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School-college Physics Notes: Waves 4. LONGITUDINAL WAVES

GCSE level Physics exam revision notes on waves

Introduction to waves: Part 4. The technical description, properties and examples of a LONGITUDINAL WAVE

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics [waves-intro- page updated Mar 31st 2026 *]

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[KEY POINTS and learning objectives for this page, after initial notes]

INDEX physics notes: Investigating & introducing properties of waves

4. The technical description and properties of a LONGITUDINAL WAVE

The oscillations/vibrations of a longitudinal wave are in the same direction as the wave is moving.

The disturbance of the medium is parallel to the direction the wave is moving.

Know and understand why longitudinal waves show areas of compression and rarefaction.

The above diagram shows the compression and decompression (rarefaction) of a longitudinal sound wave, illustrated 'visually' by the pushing pulling of a slinky spring (see below 'picture' below).

The 'to and fro' effect is due to the particles of the medium being compressed to give a point of maximum particle density (maximum pressure), squashed up to give a compression.

At the same time, further along the wave, the arrangement of particles is stretched out to give a point of minimum density (or minimum pressure) called a rarefaction.

You can appreciate this by the way the vertical lines and spaced out or compressed together - the vertical lines represent the relative density of particles in the medium (gas, liquid or solid).

Also, the diagram above illustrates longitudinal sound waves travelling at the same speed where wave B has twice the frequency and half the wavelength of wave A. You can deduce this because in wave B the distance between two compression or two rarefactions is halved, so twice as many waves will pass a given point in the same time.

The diagram above illustrates a slinky spring 'pulsed' with longitudinal waves. It also illustrates in a way what happens to the air when a sound wave passes through it and the ground with one of the types of earthquake wave (the compressional P waves), which go right through the Earth to the other side of the world!)

Examples of longitudinal waves:

Sound waves - e.g. from your vocal chords or musical instrument

Earthquake P-waves - that can go right through the Earth

Slinky spring - 'pushed and pulled' to send pulses of energy along it.

INDEX notes: Investigating and introducing the properties of waves

Key points Introduction to waves: Properties of longitudinal waves

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

Here's a comprehensive set of summary revision notes on the properties of longitudinal waves, aligned with the core requirements across major UK GCSE/IGCSE exam boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR). These notes are designed to reinforce conceptual clarity, exam technique, and real-world relevance of longitudinal waves.

Longitudinal Waves: Summary Revision Notes

Definition: A longitudinal wave is a wave in which particles of the medium vibrate parallel to the direction of energy transfer.

Key Properties of longitudinal waves

Property Description
Direction Vibrations are in the same direction as wave travel
Waveform Characterised by compressions and rarefactions
Compression Region where particles are close together
Rarefaction Region where particles are spread apart
Wavelength (λ) Distance between two consecutive compressions or rarefactions
Frequency (f) Number of waves passing a point per second (measured in Hz)
Wave speed (v) Calculated using ( v = f \times \lambda )
Medium Requires a material medium (solid, liquid, or gas) to travel

Examples of Longitudinal Waves

  • Sound waves in air, water, and solids
  • Seismic P-waves (primary waves in earthquakes)
  • Pressure waves in fluids
  • Ultrasound waves used in medical imaging

Typical Exam Board Coverage of longitudinal waves

Longitudinal Wave Properties Required Practical Sound & Ultrasound
 Compression, rarefaction, wave speed  Sound waves in air  Ultrasound applications
 Wave features & graphs  Sound & ripple tank  Medical ultrasound
 Wave behaviour & speed  Sound wave speed  Echoes & imaging
 Describing wave motion  Sound wave properties  Ultrasound & SONAR
 Sound wave structure  Sound wave speed  Seismic wave examples
 Wave equation & sound waves  Sound wave speed  Ultrasound & echoes

 Student Tips about longitudinal waves

  • Use diagrams to show compressions and rarefactions clearly.
  • Practise using the wave speed equation with sound wave data.
  • Understand how sound travels differently in solids, liquids, and gases.
  • Link wave properties to real-world applications (e.g. ultrasound scans, SONAR).
  • Review required practicals involving sound wave speed in air or solids.

Common Misconceptions about longitudinal waves

  •  “Longitudinal waves can travel in a vacuum.”
     They require particles to propagate - no medium, no wave.
  •  “Sound travels faster in air than in solids.”
     Sound travels faster in solids due to closer particle spacing.
  •  “Rarefactions are high-pressure regions.”
     Rarefactions are low-pressure regions where particles are spread out.
  •  “Longitudinal waves have crests and troughs.”
     They have compressions and rarefactions, not crests and troughs.

Keywords, phrases and learning objectives for the properties of longitudinal waves

Be able to describe and explain with a technical description, the properties and examples of a longitudinal wave.

Be able to do longitudinal wave calculations using the wave equation formula.

See Part 10. Wave calculations - formulae & how to solve wave questions

Revision notes on explaining describing examples of longitudinal waves based on the syllabus-specifications for students taking IGCSE/GCSE level physics examinations, summary revision notes and key points on explaining describing examples of longitudinal waves for students taking the AQA igcse/gcse physics notes on explaining describing examples of longitudinal waves, Edexcel gcse physics notes on explaining describing examples of longitudinal waves,  OCR 21st century GCSE physics notes on explaining describing examples of longitudinal waves, OCR gateway GCSE physics notes on explaining describing examples of longitudinal waves, WJEC gcse physics notes on explaining describing examples of longitudinal waves, CCEA gcse physics notes on explaining describing examples of longitudinal waves for students taking CIE Cambridge igcse physics, exam revision notes on explaining describing examples of longitudinal waves, useful for US grade 9-10 physics courses, importance of technical description of the properties & characteristics of  longitudinal waves in GCSE level physics, What you need to know about technical description of the properties & characteristics of  longitudinal waves for GCSE level physics, Explaining the use of technical description of the properties & characteristics of  longitudinal waves knowledge in GCSE level physics, Examples of technical description of the properties & characteristics of  longitudinal waves explained when studying GCSE level physics, What is significant about technical description of the properties & characteristics of  longitudinal waves, describing the theory of technical description of the properties & characteristics of  longitudinal waves when studying GCSE level physics, revision notes for technical description of the properties & characteristics of  longitudinal waves in exams, online exam help for technical description of the properties & characteristics of  longitudinal waves, revision notes about technical description of the properties & characteristics of  longitudinal waves, what do I need to learn about technical description of the properties & characteristics of  longitudinal waves for by GCSE physics exam? help to understand the technical description of the properties & characteristics of  longitudinal waves topic in preparation for GCSE physics exam question, how to prepare for questions involving technical description of the properties & characteristics of  longitudinal waves in a GCSE physics examination?

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INDEX notes: Investigating and introducing the properties of waves

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