Humans' Underwater Hearing Better Than Previously Thought: Study

How well can humans really hear underwater? It's apparently better than previously thought, a new study has found.

Hearing is important for all vertebrates, and it's especially crucial for those that live in the darkness or in murky waters, authors of the new study, published in the journal Hearing Research, noted. In the case of mammals, although all of them used to live on land, many such as seals have adapted to spending most, if not all, of their time underwater, so they have also evolved to have improved underwater hearing and vision.

"Aquatically adapted ears, such as the ones of marine mammals, do not only have low hearing thresholds underwater, but also acute directional hearing abilities," the researchers wrote. "For submerged humans, the usual cues to determine sound direction — interaural time and level differences — are different due to high sound velocity of water and thereby longer wavelengths."

For their new study, the researchers looked at just how well humans can actually hear underwater. There have been many studies on the matter, the University of Southern Denmark (SDU) noted in a news release.

"But common to all these scientific studies is that they all find hearing thresholds that are higher than the thresholds we have found in our new study," SDU's Jakob Christensen-Dalsgaard, an expert in animal hearing and one of the study authors, said in the news release. Furthermore, some of them had diving equipment or masks on, which can actually affect their hearing, SDU noted.

For their work, the researchers tested the underwater hearing of seven divers in a low-noise pool. They had no reported hearing disabilities in the air, and they weren't wearing hoods or SCUBA gear. All of them were also tested both in the air and underwater.

"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," Christensen-Dalsgaard said. "In fact, the threshold at 500 Hz is in line with how well animals such as cormorants and seals hear underwater."

Previous studies have suggested that human hearing underwater works through "bone conduction" or when the sound waves "vibrate the skull," SDU noted. However, the researchers said that "at least in the frequency range up to 1kHz we are confident that bone conduction cannot explain the underwater sound sensitivity."

Instead, a possible alternative pathway for underwater "sound stimulation" might actually involve the middle ear, which increases sensitivity. This mechanism has also been seen in the underwater hearing of turtles and aquatic frogs, according to the researchers.

But even though they found that humans' underwater hearing is likely better than previously thought, it doesn't mean humans can actually hear well underwater.

"You should not expect to be able to jump into the sea and orient yourself perfectly using only your sense of hearing," Christensen-Dalsgaard said, noting that hearing also entails pinpointing the direction of the sound, which is still difficult for humans underwater.

"In air we can determine the sound direction within a few degrees, but in water there is an up to 90 degrees error margin," Christensen-Dalsgaard added.

Overall, the researchers' work showed that humans' underwater hearing may actually be better than previously thought. However, our "extremely poor directional underwater hearing" shows that we're still "not adapted for underwater sounds."