A source emitting a sound of frequency $300 \mathrm{Hz}$ is fixed on block $A$ while detector is fixed on block $B$ detects the sound. The blocks $A$ and $B$ are simultaneously displaced towards each other through a distance $1 \mathrm{m}$ and then left to vibrate. Velocity of sound in air $340 \mathrm{m} {\mathrm{s}}^{-1}$ and $S_A$ and $S_B$ are identical blocks $A$ and $B$ are also identical. The frequency of vibration of $S_A$ is $2 \mathrm{Hz}$.

A train is moving on a straight track with speed $20\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ . It is blowing its whistle at the frequency of $1000 \mathrm{Hz}$. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound $=320 \mathrm{m} {\mathrm{s}}^{-1}$ ) close to:

A source of sound emits sound waves at frequency $f_0$ . It is moving towards an observer with fixed speed $v_s (v_s<\mathrm{v)}$ , where $v$ is the speed of sound in air). If the observer were to move towards the source with speed $v_0$ , one of the following two graphs (A and B) will give the correct variation of the frequency $f$ heard by the observer as $v_0$ is changed.



The variation of $f$ with $v_0$ is given correctly by:

A source of sound $\mathrm{S}$ emitting waves of frequency $\text{100 Hz }$and an observer $\text{O}$ are located at some distance from each other. The source is moving with a speed of $19.4\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$ at an angle of ${60}^{\mathrm{o}}$ with the source observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer corresponding to the wave emitted by the source at the instant shown is

(velocity of sound in air $330\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$ )
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