Sound Waves
Sound is a longitudinal wave produced by a vibrating source. It needs a medium, which is the fact almost every sound question is built around.
1. What sound is
Sound is produced by a vibrating source and travels as a LONGITUDINAL wave — the particles vibrate parallel to the direction the wave travels.
The vibration creates alternating regions of:
Compressions — particles squeezed together, high pressure Rarefactions — particles spread apart, low pressure
Sound is a PRESSURE wave — the pressure variations are what your ear detects. A recorded error denied this; the compressions and rarefactions are pressure changes.
Learn the word “rarefaction” — forgetting it was recorded, and it is the term the mark scheme wants for the low-pressure region.
Wavelength for a longitudinal wave is measured from one compression to the next.
2. Sound needs a medium
Sound CANNOT travel through a vacuum, because there are no particles to vibrate and pass the energy on.
The bell jar experiment: an electric bell ringing inside a jar becomes quieter and quieter as the air is pumped out, and is eventually inaudible — though it can still be seen vibrating.
Give both halves: “sound cannot travel through a vacuum” AND “because there are no particles to transmit the vibrations”. Tutors noted this is worth two marks — one for the statement, one for the explanation.
3. The speed of sound
In air, sound travels at about 330–340 m/s.
Speed depends on the medium:
| Medium | Speed (m/s) |
|---|---|
| Air | ~340 |
| Water | ~1500 |
| Steel / solids | ~5000 |
Sound travels FASTEST in solids and slowest in gases, because the particles are closer together and pass the vibration on more quickly. This is the opposite of light, which travels fastest in a vacuum.
Know the value for air: about 340 m/s. Guesses of 3000 or 5000 m/s were recorded — those are solid-medium values.
Speed also increases with temperature in a gas, because the particles move faster.
4. Pitch and loudness
These depend on different wave properties, and swapping them is the most common error in the topic.
PITCH depends on FREQUENCY. Higher frequency → higher pitch. LOUDNESS depends on AMPLITUDE. Larger amplitude → louder sound.
Increasing the frequency raises the PITCH, not the volume. This exact error was recorded, as was confusing amplitude with pitch. Keep the pairing: frequency–pitch, amplitude–loudness.
On an oscilloscope trace:
- A louder sound has taller peaks (greater amplitude)
- A higher-pitched sound has waves closer together (more per unit time)
5. The audible range and ultrasound
The human audible range is about 20 Hz to 20 000 Hz (20 kHz).
Ultrasound is sound with a frequency above 20 kHz — higher than humans can hear.
Ultrasound has a HIGHER frequency than audible sound. A recorded error placed ultrasound below a 750 Hz tone — but 750 Hz is well within the audible range, and ultrasound is above 20 000 Hz.
Uses of ultrasound:
- Medical imaging (prenatal scans) — it is non-ionising and therefore safer than X-rays
- Sonar / echo sounding — measuring the depth of water
- Non-destructive testing — detecting cracks inside metal
- Cleaning delicate objects
Sounds may not be heard if the frequency is outside the audible range, if the amplitude is too small, or if there is no medium between source and listener.
6. Echoes and calculations
An echo is sound reflected from a hard surface.
The key point: the sound travels to the surface and back, so:
distance to the surface = (speed × time) ÷ 2
Example: an echo returns after 0.5 s, with sound at 340 m/s.
- Total path = 340 × 0.5 = 170 m
- Distance to the wall = 85 m
Halve the total distance. Forgetting the return journey was a recorded error and doubles the answer — it is the single most common mistake in sound calculations.
Use v = f λ for other sound calculations, exactly as for any wave.
7. Measuring the speed of sound
Method: stand a measured, large distance from a wall, make a sharp sound, and time the echo with a stopwatch. Then speed = 2 × distance ÷ time.
Improving the result:
Use as large a distance as possible, and repeat and average — both flagged by tutors as the standard reliability answers, especially for the practical paper.
An oscilloscope with microphones is more accurate than human timing, because it removes reaction time — the dominant error in the stopwatch method.
8. Mistakes that cost marks
Saying sound is a transverse wave.
Forgetting the term “rarefaction”.
Saying sound can travel through a vacuum, or giving the statement without the reason.
Linking pitch to amplitude or loudness to frequency.
Wrong speed of sound in air.
Saying sound travels fastest in gases.
Forgetting the return journey in an echo calculation.
Placing ultrasound below the audible range.
Omitting units.
Frequently asked questions
What type of wave is sound? A longitudinal wave — particles vibrate parallel to the direction of travel.
What are compressions and rarefactions? Regions of high pressure (particles close) and low pressure (particles spread out).
Can sound travel through a vacuum? No — there are no particles to transmit the vibrations.
What is the speed of sound in air? About 340 m/s.
In which medium does sound travel fastest? Solids, because the particles are closest together.
What determines pitch? Frequency.
What determines loudness? Amplitude.
What is the audible range? About 20 Hz to 20 kHz.
What is ultrasound? Sound above 20 kHz — higher than humans can hear.
How do I calculate distance from an echo? (speed × time) ÷ 2 — the sound travels there and back.
Quick revision checklist
- I know sound is longitudinal and made by a vibrating source
- I can define compression and rarefaction
- I know sound is a pressure wave
- I know sound cannot cross a vacuum, and why
- I can describe the bell jar experiment
- I know the speed of sound in air is ~340 m/s
- I know the order of speeds in solids, liquids and gases
- I know pitch = frequency and loudness = amplitude
- I can interpret an oscilloscope trace
- I know the audible range and what ultrasound is
- I can give uses of ultrasound
- I halve the distance in echo calculations
- I can describe an experiment to measure the speed of sound and how to improve it
These notes cover sound waves in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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