Ultrasound

An ultrasound is a sound wave with a frequency above 20,000 Hz, beyond the audible limit for the human ear.

Discover FizziQ

How to measure it in class

With FizziQ, students can explore the limits of their hearing and detect sounds close to the ultrasound threshold.

Steps:

Open FizziQ and use the sound library to emit pure tones of increasing frequencies: 1,000 Hz, 5,000 Hz, 10,000 Hz, 15,000 Hz, 18,000 Hz, 20,000 Hz.
Ask students to raise their hand as long as they hear the sound. Note for each student the maximum perceived frequency.
Observe that the hearing limit varies among students and decreases with age. Younger people generally hear higher frequencies.
Interpret: beyond each ear’s perception limit, the sound still exists (FizziQ’s frequency spectrum confirms this) but it is no longer audible. This is the ultrasound domain.

Scientific activities on this topic

Several experiments easily achievable with a smartphone, tablet, or computer allow exploring the ultrasound domain and the limits of hearing.

1: White Noise - What frequencies make up white noise?: https://www.fizziq.org/en/activities/white-noise/

2: Instrument Timbre - Why does the same note sound different depending on the musical instrument that produces it?: https://www.fizziq.org/en/activities/the-tone-of-an-instrument/

3: The Shepard Sound Effect - Analysis of the Shepard auditory illusion: https://www.fizziq.org/en/activities/the-shepard-sound-effect/

Learn more

A bit of history

Ultrasounds have been used technologically since the early 20th century. After the sinking of the Titanic in 1912, Paul Langevin developed an ultrasound iceberg detector that would become sonar. This invention is directly linked to the discovery of the piezoelectric effect by the Curie brothers.

Animal echolocation

Bats emit ultrasound pulses from 20 to 200 kHz and analyze echoes with remarkable precision. They can detect a mosquito several meters away. Dolphins use a similar system in water, with frequencies up to 150 kHz.

Advanced medical applications

Beyond imaging, high-intensity focused ultrasound (HIFU) allows destroying tumors without surgery by concentrating acoustic energy on a precise area. Lithotripsy uses ultrasound shock waves to break up kidney stones.

Additional experiments

The experiment White Noise allows exploring the frequency spectrum and identifying the limits of the audible domain.

Formula

No specific formula applies uniquely to ultrasounds.

Ultrasounds obey the same laws as audible sounds. The fundamental relationship remains:

v = lambda x f

Meaning:

v: wave velocity in the medium (m/s)

lambda: wavelength (m)

f: frequency, above 20,000 Hz

Example: an ultrasound of 40 kHz in air has a wavelength of 343/40,000 = approximately 8.6 mm.

Application examples

Medical ultrasound imaging uses ultrasounds between 2 and 15 MHz to visualize the fetus or organs
Car parking sensors emit ultrasounds and measure echo time to calculate distance
Ultrasonic cleaning tanks use frequencies of 20 to 40 kHz to clean jewelry and instruments
Bats emit ultrasounds between 20 and 200 kHz to detect obstacles and insects
Some pest repellents emit ultrasounds to drive away rodents

FAQ

Q: Why can we not hear ultrasounds? A: The hair cells of the inner ear have a mechanical vibration limit. Beyond 20 kHz, they no longer respond. This limit decreases with age due to natural cell wear.

Q: Do animals hear ultrasounds? A: Yes. Dogs perceive sounds up to approximately 40 kHz, cats up to 60 kHz, bats up to 200 kHz. That is why an ultrasound whistle is inaudible to humans but audible to dogs.

Q: How does ultrasound imaging work? A: The ultrasound machine emits ultrasounds that pass through tissues. When they encounter an interface between two media (muscle and bone, for example), part is reflected. The device analyzes these echoes to construct an image.

Q: Are ultrasounds dangerous? A: At low intensity, ultrasounds are harmless. This is the case with medical ultrasound imaging. At high intensity, they can be destructive (lithotripsy to break up kidney stones, industrial cleaning).

Q: What is the wavelength of an ultrasound? A: In air at 20 degrees C, an ultrasound of 40 kHz has a wavelength of 8.6 mm. This small wavelength allows good spatial resolution, useful for imaging and detection.

Infrasound - Frequency - Sound wave - Echolocation - Ultrasound imaging - Sound spectrum - Hearing