Undersea audio recordings provide important information about the ocean environment, marine life, and human activities. However, capturing audio recordings underwater is not as simple as dropping a microphone below the surface. A traditional microphone not only wouldn’t work under these conditions, but it would also be instantly destroyed by the water. As such, specialized instruments called hydrophones must be used for this purpose. But compared to standard microphones, hydrophones are large and expensive, which limits the applications they can be used for.
Fortunately, researchers at MIT have developed a new type of hydrophone that is both tiny and inexpensive. Despite the low cost and small size, these new hydrophones are more sensitive than other devices currently on the market. As such, the researchers’ technology could make high quality underwater audio recording much more accessible in the near future.
The new device relies on a surprising component: a standard, commercially available MEMS microphone similar to those used in phones and other consumer electronics. By building their hydrophone around this off-the-shelf part, the team managed to sidestep the high manufacturing cost typically associated with microfabricating underwater sensors. The result is a compact, low-cost instrument that delivers sensitivity equal to, and in some cases exceeding, that of traditional high-end hydrophones.
A field test of the underwater microphone (📷: D. Freeman et al.)
While MEMS microphones are widely used on land, no existing commercial hydrophone has successfully integrated this technology. The research team initially planned to fabricate a custom sensor to fill this void. But when early designs proved too expensive, the researchers pivoted to a simpler solution: encapsulating a standard MEMS microphone in a water-resistant polymer while preserving a small air cavity around its diaphragm. Maintaining this air gap was essential, as it allows the diaphragm to respond freely to underwater sound waves.
One of the central engineering challenges was preventing excessive signal loss caused by the packaging and air cavity. Through extensive simulations and design iterations, the team found that the microphone’s inherent sensitivity compensated for these losses. Testing showed the prototype performing reliably at depths up to 400 feet and at temperatures as low as 40 degrees Fahrenheit.
To test their approach, the researchers conducted a deep-water field test at Seneca Lake in New York. There, the hydrophone consistently delivered strong signal-to-noise performance, coming within a few decibels of the quietest possible ocean conditions. Achieving such sensitivity in deep, cold water was an important test for this new hardware.
With its small size, low power consumption, and low cost, the prototype could open the door to widespread use across scientific, industrial, and other applications.
