A train is moving towards a stationary observer (at $t = 0$) with constant velocity of $20\ m/s$ and after sometime it crosses the observer. Which of the following curve best represents the frequency received by observer $f$ as a function of time ?

Two tuning forks having frequency $256\, Hz \,(A)$ and $262\, Hz \,(B)$ tuning fork. $A$ produces some beats per second with unknown tuning fork, same unknown tuning fork  produce double beats per second from $B$ tuning fork then the frequency of unknown tuning fork is :- ............ $\mathrm{Hz}$

The displacement $y$ of a wave travelling in the $x-$ direction is given by $y = {10^{ - 4}}\sin \left( {600t - 2x+\frac{\pi }{3}} \right)$ metre, where $x$ is expressed in metres and $t$ in seconds. The speed of the wave in $ms^{-1}$, is

A train whistling at constant frequency is moving towards a station at a constant speed $v$. The train goes past a stationary observer on the station. The frequency $n$ of the sound as heard by the observer is plotted as a function of time $t$. Identify the expected curve

A sound-source is moving in a circle and an observer is outside the circle at $O$ as shown in figure. If the frequencies as heard by the listener are $\nu _1, \nu _2$ and $\nu _3$ when the source is at $A, B$ and $C$ position, respectively, then

A person speaking normally produces a sound intensity of $40\, dB$ at a distance of $1\, m$. If the threshold intensity for reasonable audibility is $20\,dB$, the maximum distance at which he can be heard clearly is ..... $m$