On Wednesday, Oct. 3, Dr. Dennis Lindwall gave a lecture entitled “Directional Acoustic Observation of Whales” at the science building. Lindwall has a background in both physics and music, and physics Prof. Juliette Ioup stated in introduction, “If you go to any music performances around town, you may hear him play the violin.”
Lindwall began the talk by announcing wryly, “I don’t know anything about whales … this is probably going to be the shortest talk you’ll hear.”
His work involved the study of data acquired from sound equipment placed below the ocean’s surface, and his task was to analyze it using mathematical formulae.
Doing this, Lindwall would determine the positions of whales in the ocean.
“A colleague … said that he had access to some whale data … and that maybe we could work together,” said Lindwall.
The primary issue of his talk was how to better determine the directions of the whales as well, in order to get a more complete picture of their behavior.
He showed the audience some graphs to give examples of his data. One was titled “Spectral plots of recordings made during a 5 minute period,” and he explained that it “contains both blue whale calls and fin whale calls.”
Identifying the whales by their sounds in a recording begins with recognizing the frequencies which different species of whale use to communicate.
Lindwall introduced some of the instruments deployed to collect the data. In this case, underwater microphones called geophones and hydrophones were used to record continuously for more than three years in the Pacific Ocean.
“When you apply for funding, you have to show that the concept works. I had to show that we could get some directional information on some whale calls,” said Lindwall. “We didn’t get funding, so that was the end of it.”
“It is tremendously expensive to put these hydrophones out,” Ioup explained later.
Lindwall described to the audience his idea to help better determine the direction of sounds using the audio equipment. The concept begins with measuring a different quality of the sound movement than the one that physicists usually study.
“Imagine you’re looking at the molecules in a fluid — water or air,” he said. Both are mediums through which sound waves move. “Normally, people measure the sound with pressure. But there’s a lot more going on than just pressure,” he said, referencing kinetic energy as worth paying attention to. Kinetic energy is the energy an object, such as an air particle, has when it is moving.
“A single pressure sensor — it can tell you the amplitude and the frequency, but nothing about the direction,” Lindwall continued. Scientists will arrange an array of pressure sensors to get a better sense of the direction that the sound waves are moving, but this is still imperfect.
Lindwall suggested using a new type of instrument, an “accelerometer-based underwater vector intensity sensor.”
“It’s an accelerometer hydrophone,” he said.
Lindwall also demonstrated that other sources of sound in the area can produce noise that interfere with the data collection. These sources can include things like drill ships and even distant thunderstorms.
There are many reasons to research whale behavior in the oceans.
“We look for how many whales, which kind … with some signals, we can determine the size — whether we have adults or [young whales],” said Ioup. “If you know how many are out there, where they are and how they’re eating — what impact the oil spill had on them, for example — you can take care of them better.”
The physics talk is one of a semester-long series in the physics department. Every week, a new one occurs at 3 p.m. in science building room 1053, and three are left in the semester.
The Oct. 10 talk is entitled “DSP Physics, a Deep Learning Solution,” the Oct. 17 talk will bring Joseph Kuhner from Stennis Space Center and Oct. 24’s talk is titled “Ambient Energy-Harvesting Devices.”