Sound: a longitudinal wave needing a medium
Sound is made by a vibrating source (loudspeaker, string, tuning fork). It is a longitudinal wave — particles of the medium oscillate parallel to the direction of energy transfer (the two-element mark-scheme definition). As the source moves forward it makes a compression (higher pressure and density); as it moves back, a rarefaction (lower pressure and density). Because it needs particles, sound cannot travel through a vacuum — space is silent.
Speed of sound and the echo formula
Speed of sound ≈ 340 m/s in air, and is greater in denser media: solids > liquids > gases (water ≈ 1500 m/s, steel ≈ 6000 m/s) — the opposite of light. An echo is reflected sound. One-way distance , where is the round-trip time, so the ÷2 is always required; rearranged, . e.g. m. Sonar times reflected pulses using m/s in sea water.
Hearing range, ultrasound and the CRO
Human hearing: 20 Hz to 20 000 Hz (20 kHz); below is infrasound, above is ultrasound. Ultrasound = sound with a frequency above 20 000 Hz (state the threshold, not just "inaudible"); uses: sonar and medical scanning (non-ionising, so safer than X-rays). On a CRO: x-axis = time → period → frequency → pitch; y-axis = amplitude → loudness. Louder = a taller wave (not closer cycles); higher pitch = closer cycles (not taller).
(E) Sound at a boundary: f fixed, λ changes
(E) When sound crosses into a new medium (e.g. air → water): frequency stays the same (set by the source); speed changes (faster in a denser medium); so wavelength changes, since gives . Example — 500 Hz sound: in air m; in water m. Same principle as light refraction — only speed and wavelength change, never frequency.
Drawn from real examiner reports.
Ultrasound: state the 20 000 Hz threshold
Defining ultrasound as "sound humans cannot hear" is insufficient — infrasound (below 20 Hz) also fits. The mark scheme needs the two-element form: sound with a frequency above 20 000 Hz (20 kHz). Describing a scan ("makes images") does not define it. Ultrasound is set by frequency, not loudness; a quiet ultrasound signal is still ultrasound.
June 2024 Paper 0625/42 (Extended Theory) Q8(a): examiners noted "the strongest answer was that it is sound with a frequency higher than 20 kHz. Stating that it is a sound outside the human hearing range was insufficient as that would also apply to sounds with a frequency below 20 Hz. Many weaker candidates just referred to ultrasound being used to produce images."
Echo sums: halve the round-trip time
The top sound-calculation error is forgetting the ÷2. The measured time is the round-trip time (there and back), so the one-way distance is . Writing gives double the answer. Cambridge credits the halving step as a separate mark, so show it. To find speed, use , not .
June 2024 Paper 0625/42 Q8(b) mark scheme: working line "distance travelled = 22 × 2 OR 44 SEEN" is an explicit C1 mark, showing that the factor of 2 must appear. November 2024 Paper 0625/42 Q8(b): final answer 0.029 s requires the ÷2 step to be visible in working.
CRO: frequency from x-axis, amplitude from y
On a CRO trace: x-axis = time (gives period , frequency , pitch); y-axis = amplitude (gives loudness). Reading the wave height to find frequency scores zero — frequency is a time quantity. "A louder sound has cycles closer together" is wrong: louder = taller (y-axis), not higher frequency. Name the axis you read from to earn the method mark.
June 2024 Paper 0625/32 (Core Theory): Q examining CRO displacement graph — examiners noted "many just repeated words from the question" and candidates who "stated it showed a transverse wave" rather than correctly reading amplitude or frequency from the correct axis. November 2024 Paper 0625/32: similar longitudinal/transverse confusion on wave display questions.
Sound is longitudinal, not transverse
Sound particles vibrate back and forth along the direction of travel, not side to side — so "sound is a transverse wave" or "particles move up and down" scores zero. Measure a wavelength compression-to-compression; compression-to-adjacent-rarefaction is only half a wavelength. Draw the energy-transfer arrow parallel to the particle oscillation.
CRO: amplitude midline-to-crest; convert ms
Two precision traps when reading a CRO. Amplitude is measured midline-to-crest, not crest-to-trough (crest-to-trough doubles it). Time: if the timebase is in ms/div, convert to seconds before ( ms s) — forgetting this makes the frequency 1000× too large. Always count divisions for one full cycle to get .
Sound is faster in denser media, unlike light
Sound travels faster in denser media (gas < liquid < solid), because closer, bonded particles pass the vibration on faster — the opposite of light, which slows in a denser medium. Match the speed to the medium: if a question gives water (≈ 1500 m/s) or steel (≈ 6000 m/s), do not use 340 m/s (the air value) — the wrong medium speed scores zero.
Echo sums: show the ÷2 on its own line
Use three lines: (1) write (or ); (2) substitute with the halving shown, e.g. ; (3) state the answer with unit, m. Same structure for a sonar question ( m/s).
CRO: get T from the x-axis, then f = 1/T
Count the divisions for one complete cycle on the x-axis and multiply by the time-per-division to get ; convert ms to s first ( ms s). Then apply and state in Hz. Name the axis you read from to secure the method mark.
Wave speed: formula, substitute, unit
Write first, rearrange if needed, then substitute on a new line, e.g. , and finish with the value and unit ( Hz, to suitable sig figs). The equation earns method marks even if the arithmetic slips.
Practical: equipment, measure, calculate
For "describe an experiment" questions (P5/P6), give four parts: (a) the equipment; (b) what you measure; (c) how you calculate the speed, quoting the formula; (d) one precaution to improve accuracy. Missing the formula or the precaution is where marks are lost.
Wave speed:
= speed of sound (m/s), = frequency (Hz), = wavelength (m).
Period and frequency:
Read from the time axis (x-axis) of a cathode-ray oscilloscope (CRO) display.
Echo / sonar distance:
= one-way distance to reflector (m), = speed in the medium (m/s), = total round-trip time (s). The factor of 2 accounts for the outward and return journey.
| Find | Formula |
|---|---|
| Distance | |
| Speed | |
| Time |
Reference speeds: air m/s; water m/s; steel m/s.
State two properties of sound that must be included in any Cambridge mark-scheme answer about its nature.
A student measures the speed of sound in a large gymnasium. The student claps two wooden blocks together 25 m from one wall, while a partner 25 m further away starts a stopwatch when the clap is heard and stops it when the echo from the far wall is heard.
The student repeats the experiment three times and records the echo times: , , .
(a) Identify one source of random error in this experiment. (1 mark)
(b) Suggest one specific improvement to reduce the error you identified in (a). (1 mark)
