Describe the longitudinal nature of sound waves
3.4 Sound – Longitudinal Waves
What is a Sound Wave?
Sound is a mechanical wave that travels through a medium (air, water, solids) by the vibration of particles. Unlike transverse waves (like waves on a string), sound waves are longitudinal – the particles move back and forth in the same direction as the wave travels.
How Particles Move
- When a sound source (e.g., a drum) vibrates, it pushes on the surrounding air molecules.
- These molecules collide with their neighbours, creating a series of compressions (high pressure) and rarefactions (low pressure).
- The pattern of compressions and rarefactions moves outward from the source.
Analogy: The Ripple in a Crowd
Imagine a crowd at a stadium doing the wave. Each person stands up and sits down in sequence. The movement travels through the crowd, but the people themselves only move up and down (not sideways). Similarly, in a sound wave, the air molecules move forward and backward along the direction the sound travels.
Key Parameters
| Parameter | Symbol | Units |
|---|---|---|
| Wavelength | $\lambda$ | m |
| Frequency | $f$ | Hz |
| Speed of Sound | $v$ | m s⁻¹ |
| Amplitude | $A$ | m or Pa |
Relationship Between Parameters
The speed of a sound wave in a given medium is related to its frequency and wavelength by the equation:
$$v = f \lambda$$
For example, in air at 20 °C, $v \approx 343\;\text{m s}^{-1}$. If a note has a frequency of 440 Hz (A4), its wavelength is:
$$\lambda = \frac{v}{f} = \frac{343}{440} \approx 0.78\;\text{m}$$
Why Longitudinal Matters
- Sound can travel through solids, liquids, and gases because particles can push against each other.
- Transverse waves cannot propagate in a gas because particles would need to move sideways, which is not supported.
- Understanding longitudinal waves helps explain phenomena like echo, Doppler effect, and musical instrument behaviour.
Quick Quiz 🎓
- What happens to the wavelength if the frequency doubles while the speed stays the same?
- Why can sound travel through a vacuum? (Hint: think about particles)
Summary
Sound waves are longitudinal mechanical waves where particles oscillate parallel to the direction of propagation, creating alternating compressions and rarefactions. Their key properties—frequency, wavelength, speed, and amplitude—are linked by $v = f \lambda$, and they can travel through any medium that allows particle interaction.
Revision
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