How does sound travel and how are sound and other waves used in technology?
Sound and uses of waves: how sound travels through solids and is heard, the range of human hearing, ultrasound, and the use of waves in detection and imaging (separate physics).
A focused answer to AQA GCSE Physics 4.6.1 and 4.6.2, covering how sound travels as a longitudinal wave and is heard, the human hearing range, ultrasound and its uses, and how reflected waves are used for detection and imaging such as echo sounding and seismic waves.
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What this dot point is asking
AQA wants you to explain how sound travels as a longitudinal wave and is detected by the ear, state the human hearing range, describe ultrasound and its uses, and explain how reflected waves are used for detection and imaging. This is part of topics 4.6.1 and 4.6.3 of the AQA GCSE Physics (8463) specification, with ultrasound and seismic waves being separate physics only.
How sound travels
The speed of sound depends on the medium. It travels fastest in solids, where the particles are close together and pass on the vibration quickly, slower in liquids, and slowest in gases such as air (around ). This is the opposite ordering to how we might first guess, and it follows from how tightly the particles are coupled. It is also why you can hear an approaching train sooner through the rail than through the air.
How sound is heard
Because hearing depends on the eardrum and the structures behind it vibrating, the limited frequency range is set by how those parts can respond. The pitch of a sound is determined by its frequency (higher frequency means higher pitch), while the loudness is determined by its amplitude. As people age, the upper limit of their hearing range tends to fall, so older people often cannot hear the highest frequencies that children can.
Ultrasound
Detection and imaging with reflected waves
Ultrasound is preferred over X-rays for scanning a developing baby because it does not ionise tissue and so is not thought to damage cells, whereas X-rays are ionising and could harm the baby. This is a good example of choosing the right type of wave for a job based on its physical properties.
Try this
Q1. State the approximate range of human hearing. [1 mark]
- Cue. About to .
Q2. Explain why sound cannot travel through a vacuum. [2 marks]
- Cue. Sound needs particles of a medium to vibrate; a vacuum has no particles, so the wave cannot travel.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20205 marksA ship uses echo sounding to measure the depth of the sea. It sends a pulse of ultrasound towards the seabed and detects the reflected pulse later. The speed of sound in seawater is . Calculate the depth of the sea.Show worked answer →
The ultrasound pulse travels down to the seabed and back, so the total distance travelled is twice the depth. Use distance speed time (2 marks). This is the total there-and-back distance (1 mark). The depth is half of this, because the pulse covers the depth twice: depth (2 marks). Markers reward calculating the total distance with , recognising the pulse travels to the seabed and back (so dividing by two), and the final depth. The single commonest error is forgetting to halve the distance.
AQA 20214 marksExplain why sound is described as a longitudinal wave and why it cannot travel through a vacuum, and describe one use of ultrasound.Show worked answer →
Sound is a longitudinal wave because the particles of the medium vibrate back and forth in the same direction as the wave travels, creating regions of compression (particles close together) and rarefaction (particles spread apart) (1 mark). It cannot travel through a vacuum because it relies on particles of a medium vibrating to pass the wave on, and a vacuum has no particles (1 mark). One use of ultrasound is prenatal scanning, where pulses of ultrasound are sent into the body and reflected at boundaries between different tissues, and the reflected pulses are used to build up an image of the developing baby (1 mark for the use, 1 mark for how the reflection is used). Markers reward the longitudinal definition, the need for particles, and a valid ultrasound use with the reflection idea.
Related dot points
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- Reflection and refraction: how waves are reflected, transmitted or absorbed at a boundary, the law of reflection, and why refraction occurs (separate physics).
A focused answer to AQA GCSE Physics 4.6.1 and 4.6.2, covering how waves are reflected, transmitted or absorbed at a boundary, the law of reflection with specular and diffuse reflection, and why refraction occurs when a wave changes speed.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)