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How does sound travel and how do we hear it, and what are the uses of ultrasound?

Sound as a longitudinal wave, how sound travels through a medium and is heard, the human hearing range, ultrasound and its uses in imaging and measuring distance, and echoes.

A focused answer to OCR Gateway GCSE Physics A topic P5 on sound and ultrasound, covering sound as a longitudinal wave, how it travels and is heard, the human hearing range, ultrasound and its uses in imaging and measuring distance, and how echoes are used.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. How sound travels
  3. How we hear
  4. The hearing range and ultrasound
  5. Uses of ultrasound
  6. Echoes
  7. Try this

What this topic is asking

OCR wants you to describe sound as a longitudinal wave, explain how it travels and is heard, state the human hearing range, define ultrasound, and give its uses including imaging and measuring distance. This is part of topic P5.3 of the OCR Gateway Physics A (J249) specification.

How sound travels

This is why you cannot hear sounds in the vacuum of space, and why sound carries well through a solid wall or along a metal pipe. A higher frequency sound is heard as a higher pitch, and a larger amplitude is heard as a louder sound.

How we hear

When a sound wave reaches the ear, it makes the eardrum vibrate. These vibrations are passed on through the ear and converted into signals that the brain interprets as sound. Because hearing depends on the eardrum and other parts vibrating, very loud sounds (large amplitude) can damage hearing.

The hearing range and ultrasound

Ultrasound behaves like other waves: it can be reflected at a boundary between two different materials, which is the basis of its uses.

Uses of ultrasound

  • Medical imaging. Ultrasound is sent into the body and reflects at the boundaries between different tissues; the reflections are used to build an image, for example scanning a fetus, safely (no ionising radiation).
  • Measuring distance and depth (sonar). A pulse is sent out and the time for the echo to return is measured; the distance is found from distance=speed×time\text{distance} = \text{speed} \times \text{time}, then halved because the pulse travels there and back. This is used to find the depth of the sea or detect objects underwater.
  • Cleaning and industry. Ultrasound cleans delicate objects and detects flaws inside metals.

Echoes

An echo is a reflected sound. Timing how long an echo takes to return lets you work out the distance to the reflecting surface, using the speed of sound. The key step is to remember the sound travels to the surface and back, so the distance to the surface is half the total distance the sound travels.

Try this

Q1. State why sound cannot travel through a vacuum. [1 mark]

  • Cue. There are no particles to vibrate and pass the energy along.

Q2. Define ultrasound. [1 mark]

  • Cue. Sound with a frequency above the upper limit of human hearing (above 20 000 Hz).

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksAn ultrasound pulse is sent into the sea and the echo returns from the seabed after 0.30 s0.30\,\text{s}. The speed of sound in seawater is 1500 m/s1500\,\text{m/s}. Calculate the depth of the sea.
Show worked answer →

A P5 Calculate question using distance=speed×time\text{distance} = \text{speed} \times \text{time}. The pulse travels to the seabed and back, so the total distance is 1500×0.30=450 m1500 \times 0.30 = 450\,\text{m} (2 marks for the total distance there and back). The depth is half the total distance, because that distance covers the journey down and up: depth =4502=225 m= \dfrac{450}{2} = 225\,\text{m} (2 marks for halving and the answer). Markers reward the total distance, the halving for the one-way depth, and the answer with units. A common error is to forget to halve and give 450 m450\,\text{m}.

OCR 20213 marksState the approximate range of frequencies that a healthy young human can hear, define ultrasound, and give one use of ultrasound.
Show worked answer →

A P5 question worth three marks. The human hearing range is approximately 20 hertz to 20 000 hertz (20 kHz) (1 mark). Ultrasound is sound with a frequency above the upper limit of human hearing, that is above 20 kHz20\,\text{kHz} (1 mark). A use of ultrasound is medical imaging (for example, scanning a fetus), measuring distances or depths (sonar), or cleaning delicate objects (1 mark). Markers reward the 20 Hz to 20 kHz range, the definition above 20 kHz, and a valid use. A common error is to state the range the wrong way round or in the wrong units.

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