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.jpg

Wavelength of a sound wave.gif



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)

Wavelength of a Sound Wave - teachoo.png


It is denoted by lamda (λ)

Its SI Unit is meter

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

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CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 9 years and a teacher from the past 16 years. He teaches Spoken English, Written English, Grammar and Vocabulary at Englishtan.