MYP Physics Sound Waves: What Do Students Need to Know?

In practice, that breaks down into four checkable skills:

1. Describe sound as a series of compressions and rarefactions, not a side-to-side wiggle.
2. Rearrange and use v = fλ for at least three variable combinations.
3. Explain why sound can't travel through a vacuum (no particles to compress).
4. Link resonance to a real example — a wine glass shattering, or an instrument's soundbox.

I've marked hundreds of MYP unit tests, and the students who lose marks almost never get the equation wrong — they lose them describing why sound behaves the way it does.

Quick tip: if a diagram shows particles bunching and spreading along the direction of travel, it's longitudinal — draw the compressions as darker lines, rarefactions as lighter ones. Examiners in Criterion A tasks often ask you to label a displacement-distance graph, so practise sketching both wave types from memory, not just recognising them.

A common mistake: assuming sound speeds up when it's hotter and denser — actually within gases, warmer air does carry sound slightly faster because particles collide more often, but the density comparison across states of matter is the one MYP tests most.

Students regularly confuse a loud, low sound with a high-pitched one because both feel intense. Test yourself: a bass drum is low frequency, high amplitude. A dog whistle is high frequency, often low amplitude to human ears (we can't hear it at all above ~20 kHz).

Command term to watch for: explain. An MYP-level answer needs cause and effect, not just a definition — e.g. "the singer's note matched the glass's natural frequency, so energy transferred efficiently and amplitude increased until the glass fractured," not just "resonance broke the glass."

Worked example (wave equation): A tuning fork vibrates at 440 Hz (concert A) and sound travels through air at 343 m/s. $$\lambda = \frac{v}{f} = \frac{343}{440} \approx 0.78 \text{ m}\

Worked example (echo): A ship sends a sonar pulse and hears the echo 4 seconds later. Sound travels at 1,480 m/s in water. $$d = \frac{v \times t}{2} = \frac{1480 \times 4}{2} = 2,960 \text{ m}$$ That's the depth to the seabed — a classic Criterion C data-processing question.

Common mistakes checklist — check before submitting any sound wave answer:

1. Did you halve the distance in an echo calculation?
2. Are your units consistent (all in Hz, not mixing kHz and Hz)?
3. Did you state sound needs a medium, especially in "explain why" questions about space?
4. Have you rounded to a sensible number of significant figures, matching the data given?

According to the MYP: From Principles into Practice guide, each of the four criteria (A–D) is marked out of 8, and your subject grade is derived from the sum across all four. A student aiming for levels 7-8 in Criterion A on a sound waves task should be able to:

• Define wave properties (frequency, amplitude, wavelength) with correct units
• Apply the wave equation to unfamiliar contexts, not just textbook numbers
• Justify their reasoning using scientific vocabulary rather than everyday language

I tell every student I teach this unit to: write the equation, substitute the numbers, show the units at every step. Markers can't award a method mark for arithmetic they can't see.

MYP eAssessment on-screen exams are set once per session (May and November) and use interactive question types — dragging labels, entering numeric answers, short written responses — rather than pure multiple-choice. If your school uses eAssessment, ask your teacher for the specimen papers; the format differs from a typical classroom unit test.

Think of MYP as teaching you the vocabulary and the basic equation; DP asks you to derive relationships, interpret standing wave diagrams, and apply the Doppler effect to moving sources — genuinely harder maths, same underlying concepts.

A few low-effort ways to help at home: ask them to explain resonance using a real object in the room, time an echo off a wall and check their calculation together, or simply ask "why" after every answer they give you. On RevisionPrep, our Physics Revision Notes and Topical Worksheets for MYP break this unit into practice questions with worked solutions, which is useful for building calculation confidence without you needing a physics background yourself.