Humans or seals? Who has the finest underwater hearing?
All mammals existed on land millions of years ago, but certain species gradually abandoned the land and adapted to living in the sea, such as seals and whales, which can now dwell underwater.
The remaining species that have survived on land have similarly adapted to a life on land. That is why, in a recent research, a panel of specialists concluded that individuals nowadays hear better on land than under water. However, the study also provides unexpected facts about human hearing.
Jakob Christensen-Dalsgaard, an expert in animal hearing, studies the hearing of organisms such as cormorants, geckos, frogs, and crocodiles, as well as humans, at his laboratory at the University of Southern Denmark. This time, he is collaborating with Ph.D. candidate Kenneth Srensen and biologist Magnus Wahlberg, a specialist in animal underwater hearing who also studies at the University of Southern Denmark.
Decades of hearing tests
Several attempts have been undertaken since the 1950s to test human hearing underwater. The US military, for example, has been interested in learning how underwater explosions effect divers, and the hearing tests have often been extremely different.
Some people were tested while wearing diving equipment, some while wearing neoprene caps, and yet others while wearing air-filled diving masks – all of which might impair the test subjects' hearing.
However, the authors claim that one thing all of these scientific studies have in common is that they all identify hearing thresholds that are greater than the thresholds reported in their recent study.
We hear as well as seals underwater
The average hearing threshold in the latest research, which included 7 participants, is 71 dB (3.5 mPa) at 500 Hz. The sound level below which a person's ear cannot hear anything is referred to as the hearing threshold.
“It is 26 dB lower than hypothesized in previous studies, so we must conclude that humans hear significantly better under water 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,” adds Jakob Christensen-Dalsgaard.
It is worth emphasizing in this context that seals and dolphins, unlike humans, can hear very loud noises underwater, even ones that people cannot hear.
Previous research theorized that the human ear functions underwater by bone conduction, in which sound waves shake the skull. This idea fits the high hearing thresholds discovered in prior investigations.
“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,” says Jakob Christensen-Dalsgaard.
“You should not expect to be able to jump into the sea and orient yourself perfectly using only your sense of hearing,” explains Jakob Christensen-Dalsgaard. “Sense of hearing is not just about being able to pick up a sound. It is also about determining the direction of the sound — and this is very difficult for a person underwater.”
“In the 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,” Jakob Christensen-Dalsgaard explains.
“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.”
Reference: “Is human underwater hearing mediated by bone conduction?” by K.Sørensen, J.Christensen-Dalsgaard and M.Wahlberg, 19 March 2022, Hearing Research.
Reviewed by Lilit
on
July 04, 2022
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