We humans are perfectly adapted to life on land. In or under water, on the other hand, we quickly reach our limits. We cannot compete with aquatic mammals in any respect. This is also true for hearing. At least, this was the view of science until now. In a new study, researchers at the University of Southern Denmark have now found that we humans can hear better underwater than we thought.
That humans are better adapted to hearing on land than underwater is not surprising. From an evolutionary biology perspective, being able to hear well underwater has little benefit to us. Nevertheless, our hearing does not seem to compare as poorly with marine mammals such as seals as previously thought by scientists. According to the new study, we can hear just as well as seals at certain frequencies.
Human unterwater hearing has been studied since the 1950s. Especially in the military, people wanted to understand how divers were affected by underwater explosions. All of these studies, conducted under very different conditions – with diving equipment, with neoprene caps or with air-filled diving masks – found higher hearing thresholds than the current study, explains Jakob Christensen-Dalsgaard, an expert in animal hearing and professor at the University of Southern Denmark and lead author of the study.
Seven people participated in the new study, and the tests revealed that underwater, the average hearing threshold of 71 decibels is 500 hertz. The hearing threshold indicates the volume that one can only just hear. Over water, sounds such as a running shower or human coughing correspond to a volume of 71 decibels.
“It is 26 dB lower than hypothesized in previous studies, so we must conclude that humans hear significantly better underwater than previously reported by science. In fact, the threshold at 500 Hz is in line with how well animals such as cormorants and seals hear underwater,” Christensen-Dalsgaard says.
In the earlier studies, it was assumed that the human ear works under water through the so-called bone conduction, which means that the sound waves vibrate the skull. This hypothesis would fit with the high hearing thresholds found earlier.
“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.
However, even though we can match seals at certain frequencies underwater, this does not mean that we can also orient ourselves just as well. For this, our hearing lacks a crucial ability: the precise determination of the direction from which the sound waves come.
“In air we can determine the sound direction within a few degrees, but in water there is an up to 90 degrees error margin. This is not so strange, because we are trained to react to the small time differences between the ears, which are due to the speed of sound in air. In water, the speed of sound is four times greater, and the time differences are much smaller,” Christensen-Dalsgaard concludes.
“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 underwater.“
Julia Hager, PolarJournal