ODENSE, Denmark — Say what? It may not surprise many people to learn that humans hear better on land than they do underwater. However, researchers from the University of South Denmark have found that people do surprisingly hear just about as well as seals underwater.
Dating back to the 1950s, there have been plenty of attempts to gauge just how adequately humans can hear underwater. The United States military, in particular, was interested in better understanding how underwater explosions impact divers’ hearing.
Generally speaking, however, these underwater hearing tests came in a variety of shapes and sizes. Some projects asked participants to wear diving equipment, others involved neoprene caps, and some featured air-filled diving masks. All of those objects influence hearing to varying degrees.
Interestingly, all of those earlier studies had one thing in common. They all settled on underwater human hearing thresholds that were “higher than the thresholds we have found in our new study,” says lead study author Jakob Christensen-Dalsgaard in a university release.
Human underwater hearing is surprisingly good
This new project featured seven people, and the average hearing threshold of 71 dB (3.5 mPa) is at 500 Hz. For reference, a “hearing threshold” is defined as a measurement of which volumes you can only just hear. In other words, less is better.
That’s quite notable because 71 dB is 26 dB lower than hypothesized in earlier studies. This suggests humans have much better hearing underwater than previously thought. In fact, study authors explain that threshold at 500 Hz is pretty much how well animals such as cormorants and seals hear underwater.
Of course, before you go and challenge a seal to a hearing contest, keep in mind that seals and dolphins can hear very loud sounds underwater (we can’t), as well as other sounds that humans can’t.
Earlier studies had hypothesized that human underwater hearing works via so-called bone conduction, meaning sound waves vibrate the skull. While that theory would work with earlier estimated human hearing thresholds, this new work complicates matters.
“But we believe that resonance in the enclosed air in the middle ear amplifies the sound and makes the ear more sensitive. We have also shown this in previous studies of cormorants, turtles, and frogs,” Christensen-Dalsgaard explains.
There are more components of hearing than just hearing
Study authors caution that strong hearing involves more than simply picking up sounds. One must determine which direction the sound is coming from. That isn’t easy for a human while underwater.
On solid ground, we can usually ascertain a sound’s direction within a few degrees, give or take. Underwater, however, there is “an up to 90 degrees error margin.” Study authors explain this is to be expected. Why? Our brains are trained to react to small time differences between the ears caused by the speed of sound in air. Underwater, the speed of sound is four times greater, which means the time differences are much smaller.
“The results tell us that humans have a reduced ability to determine the direction of sounds underwater, thus confirming that human hearing is not adapted to work well under water,” Christensen-Dalsgaard concludes.
The study is published in the journal Hearing Research. Continue below for some spectacular photographs of seals.