Two sound sources emitting sound each of wavelength λ

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Two sound sources emitting sound each of wavelength λ are fixed at a given distance apart. A listener moves with a velocity u along the line joining the two sources. The number of beats heard by him per second is

Frequency received by listener from the rear source,

n' =	v - u	×	v - u	×	v	=	v - u
	v		v		λ		λ

Frequency received by listener from the front source,

n" =	v + u	×	v	=	v + u
	v		λ		λ

No. of beats

= n'' – n' =	v + u	-	v - u	=	v + u - v + u	=	2u
	λ		λ		λ		λ