Define ultrasound as sound with a frequency higher than 20 kHz

3.4 Sound

What is Sound?

Sound is a mechanical wave that travels through a medium (air, water, solids) by the vibration of particles. When you clap your hands, the air molecules vibrate and create a wave that reaches your ears, letting you hear the sound. 🎧

Frequency and Pitch

The frequency of a sound wave determines its pitch. Higher frequency = higher pitch; lower frequency = lower pitch. Frequency is measured in hertz (Hz).

Ultrasound

Ultrasound is sound with a frequency higher than $20\,\text{kHz}$. Because humans cannot hear it, we use it in many practical ways:

• Medical ultrasound imaging: waves bounce off tissues to create pictures of the inside of the body. 🩺
• Cleaning: high‑frequency vibrations dislodge dirt from jewellery or surgical instruments. 🧼
• Animal communication: some animals (e.g., bats, dolphins) use ultrasound for echolocation. 🦇🐬
• Industrial testing: detecting cracks in metal by sending ultrasound waves through it. 🛠️

Analogies & Everyday Examples

• Echo in a canyon – The time it takes for sound to return tells you how far away the wall is.
• Dog whistles – Dogs hear the high‑frequency sound, but we don’t. Think of it as a secret language.
• Ultrasound in a kitchen – The high‑frequency vibrations that clean your dishes are like invisible waves that shake off grime.