +9, “tomorrow” for me – UTC-7).”

We did get a lot of work done last week, for which I thank the conveners, Michael Ainsley and Michele Halvorsen, both from JASCO Applied Science – and I am honored to be in the company of some really facile and informed minds from other organizations who are all above my institutional pay grade.

Our remit is to make sure that when we express a word or term that refers to sound in the sea, we all know what we’re talking about. And we can substantiate that knowledge with clear and unambiguous metrics.

And the testimony to our success is that quite a number of the papers we synthesized in 2008 would not have survived peer review had they been submitted in 2024.