Echolocation, Infrasonic and Ultrasonic Hearing

Seeing with sound

On a summer evening near a lake in Ontario, watch bats swooping over the water. They catch flying insects in near-darkness. They are not relying on their eyes: they are listening. In this last teaching lesson of the unit, you will use everything you know about waves (frequency, wavelength, reflection, speed and the Doppler effect) to explain how animals hear sounds we cannot, and how people copy them.

Echolocation

An echolocating animal sends out short, loud calls and listens for the echoes:

• The time delay of an echo gives the distance: d = v t ÷ 2.
• The loudness and direction of the echo give the size and position of the target (two ears compare arrival times and loudness).
• A Doppler shift in the echo tells whether the target is moving toward or away.

Bats that hunt insects use mostly ultrasonic calls, above 20 kHz. Why so high? The wavelength must be about as small as the target, or the wave mostly passes around it instead of reflecting. A 20 kHz call in air has λ ≈ 17 mm, too big to give a strong echo from a small mosquito; an 80 kHz call has λ ≈ 4.3 mm. The trade-off: high-frequency sound is absorbed by air more quickly, so it only works over short distances (a few metres to tens of metres). As a bat closes in on an insect, it calls faster and faster, a rapid "feeding buzz", to update the target's position.

Toothed whales such as dolphins, porpoises and the belugas of the Arctic and the St. Lawrence River produce rapid clicks and focus them with a fatty organ in the forehead called the melon. Sound travels about 4.5 times faster in water than in air, so the same frequency has a longer wavelength in water.

Some insects fight back: many moths can hear bat calls and dive or swerve when they detect them, and some tiger moths make clicks of their own.

Infrasonic and ultrasonic hearing

Ultrasonic hearing (above 20 kHz): bats, dolphins, dogs and cats all hear above the human range. That is why a dog whistle, which sounds silent to us, makes a dog's ears perk up.

Infrasonic hearing (below 20 Hz): elephants make very low rumbles, partly below 20 Hz, and large whales such as blue and fin whales make very low-frequency calls. Low frequencies have long wavelengths and are absorbed very little, so they travel far: elephant rumbles can be detected kilometres away, and some whale calls travel hundreds of kilometres through the ocean. Long waves also bend easily around obstacles such as trees and hills.

The pattern: high frequency → short wavelength → fine detail, short range. Low frequency → long wavelength → long range, little detail.

People using sound

• Sonar on ships sends sound pulses down and times the echoes to map the sea floor and find fish or submarines.
• Medical ultrasound uses frequencies of millions of hertz (MHz). Echoes from the boundaries between tissues build an image of a baby before birth, the heart or other organs, without the ionizing radiation of X-rays. Doppler ultrasound measures blood flow.
• Ultrasonic sensors on cars beep as you back toward an obstacle; ultrasonic cleaners shake dirt off jewellery and tools.
• Some people who are blind have learned to echolocate with tongue clicks, sensing walls, cars and doorways from the echoes.

A Canadian pioneer

Reginald Fessenden (born in Quebec in 1866) was an inventor best known for pioneering radio broadcasting. After the sinking of the Titanic in 1912, he developed an underwater sound device, the Fessenden oscillator, for signalling between ships, and in 1914 he demonstrated that it could detect an iceberg by its echo, an early step toward sonar.

Careers in sound (A2.1)

• Audiologist: tests hearing and fits hearing aids; usually a university degree followed by a master's degree in audiology.
• Diagnostic medical sonographer: makes ultrasound images; usually a college or specialized post-secondary program and certification.
• Acoustical engineer: designs quiet buildings, concert halls, noise barriers and products; an engineering degree.
• Hydrographer or sonar technician: maps waters and the sea floor with sonar; college or university training, sometimes through the Canadian Armed Forces or the Canadian Hydrographic Service.