Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
4.14P Recall that sound with frequencies less than 20 hertz, Hz, is known…
Full specification wording
Topic 4 – Waves: 4.14P Recall that sound with frequencies less than 20 hertz, Hz, is known as infrasound
Sound is a longitudinal wave, and its frequency tells you how many complete oscillations pass a point each second, measured in hertz (Hz).
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 13 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
Sound is a longitudinal wave, and its frequency tells you how many complete oscillations pass a point each second, measured in hertz (Hz). Human hearing only covers part of the whole sound spectrum: most people hear roughly from 20 Hz up to about 20 000 Hz. Any sound whose frequency is below 20 Hz is called infrasound. So the boundary value is 20 Hz, and the defining condition is 'less than', not 'less than or equal to'. A vibration at 12 Hz is infrasound; one at 25 Hz is audible sound; one at 20 Hz sits exactly on the boundary and is not counted as infrasound under this definition. Infrasound can come from earthquakes, volcanoes and large machinery, and it travels through solids, liquids and gases just as audible sound does. In a multiple-choice question, read the number and the inequality carefully, then match the frequency to the correct named band.
What earns the marks
- Selects the option that states sound below 20 Hz is infrasound.
- Identifies 20 Hz as the upper boundary of the infrasound range.
- Rejects an option that labels frequencies above 20 Hz as infrasound.
- Recognises that infrasound is still sound and travels as a wave through a medium.
Common mistakes and how to put them right
Confuses infrasound with ultrasound.
Put it right: Keep infrasound below 20 Hz and ultrasound above the audible range.
Treats 20 Hz itself as infrasound.
Put it right: Apply the strict 'less than 20 Hz' condition.
Assumes infrasound cannot travel through materials.
Put it right: It propagates like any sound wave.
Exam tips
- Underline the inequality word 'less than' before choosing an option.
- Check the unit is hertz, Hz, and not kilohertz.
- Eliminate options that describe frequencies above the audible range, since those are ultrasound.
Try the lesson
The start of the Topic 4 – Waves: 4.14P Recall that sound with frequencies less than 20 hertz, Hz, is known as infrasound lesson, made from checked teaching notes for this specification point.
Have a go first
A sound wave has a frequency of 12 Hz. Which term describes this sound?
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