IGCSE Physics: Sound Practice Questions
Sound is a longitudinal wave produced by a vibrating source. It travels as compressions and rarefactions through a medium, so it cannot travel through a vacuum, and it travels fastest in solids and slowest in gases.
Sub-topic 3.4 of Cambridge IGCSE Physics 0625 is short and calculation friendly. Echo questions are the most common, and the single most frequent error is forgetting that the sound travels to the reflector and back. The questions below make that explicit.
What you need to know for Sound
- Sound as a longitudinal waveParticles vibrate parallel to the direction of energy transfer, producing regions of compression where particles are close together and rarefaction where they are further apart.
- Sound needs a mediumSound requires particles to transmit the vibration, so it cannot travel through a vacuum. A ringing bell in a bell jar becomes silent as the air is pumped out.
- Speed in different mediaSound travels fastest in solids, more slowly in liquids and slowest in gases, because particles are closer together in solids and pass the vibration on more quickly. In air it is roughly 330 to 350 m/s.
- Pitch and loudnessPitch depends on frequency. A higher frequency gives a higher pitch. Loudness depends on amplitude. A larger amplitude gives a louder sound.
- Audible rangeThe human audible range is approximately 20 Hz to 20000 Hz. Sound above 20000 Hz is ultrasound.
- Ultrasound usesPrenatal scanning, detecting flaws in metal castings, and sonar for measuring the depth of water. All rely on measuring the time for a reflected pulse to return.
IGCSE Physics Sound questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A student stands 165 m from a large wall and claps once. She hears the echo 1.0 s later. Calculate the speed of sound in air.
Show the worked answer
- The sound travels from the student to the wall and back again, so the total distance is twice the separation.
- Total distance = 2 x 165 = 330 m.
- Speed = distance divided by time = 330 divided by 1.0.
- Speed = 330 m/s.
Explain why sound cannot travel through a vacuum, and describe an experiment that demonstrates this.
Show the worked answer
- Sound travels as a series of compressions and rarefactions, which requires particles that can be pushed together and pulled apart.
- A vacuum contains almost no particles, so there is nothing to transmit the vibration and no sound is carried.
- To demonstrate this, place an electric bell inside a sealed bell jar connected to a vacuum pump, suspended so it does not touch the sides.
- Switch the bell on and gradually pump the air out. The sound becomes quieter and eventually cannot be heard, even though the hammer is still visibly striking, showing that sound requires a medium.
A ship uses sonar to measure the depth of water. It emits a pulse and receives the reflection 0.24 s later. The speed of sound in seawater is 1500 m/s. Calculate the depth.
Show the worked answer
- The pulse travels down to the seabed and back, so the time for the one way journey is half the total.
- One way time = 0.24 divided by 2 = 0.12 s.
- Depth = speed x time = 1500 x 0.12.
- Depth = 180 m.
Two sounds are displayed on an oscilloscope. Sound B has waves that are half as far apart and twice as tall as those of sound A. Describe how sound B differs from sound A.
Show the worked answer
- Waves half as far apart on the screen means the period is halved, so the frequency of sound B is twice that of sound A.
- Pitch depends on frequency, so sound B has a higher pitch than sound A.
- Waves twice as tall means the amplitude of sound B is twice that of sound A.
- Loudness depends on amplitude, so sound B is louder than sound A.
Common mistakes in this topic
- Forgetting the double journey in echo and sonar questions.
- Describing sound as a transverse wave.
- Saying sound travels faster in air than in solids.
- Confusing amplitude with frequency when reading an oscilloscope trace.
- Quoting the audible range in the wrong units. It is 20 Hz to 20 kHz, not 20 to 20000 m/s.
Exam tips
- In every echo or sonar question, write the double journey on its own line before calculating.
- Use the terms compression and rarefaction whenever describing how sound travels.
- Link pitch to frequency and loudness to amplitude, and never swap them.
- For a bell jar experiment, mention that the bell is suspended so no solid path exists.
- Ultrasound applications all work the same way: emit a pulse, time the reflection, calculate distance using half the time.
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Sound FAQs
Why can't sound travel through a vacuum?
Sound travels as compressions and rarefactions, which requires particles that can be pushed closer together and pulled further apart. A vacuum contains almost no particles, so there is nothing to transmit the vibration. This is demonstrated by a ringing bell in a bell jar falling silent as the air is pumped out.
How do I calculate the speed of sound from an echo?
The sound travels to the reflecting surface and back, so the total distance is twice the separation. Divide that total distance by the time taken for the echo to return. Alternatively, halve the time and use the one way distance. Forgetting the double journey is the most common error in this topic.
What is the difference between pitch and loudness?
Pitch depends on the frequency of the sound wave. A higher frequency produces a higher pitch. Loudness depends on the amplitude. A larger amplitude produces a louder sound. On an oscilloscope trace, frequency is shown by the horizontal spacing and amplitude by the height.
What is ultrasound used for?
Ultrasound is sound above 20000 Hz, beyond the human audible range. It is used for prenatal scanning, for detecting cracks and flaws inside metal castings, and for sonar to measure water depth. All of these work by emitting a pulse and timing how long the reflection takes to return.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.