When both the source of sound and the observer approach each other with a speed equal to $10 \%$ of the speed of sound,then the percentage change in frequency heard by the observer is nearly (in $\%$)

An observer moves towards a stationary source of sound with a velocity equal to one-fifth of the velocity of sound. The percentage change in the frequency will be $\dots \;$%.

When a source of sound moves towards a stationary observer,the apparent frequency heard by him

$A$ railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n)$ of the sound heard by the observer is plotted against time $(t).$ Which of the following best represents the resulting curve?

Two sources of sound of same frequency are placed at a distance of $100\,m$ from each other. An observer moving in between them hears $4$ beats per second. If the distance between the sound sources is increased to $400\,m$,then the number of beats heard by him will be:

An observer starts moving with uniform acceleration $a$ towards a stationary sound source of frequency $f$. As the observer approaches the source,the apparent frequency $f'$ heard by the observer varies with time $t$ as: