When two waves of same frequency, same wavelength, same velocity (nearly equal amplitude) moves in the same direction, Their superimposition results in the interference. Due to interference the resultant intensity of sound at that point is different from the sum of intensities due to each wave separately. This modification of intensity due to superposition of two or more waves is called interference.

Let at a given point two waves arrives with phase difference and the equation of these waves is given by

and since    Intensity

Important Points

 (1) Constructive interference : Intensity will be maximum

when

It means the intensity will be maximum at those points where path difference is an integral multiple of wavelength These points are called points of constructive interference or interference maxima.

(2) Destructive interference : Intensity will be minimum

where (3) All maxima are equally spaced and equally loud. Same is also true for minima. Also interference maxima and minima are alternate as for maximum

then I_max =  and I_min = 0

(6) In interference the intensity in maximum exceeds the sum of individual intensities (I₁ + I₂) by an amountwhile in minima

Hence in interference energy is neither created nor destroyed but is redistributed.