3.4 Sound
Objective
Define ultrasound as sound with a frequency higher than \$20\ \text{kHz}\$.
Key Concepts
- Sound is a longitudinal wave that propagates through a medium by particle vibrations.
- The frequency (\$f\$) of a sound wave determines its pitch and is measured in hertz (Hz).
- Human hearing typically ranges from about \$20\ \text{Hz}\$ to \$20\ \text{kHz}\$.
- Sound with a frequency above the upper limit of human hearing is called ultrasound.
Frequency Ranges
| Type of Sound | Frequency Range | Common Examples |
|---|
| Infrasound | \$< 20\ \text{Hz}\$ | Earthquakes, volcanic eruptions, large explosions |
| Audible Sound | \$20\ \text{Hz}\$ – \$20\ \text{kHz}\$ | Speech, musical instruments, everyday noises |
| Ultrasound | \$> 20\ \text{kHz}\$ | Medical imaging, industrial cleaning, sonar |
Why Ultrasound Is Useful
- It can penetrate materials that visible light cannot, allowing internal imaging (e.g., medical ultrasound).
- Higher frequencies provide better resolution because the wavelength \$\lambda\$ is shorter (λ = v / f).
- Ultrasound waves can be focused to a small spot, enabling precise cleaning or cutting in industrial applications.
Relevant Formula
The relationship between speed (\$v\$), frequency (\$f\$), and wavelength (\$\lambda\$) is:
\$ v = f \lambda \$
Suggested Diagram
Suggested diagram: A schematic showing the audible range (20 Hz–20 kHz) with the ultrasound region (>20 kHz) highlighted.