Question

Two sound sources each emitting sound of wavelength $\lambda$ are fixed some 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

• A. $\frac{2u}{\lambda }$
• B. $\frac{u}{\lambda }$
• C. $\frac{u}{3\lambda }$
• D. $\frac{2\lambda }{u}$

Answer 1

The correct option is A $\frac{2u}{\lambda }$

Number of extra waves receives from each side, $n=\pm \frac{u}{\lambda }$

where,

$u=$ speed of the observer

$\lambda =$ Wavelength of sound

The number of beats is given by the difference between the number of extra

waves received on either sides

So,

Number of beats $=\frac{u}{\lambda }-(-\frac{u}{\lambda })=\frac{2u}{\lambda }$