For a long time, it was thought that giraffes were mute. Eventually, it was discovered that giraffes murmur at such a low pitch that humans have difficulty detecting it.
A lot more is going on outside the reach of our ears. A host of animal species communicate among themselves using sounds that are either too high or low for human perception.
Human beings hear sounds whose frequency is between 20 and 20,000 hertz. Sounds below the lower limit are known as infrasound, while those above the other extreme are called ultrasound.
Usually, large animals emit sounds in the lower frequencies and smaller animals in the higher. The hearing range is affected by the structure and size of the ear.
Is lower voice more convincing?
“To hear high sounds, you have to have a small middle ear and light auditory ossicles,” says Sirpa Nummela, a docent in zoology.
The human speaking voice ranges between 100 and 300 hertz, depending on the age and gender of the speaker. Some people strive to speak in a voice lower than natural to them, as it is considered more convincing. This might be because people unconsciously associate a low voice with a larger stature.
Culture also affects pitch. Over the decades, the fundamental frequency of women's speaking voice has markedly fallen in the West. However, in certain cultures, women are still expected to speak in a more high-pitched voice than they normally would.
Very powerful high or low sounds can be perceived through tactile sensation, even without actually hearing them. This is why a risk of adverse health effects has been associated with infrasound.
However, a study was published in the spring on the effects on humans of infrasound generated by wind power plants. The study demonstrated that there were none: humans did not perceive the infrasound produced by wind power, nor did they react when exposed to the sound in a range of listening experiments.
New technology has allowed us to hear sounds that we have earlier been unable to detect. Elephants have been filmed receiving messages sent by members of the same species. The infrasound they emitted could be identified in the recording only in the laboratory.
Adult male elephants live apart from females. When the females are in heat, they communicate it to the males using infrasound that travels through air, or airwaves that travel across the soil.
A cacophony on waves
The benefit of infrasound is that it carries far and fades away slowly. Acoustic energy progresses in wave motion. The greater the wavelength, the lower the frequency. Low frequencies require less energy than high ones.
In water, sound travels nearly five times as fast as in the air, with the length of the wave growing in proportion. Baleen whales are able to communicate across hundreds of kilometres relying on infrasound. However, the noise generated by shipping interferes with whale communication and navigation.
“Whales beach every now and then, and it is suspected that the cacophony in seas is behind this. When inspecting the inner ears of dead whales, researchers have found damage caused by noise,” Nummela says.
Sharp sounds increase focus
Nummela studies the sound communication of mammals by examining the anatomy of their ears. For instance, she has compared the middle ear structure of whales and seals. Through evolution, whale ears have completely adapted to marine life, whereas seals also spend time on land and ice floes, and their ears have not undergone the same changes as those of whales.
According to Nummela, the changes in the ears of whales and seals have been necessary for their survival. For them, hearing is essential for communication, but also for navigation and finding food. Dolphins and bats use ultrasound for hunting. They make high-pitched sounds to locate their prey with the help of echoes.
“The higher the frequency, the more accurate the image generated through echo-sounding,” Nummela explains.
Lower sounds can also be used for echo-sounding. If a person comes across a dark cave, they usually instinctively start making sounds to picture the shape and size of the space.
A sound from the dentist’s office
Ultrasound is familiar also from the maternity clinic and the dentist's office. Sound at a sufficiently high frequency can be utilised in imaging and cleaning. However, this ultrasound differs significantly from the one that animals use.
Animals that produce the highest sounds reach roughly 100–200 kilohertz, while medical imaging requires frequencies exceeding one megahertz. Making such a sound would require more energy than any animal would have at its disposal.
Small animals use high-frequency sounds that their predators cannot hear to communicate with one another. Mice and rats rely on ultrasound, with rats even emitting high-pitched sounds resembling laughter when they play.
However, evolution has expanded the auditory range of predatory mammals, enabling them at times to hear their prey. In a way, it comes down to an arms race between sounds and hearing.
There are ultrasound devices that are used to keep rodents away from summer cottages. Nummela doubts their effectiveness: although mice hear the sound, they get used to it quickly since nothing unpleasant follows the sound.
Hearing degenerates in humans as well as in animals as they age, making elderly people unable to hear crickets anymore.
“After turning 20, we lose roughly one hertz a day from the upper register,” Sirpa Nummela says.
The article has been published in Finnish in the 8/2020 issue of the Yliopisto magazine.