International definitions of “Hearing” and “Sounding”

In our last newsletter I provided an account of a International Standards Organization (ISO) meeting where we worked on ISO 18405 “Underwater acoustical terms,” and I elaborated on how much of the meeting was spent on what terms were needed to define biological sound production, and what terms were needed to define sound perception.

Regarding sound production, I am sticking with the terms “phonation” – as an “intentional creation of sound,” “vocalization” – as “a deliberate creation of sounds by way of passing air through an organ specifically used to make sounds.” For me these two terms are nested in the term “sonation”- “a biological activity that makes sound”

So the song of a humpback whale is a vocalization a phonation, and a sonation; the cricking noise produced by spiney lobsters scratching their antennae on their shell structures is phonation and sonation; and the “snapping” of a pistol shrimp is just a sonation.

Perceiving acoustical energy becomes a much more complicated affair because it is conveyed at a molecular level though all three material states – gasses, liquids, and solids, with an energy transfer continuum between “pressure gradient” and “particle motion,” depending on the density and elasticity of the transferring material. Given that all living habitats occur as some mix of these material states, biological adaptations reflecting these mixed-habitat states get complicated really fast.

So let’s dig into some of these a bit. The image array above – from to left to right, from upper left to lower right:

• Sperm whale phonic lips  (called “monkey lips” by the whalers) are extremely dense and powerful muscles that modulate the passage of air through them by way of bladders adjacent to the breathing system, producing the complex sounds these whales use for both echolocation and communication.

• Acoustical lipids (orange) in the head of a dolphin that conduct and direct sounds (from Ketten). Echolocation clicks and communication sounds are produced by phonic lips along the breathing passage and behind the “melon” which directs outgoing sounds. Acoustical lipids in the jaws conduct sound into the hearing system.

• Gray whale skull with the mandibles, maxilla, and pre-maxilla removed revealing a rostral channel of surprisingly thin bone which envelopes a fatty (acoustical?) lipid that terminates in a porous anterior-cranial process.

• Pharyngeal teeth of a “Black Drum” located on the tongue and palate. While used to process food, they also make a lot of noise with these – particularly during breeding season.

• Statocysts encircle the mantle of jellyfish, each containing a statolith providing a rudimentary inertia-sensing organ – the precursors to semi-circular canals found in vertebrates.

• Barnacle shell structures creak as they articulate them, which collectively as a colony can sound like a “whistle.”

• Barnacle nauplius (larvae) are free floating zooplankton that probably use their fine cilia to sense the whistling and creaking of barnacle colonies where they can productively recruit.

• “Cupula” are a gelatinous envelope enclosing sensory cilia along the lateral line of fishes used to sense acoustical particle motion.

• “Pistol” or snapping shrimp snap their large claws, producing a cavitation bubble that pops and stuns their prey. This is one of the more ubiquitous sounds heard in coastal waters.

• Harbor porpoise mandibles and otic capsules. The mandible on the top reveals the thin bone pocket that encloses the acoustical lipids which confer sound to the otic capsules (red dot in #2).

• Harbor seal whiskers, or “vibrassae” sense particle motion variations in water caused by the turbulence paths left by swimming fish – a perturbation they can sense and follow from up to 180 meters distant from their prey.

• The beluga whale is probably the most acoustically complex animal on the planet.

• The midshipman oscillates their swim bladder at ~180Hz during breeding season. Two or more males seem to phase these signals.

• The angler fish have all sorts of filaments and cilia protruding. It would not be surprising if some of these facilitated their sensing acoustical particle motion in their deep, dark habitats.

• Graphic of the interior hearing systems of various fishes. In some cases the swim bladder is coupled to these structures – making their entire body a swimming ear.

• California sea lion. She has a haughty expression because she can sense acoustical energy in at least three different ways: Pressure gradient above and below water through her ears, as well as sensing particle motion through her vibrassae.

…and then there is the added dimension that the density and elasticity of marine animal tissues are close to that of their aqueous habitats – offering another pathway for acoustical perception.

Each of these bullet points above is a much longer discussion. So while the specifics of how underwater acoustical energy is described in ISO 18405, the particulars get pretty wooly, and why after a couple hours of discussion, a collection of Physicists, Biologists, and Acousticians have still not settled on clear definitions of the particulars of how marine animals make and perceive acoustical energy.

Thanks for reading!