3.4 Sound
Objective
Define ultrasound as sound with a frequency higher than \$20\,\text{kHz}\$.
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).
| Range | Frequency (Hz) | Common Example |
|---|
| Low | 20–2000 | Bass drum, human voice |
| Mid | 2000–20000 | Guitar, piano |
| High | 20000–40000 | Bird chirps, high‑pitched alarms |
| Ultrasound | \$>20\,\text{kHz}\$ | Medical imaging, cleaning, dog whistles |
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.
Exam Tips
- Remember the key definition: Ultrasound = sound with \$f > 20\,\text{kHz}\$.
- Use the frequency ranges table to quickly answer questions about pitch.
- When asked about uses of ultrasound, list at least two and give a short example.
- Draw a simple diagram of a sound wave and label frequency and wavelength.
- Practice converting between frequency and wavelength using \$c = f \lambda\$ (speed of sound in air ≈ \$340\,\text{m/s}\$).