We have already learnt that Sound waves cause Compression and Rarefaction

In compression

Particles of medium (air particles) are tightly packed, hence there is more density and more pressure

In refraction,

Particles of medium (air particles) are loosely packed, hence they have less density and less pressure


Plotting them on Graph

We get peaks and trough as follows


Wavelength of Sound Wave




The peak represents region of highest compression. It is called the crest.

The valley represents region of rarefaction. It is called the trough.

What is Wavelength of Sound Wave

The distance between any two consecutive compressions or rarefactions is called wavelength.

SI unit of wavelength is metre (m)

It is denoted by lambda. (λ)


Wavelength is the distance between

2 consecutive compressions (2 Crests)


2 consecutive Rarefactions (2 Troughs)



It is denoted by lamda (λ)

Its SI Unit is meter

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  1. Class 9
  2. Chapter 12 Class 9 - Sound

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.