Audible, Infrasonic, and Ultrasonic Waves

Not every sound wave can be heard by the human ear. Sound waves are classified by frequency into audible waves, infrasonic waves, and ultrasonic waves. Audible waves lie in the range our ears can detect; infrasonic waves are too low, and ultrasonic waves are too high for us to hear.

This classification matters because animals, machines, and natural events use frequencies outside our range all the time. Elephants communicate with infrasound over kilometres, bats navigate with ultrasound, and doctors image unborn babies with ultrasonic waves. Knowing the bands helps us use the right sound for the right job.

Key Concepts

Audible waves have frequencies from about 20 Hz to 20,000 Hz (20 kHz) for a healthy young ear. Infrasonic waves are below 20 Hz; you may feel them as vibration more than hear them. Ultrasonic waves are above 20 kHz. Frequency (f) is measured in hertz. Higher frequency means shorter wavelength for the same speed, since λ = v/f. Sensitivity varies with age and exposure; most adults lose the highest audible frequencies first.

Formulas

The same wave relation applies to all three types: v = f × λ, with v in m/s, f in Hz, and λ in m. For sound in air at room temperature v is about 343 m/s irrespective of the band, provided temperature is constant. The boundary values 20 Hz and 20 kHz are approximate biological limits, not sharp physical changes; the wave physics is identical across the spectrum.

Worked Example

Take sound speed in air as 340 m/s. For the lowest audible frequency 20 Hz, λ = 340/20 = 17 m, about the length of a bus. For the highest audible 20,000 Hz, λ = 340/20,000 = 0.017 m = 1.7 cm. An ultrasonic wave at 40,000 Hz used by a bat has λ = 340/40,000 = 0.0085 m = 8.5 mm. The tiny wavelength explains why ultrasound can reflect off small insects and give detailed images, while infrasound wavelengths of hundreds of metres bend easily around hills.

Common Misconceptions

• Ultrasound is not loud sound. Ultra refers to frequency above hearing, not to high amplitude or power.
• Infrasound is not silence. It is real sound with very low frequency; powerful infrasound can rattle windows and be felt in the chest.
• Everyone hears the same range. Hearing range shrinks with age and noise damage, so a teenager may hear a 17 kHz tone that an older adult cannot.

Real-World Applications

Infrasound monitors detect volcanic eruptions, earthquakes, and distant explosions for early warning. Elephants and whales use infrasound for long-distance communication. Ultrasound cleans jewellery, welds plastics, breaks kidney stones, and produces medical scans without radiation. Pest repellers and dog whistles also work in the ultrasonic band.

Audible, infrasonic, and ultrasonic waves are the same kind of wave divided only by frequency relative to human hearing. Their different wavelengths make each band suited to distinct natural and technological uses.