C.2.3—Nature of sound waves

Sound needs a mechanical medium

Sound is produced by a vibrating source and travels through matter as a mechanical wave. In air it is longitudinal: air molecules oscillate back and forth parallel to the direction in which the disturbance and energy propagate.

Compressions and rarefactions

A compression is a region of higher particle density and pressure; a rarefaction is a region of lower density and pressure. One wavelength is the distance between neighbouring compressions or neighbouring rarefactions.

Exam-language calibration from local practice question 2

A complete description connects all four ideas: longitudinal particle motion, propagation through air, alternating compressions/rarefactions, and energy transfer away from the source.

Boundary

The air molecules oscillate locally; they are not carried from loudspeaker to listener. Sound cannot propagate through a vacuum because there are no particles to sustain the mechanical disturbance.

How it is assessed

Questions ask you to calculate sound wavelength or wave speed from frequency and wavelength. The evidence rewards the equation, correct medium speed and consistent units.

Command terms

Calculate

What earns marks

Choose the form of v = fλ that matches the requested quantity, convert all units first, and use the wave speed in the stated medium. Show the substitution and report appropriate significant figures.

Watch for

Using the wrong wave speed for the medium or mixing centimetres and metres before applying v = fλ.

Representative question

[Maximum number: 1]

Calculate the wavelength of the sound wave in air.

《 $\frac{340}{1700}=$ 》 0.20 m

Unit is not required.

Model the transfer

A travelling wave propagates a disturbance and transfers energy without a resultant transport of the medium. Use the local particle/field motion to describe oscillation, and the wave direction to describe propagation.

Connect the quantities

Describe waves with wavelength $\lambda$ , frequency $f$ , period $T$ , amplitude and speed $v$ , linked by $v=f\lambda=\lambda/T$ . Measure wavelength between adjacent in-phase points.

Classify the wave

Transverse oscillations are perpendicular to propagation; longitudinal oscillations are parallel. Mechanical waves require a medium, while electromagnetic waves are transverse field oscillations and travel at $c$ in vacuum.

Final check

At a boundary, identify which quantity is fixed by the source and which properties change in the new medium. Keep units consistent and distinguish propagation speed from the local oscillation speed.

C.2.3—Nature of sound waves

Explain the Nature of Sound Waves

C.2.3 Exam Analysis

Assessment in practice

Retrieve the C.2 Wave Model

Question 1