What do acousticians do?

(This post was originally published September 24, 2021. Posts have been manually reordered for more logical storytelling. To go to the next post in the sequence, click “Previous Post” at bottom.)

Think of a command center — a single person monitoring many screens all around them. This is what we acousticians call home while at sea. On the Gyre Expedition, we listen to sounds in the ocean in real time, detected by a long cable containing underwater microphones (known as hydrophones) towed behind the research vessel Pacific Storm. We use a variety of software tools to visualize these sounds, because our human sense of vision is much better than our sense of hearing (unlike the beaked whales and other marine mammals we study).

Acoustician Annamaria DeAngelis (left) calls up to the bridge to give Captain Yogi Briggs instructions on where to steer the vessel. Acoustician Daniel Gillies (right) is viewing actual beaked whale calls depicted graphically on the television on the wall. The lines on the map will intersect where the whales are located.

Sound is measured on a frequency scale, and our hearing has evolved to be sensitive to frequencies important to us. As we age (starting as early as 20 years old), our hearing deteriorates over time, especially for very high frequency sounds. Do you live in a rural area with very little background noise? Do you wear earbuds often? Are you a construction worker listening to the constant sounds of engines and machinery? Our individual experiences and interactions with sound in our daily lives make our hearing capabilities unique to each of us.

This is a sound clip recorded from the Baird’s beaked whale sighting earlier on our expedition. Watch the image scroll by as the sound plays. Each of these lines is a click from a Baird’s beaked whale. Baird’s use these clicks to look for prey items, and each click presents them with a “picture” of their environment, much like sonar or radar. Can you hear something when you see the green bar pass over vertical lines? Probably not. (Nor can most acousticians!)
Here is the same sound clip, slowed so that the frequency is more suitable for human ears. Can you hear them now? (You may need to turn up your volume a bit.)

Most marine mammals vocalize outside of the human hearing range. This is why we need so many displays — to see this “ultrasonic” sound. Each display provides specific information about these vocalizations, maps indicating the location of the sounds, and forms for recording additional data.

Listening to beaked whales from our “command center” reminds me of playing whack-a-mole. Our primary display consists of little dots, each representing a click, which scroll by telling us about the time and location of the animal that produced them. Clicking on each dot produces three diagnostic plots that provide more information, as seen below. These dots flash by and are typically visible for about one minute as new clicks continue to stream in.

Images produced by our acoustic software (Pamguard) while listening to beaked whales. The top display shows clicks produced through time (most recent on the left), with each color representing a different frequency. Clicking on any dot produces three additional plots below. Beaked whale clicks have a unique shape visible in the Wigner plot (with green background). The shape of the curve and placement of the peaks in the Click Spectrum (to the left of the Wigner plot) indicate the species of beaked whale producing the sounds.

There is still much to learn about species-specific characteristics of beaked whale vocalizations. This expedition is providing us data to add to a growing library of knowledge, ultimately with a goal of identifying beaked whale species based solely on the sounds they make. The recent Baird’s encounter was one such fortunate event where the whales dove within two to three football fields of us and stayed nearby, allowing us to capture thousands of clicks to study. A very rare treat!

~Annamaria DeAngelis

2 comments

  1. Wow!! This is really cool to learn how sounds are recorded, data is collected & translated to be heard by humans!
    Was not aware our human hearing sense could not hear the beaked whales sounds.
    Hugely important to learn more for their well-being & understand better how our human pollutions, including noise, must be causing major problems for them.
    Grateful for your sharing!
    Thank you for doing such important & amazingly intricate work ❤ 💙

  2. All of the beaked whale signals recorded to date which have yet to be linked to actual species is fascinating….hope your cruise finds more links to add data to this field of study.

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