"It’s kind of a mystery how you can make ultrasonic sound without metal, just using soft tissue," said Thode, who presented his as-yet-unpublished research this week at the 171st meeting of the Acoustical Society of America. He was noodling around with ideas for the artificial production of these noises when he came up with an idea for how to model the process.
Along with his father, retired physicist Lester Thode — who was recruited because "he was getting a little restless" — Thode adapted a simple computer model of human vocal cords to fit the physics of dolphin nasal pathways. The model was simple, he said, basically converting the vocal system into "a bunch of legos connected by string".
When they converted the vibration of their simulated blocks and string into sound, the results were a good match for dolphin recordings. The model made a loud thump followed by a long ring, which are the two distinct halves of a dolphin's signature click. The thump, Thode said, happens when the dorsal bursae collide, and the ring is created by lingering vibrations after they pull apart.
But to get the highest frequencies of a typical dolphin call, the Thodes found, he needed to make things a little sticky.
"When these surfaces bounce, they need to stick together for just a moment," Thode said. And a dolphin's natural mucus seems like the most obvious solution to that sticky situation.
The paper still has to be submitted for peer review, so other acoustical scientists and oceanographers will have to wait on the data before they can weigh in on Thode's findings. And while Thode and Thode plan on continuing the project with the support of the US Navy Marine Mammal Program — and hopefully studying some actual dolphins full of actual mucus – it might be tricky to prove their hypothesis.
"We’re predicting that there has to be this mucus, but directly observing it will be difficult because the timescale it happens on is very short," Thode said. Things that happen in fractions of a second are hard enough to observe when they don't occur all up in the delicate nasal folds of a slippery, powerful animal. Still, they're brainstorming possible observation techniques with other researchers.
In the meantime, Thode thinks the study is a great example of the things one can do with a computer model – even one so simple it's basically a digital pile of legos and string.
"The value of a model isn't always its ability to replicate reality," he said. "Sometimes it just has to make you ask the right questions."