Human Hearing Range: Infrasound and Ultrasound

The human ear can detect sound frequencies from about 20 Hz to 20,000 Hz; sound below 20 Hz is called infrasound, and sound above 20 kHz is called ultrasound. In brief: hearing range narrows with age, with high frequencies lost first. Elephants and whales use infrasound to communicate over long distances, and bats and dolphins use ultrasound for echolocation. Ultrasound is used in medical imaging, cleaning and industrial testing.

Dogs respond to whistles that people cannot hear, and sonar maps the sea floor with sounds well above the audible range. This guide describes the hearing range and how the ear works, the properties and uses of infrasound and ultrasound, compares the ranges in a table, works through wavelength and echo calculations and corrects common mistakes.

What is the human hearing range?

The ear collects sound waves with the outer ear, which directs them through the ear canal to the eardrum. Three small bones (hammer, anvil and stirrup) transmit and amplify the vibrations to the fluid-filled cochlea, where hair cells convert the movement into nerve signals. A healthy young person can hear frequencies from about 20 Hz to 20 kHz. The ear is most sensitive between 2 kHz and 5 kHz, which includes the frequencies of speech. Hearing loss with age (presbycusis) usually reduces the upper limit to about 12 to 15 kHz by middle age. Exposure to loud noise (above about 85 dB for long periods) can damage the hair cells permanently. The threshold of hearing depends on the frequency: sounds at very low or very high frequencies must be more intense to be heard.

What is infrasound?

Infrasound has frequencies below 20 Hz and wavelengths of more than about 17 m in air. Such waves are attenuated very little and diffract around obstacles, so they travel long distances. Sources include earthquakes, volcanoes, storms, avalanches, wind turbines, and large machinery. Animals use it: elephants communicate with calls of about 14 to 35 Hz that may travel several kilometers, and blue and fin whales produce calls at about 10 to 40 Hz that are detected over hundreds of kilometers. The International Monitoring System uses infrasound detectors to detect nuclear explosions. Very intense infrasound can cause discomfort, though the claims that it causes ghostly feelings are not well established.

What is ultrasound?

Ultrasound has frequencies above 20 kHz. Because the wavelength is short, it can be formed into narrow beams and reflects from small objects. It is produced by piezoelectric crystals, which vibrate when an alternating voltage is applied to them (and generate a voltage when they are deformed, so they can also be used as detectors). Applications:

• Medical imaging: frequencies of 2 to 15 MHz produce images of the fetus, heart and organs, from the echoes at boundaries between tissues. It uses no ionizing radiation.
• Therapy: focused ultrasound can break up kidney stones (lithotripsy) and warm tissue.
• Cleaning: ultrasonic baths clean jewelry and instruments by cavitation, the formation and collapse of tiny bubbles.
• Industrial testing: detecting cracks in metals, and measuring the thickness of materials.
• Sonar and echolocation: ships map the sea floor, and bats (frequencies 20 to 100+ kHz) and dolphins locate prey.

Hearing ranges compared

Worked example: wavelengths and ultrasound imaging

1. Limits of human hearing in air (v = 343 m/s). At 20 Hz: λ = 343 ÷ 20 = 17.2 m. At 20 kHz: λ = 343 ÷ 20,000 = 0.0172 m = 17.2 mm.
2. Elephant infrasound. At 15 Hz: λ = 343 ÷ 15 = 22.9 m.
3. Bat. At 100 kHz: λ = 343 ÷ 100,000 = 3.4 mm, so it can detect an insect of a few millimeters.
4. Medical ultrasound. At 5.0 MHz in tissue (v = 1,540 m/s): λ = 1,540 ÷ 5.0 × 10⁶ = 3.1 × 10⁻⁴ m = 0.31 mm. Higher frequencies give a better resolution but are absorbed more strongly in tissue.
5. Echo time. An organ boundary is 10 cm deep. The pulse travels 20 cm in all: t = 0.20 ÷ 1,540 = 1.3 × 10⁻⁴ s = 130 µs. The scanner measures this delay to find the depth.

How were these sounds discovered?

Lazzaro Spallanzani showed in 1793 that bats navigate by hearing and not by sight. The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880, and Paul Langevin developed the first ultrasonic sonar around 1917 to detect submarines. Karl Dussik used ultrasound for medical diagnosis in the 1940s, and Ian Donald introduced obstetric ultrasound in Glasgow in the 1950s.

Which mistakes are common?

Mistake 1: Saying that ultrasound is louder than normal sound

Ultrasound means a frequency above the audible range. It is not about loudness.

Mistake 2: Thinking that all animals hear the same range

Many animals hear higher or lower frequencies than humans.

Mistake 3: Mixing up infrasound and ultrasound

Infra means below (low frequency), and ultra means beyond (high frequency).

Mistake 4: Using the speed of sound in air for tissue

Ultrasound in the body travels at about 1,540 m/s.