Scientists at MIT’s Lincoln Laboratory have created a new type of underwater microphone, called a hydrophone, that is smaller, cheaper, and just as sensitive as the ones used today.
This device is built from a simple, commercially available MEMS microphone—the same kind of tiny microphone used in smartphones—and could be valuable for the U.S. Navy, researchers, and industries that rely on underwater sound.
Hydrophones work like microphones in the air, but they record sounds underwater.
These sounds help scientists and engineers understand the ocean environment, track marine animals, study natural events, and support naval operations.
Traditional hydrophones can be large and expensive, so the research team set out to design something smaller and more affordable without losing high performance.
MEMS, or microelectromechanical systems, are miniature devices that contain tiny moving parts. They are used in many modern technologies, but surprisingly, no commercial hydrophones today use MEMS technology.
The team initially planned to build their new device using advanced microfabrication techniques.
However, that approach turned out to be too costly and complicated. Instead, they found a simpler solution: build the hydrophone around an off-the-shelf MEMS microphone.
To turn a standard microphone into a deep-water sensing device, the researchers worked with Tufts University and industry partners.
They placed the microphone inside a special waterproof polymer while leaving a small air cavity around the vibrating part of the microphone, called the diaphragm.
The biggest concern was that the air gap and packaging might weaken the signal. But after many simulations and tests, the team found that the microphone’s naturally high sensitivity balanced out any loss in sound.
The device went through careful engineering, including computer modeling, electronic system design, and prototype manufacturing. In July, the team tested the hydrophones in Seneca Lake in New York.
They lowered the devices into depths ranging from 100 to 400 feet and sent out underwater signals to measure how well the hydrophones detected them.
This deep-water test was an important step because earlier tests had only been done in indoor water tanks.
The researchers wanted to see if the hydrophone would perform well under real conditions, including high pressure and cold temperatures. The results exceeded expectations. The new hydrophone picked up sound levels close to the quietest possible ocean conditions—known as “sea state zero”—even at 400 feet underwater and in 40°F water.
The small size, low cost, and strong performance of this prototype make it promising for many uses, from scientific studies to military operations. The team is now discussing how to bring this technology into government and commercial use and plans to continue refining the design.
