How are sound waves produced and heard, and how is ultrasound used?
Sound as a longitudinal wave, the link between pitch and frequency and loudness and amplitude, the human hearing range, and uses of ultrasound.
A CCEA GCSE Physics answer on sound as a longitudinal wave, how pitch depends on frequency and loudness on amplitude, the range of human hearing, and the uses of ultrasound in medicine and depth measurement.
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What this dot point is asking
CCEA wants you to describe sound as a longitudinal wave, link pitch to frequency and loudness to amplitude, state the human hearing range, and describe uses of ultrasound including echo (depth) measurements. The echo calculation is a frequent exam question.
The answer
Sound as a longitudinal wave
Sound travels faster in solids and liquids than in air, because the particles are closer together and pass on vibrations more readily. In air it travels at about .
Pitch and loudness
The hearing range
The healthy human ear hears frequencies from about 20 Hz to 20000 Hz (20 kHz). Sound above this range is called ultrasound. The upper limit falls with age and after exposure to loud noise.
Uses of ultrasound
For an echo, the pulse travels to the reflecting surface and back, so the distance to the surface is half the total distance travelled.
Worked example: a cliff echo
Examples in context
- Example 1. A guitar string
- A thinner, tighter or shorter string vibrates at a higher frequency, giving a higher-pitched note; plucking it harder increases the amplitude, making the note louder, not higher.
- Example 2. A pregnancy scan
- Ultrasound pulses are sent into the body and reflect at boundaries between tissues. The reflected pulses are processed into an image, giving a safe, non-ionising way to view an unborn baby.
- Example 3. A bat hunting
- A bat emits high-frequency ultrasound and listens for the echoes that bounce back off insects and obstacles. The time for each echo to return tells the bat how far away the object is, the same principle as sonar and the cliff echo, allowing it to hunt in the dark.
It is worth remembering why we halve the distance in echo problems: the sound travels out to the reflecting surface and then back again, so it covers twice the distance to the surface. Forgetting this is one of the most common errors in these questions, so always ask whether the pulse made a round trip.
Try this
Q1. What type of wave is sound? [1 mark]
- Cue. A longitudinal wave.
Q2. State the approximate range of human hearing. [1 mark]
- Cue. About 20 Hz to 20000 Hz (20 kHz).
Q3. A sonar pulse returns to a boat after being sent. The speed of sound in water is . Find the depth. [3 marks]
- Cue. Total distance ; depth .
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksA ship sends an ultrasound pulse to the seabed and detects the echo 0.30 s later. The speed of sound in seawater is 1500 m/s. Calculate the depth of the seabed.Show worked answer →
The pulse travels to the seabed and back, so the total distance is twice the depth.
Total distance
Depth is half of this:
Markers reward total distance = speed times time, halving for the there-and-back trip, and the depth 225 m.
CCEA style3 marksExplain how the pitch and loudness of a sound are related to the frequency and amplitude of the sound wave.Show worked answer →
Pitch depends on frequency: a higher frequency gives a higher pitch, and a lower frequency a lower pitch.
Loudness depends on amplitude: a larger amplitude gives a louder sound, and a smaller amplitude a quieter sound.
Markers reward pitch linked to frequency (higher frequency, higher pitch) and loudness linked to amplitude (larger amplitude, louder).
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)