11.4. Range of Hearing

The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s). Children under the age of five and some animals, such as dogs, can hear up to 25 kHz (1 kHz = 1000 Hz).

The range of sound that humans can hear is measured in hertz (Hz), which quantifies how many sound wave cycles pass a point in one second. While most adults can hear sounds between 20 Hz and 20,000 Hz, younger children and specific animals can detect even higher frequencies up to 25,000 Hz. As people age, the higher frequencies become harder to hear, which is why some sounds that are easily heard by children are often imperceptible to older adults.

Think of a dog whistle, which operates at frequencies above 20,000 Hz. Adults may hear nothing when the whistle is blown, but dogs can hear it clearly. This illustrates how different species have varying hearing ranges, allowing them to perceive noises that humans cannot.

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. If we could hear infrasound, we would hear the vibrations of a pendulum just as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.

Infrasonic sounds are those that fall below the range of human hearing, below 20 Hz. These frequencies can be produced by natural events, like earthquakes, and by certain animals, like rhinoceroses, which use these low sounds for communication. On the other end of the spectrum, ultrasonic sounds are those above 20 kHz, and are commonly used by animals like bats for echolocation, as well as in human technologies such as medical imaging.

Consider how elephants communicate over long distances using infrasound that can travel through the ground. Meanwhile, bats use ultrasonics to navigate and hunt in the dark, emitting sounds that bounce back from objects, allowing them to determine distance and size even without sight.

As people grow older, their ears become less sensitive to higher frequencies.

Aging affects our ability to hear higher frequency sounds. This is typically due to changes in the inner ear, which can lead to hearing loss, particularly for frequencies that would be easily detected in youth. This gradual decline means that older adults may struggle to hear sounds such as birds chirping or high-pitched electronic beeps.

Imagine being at a crowded place where a child shouts or an electronic toy beeps. A young individual may notice these sounds immediately, but an older adult might struggle to hear them clearly due to the high frequencies. This can lead to misconceptions about the volume of the sounds being made.

It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.

Many animals exhibit heightened sensitivity to infrasound, which allows them to detect natural disasters such as earthquakes before they occur. This ability may be due to evolutionary adaptations that help them survive in their environments. The detection of such sounds is crucial since these frequencies travel long distances and can provide early warning of impending danger.

Imagine a group of elephants migrating. If they sense low-frequency vibrations from an earthquake through the ground before humans can perceive any danger, they may change direction and avoid the disaster. This instinctive behavior showcases the advantage of having sensitive hearing capabilities beyond that of humans.

Ultrasound is produced by animals such as dolphins, bats, and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high-frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture.

Ultrasonic sounds play an essential role in both animal communication and human technology. Many animals, including dolphins and bats, use ultrasonic frequencies for echolocation, enabling them to navigate and locate prey in their environments. In humans, ultrasound has a wide range of applications, from medical imaging to cleaning delicate items. The adaptability of these sound waves allows them to permeate various fields effectively.

Consider a doctor using an ultrasound machine to visualize a baby during a mother's pregnancy. The machine sends high-frequency sound waves into the body, which bounce off tissues and create an image on a monitor. This process is akin to how bats use sound to 'see' their surroundings, navigating in total darkness by interpreting the echoes of their ultrasonic calls.

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