The source producing sound and an observer are both moving along the direction of propagation of sound waves. If the respective velocities of sound,source,and an observer are $v$,$v_s$,and $v_o$,then the apparent frequency heard by the observer will be ($n =$ frequency of sound).

This question has Statement $1$ and Statement $2$. Of the four choices given after the Statements,choose the one that best describes the two Statements.
Statement $1$: Bats emitting ultrasonic waves can detect the location of a prey by hearing the waves reflected from it.
Statement $2$: When the source and the detector are moving,the frequency of reflected waves is changed.

Two loudspeakers $M$ and $N$ are located $20 \ m$ apart and emit sound at frequencies $118 \ Hz$ and $121 \ Hz$,respectively. $A$ car is initially at a point $P$,$1800 \ m$ away from the midpoint $Q$ of the line $MN$ and moves towards $Q$ constantly at $60 \ km/h$ along the perpendicular bisector of $MN$. It crosses $Q$ and eventually reaches a point $R$,$1800 \ m$ away from $Q$. Let $v(t)$ represent the beat frequency measured by a person sitting in the car at time $t$. Let $v_P, v_Q$ and $v_R$ be the beat frequencies measured at locations $P, Q$ and $R$,respectively. The speed of sound in air is $330 \ m/s$. Which of the following statement$(s)$ is(are) true regarding the sound heard by the person?
$(A)$ $v_P + v_R = 2v_Q$
$(B)$ The rate of change in beat frequency is maximum when the car passes through $Q$
$(C)$ The plot below represents schematically the variation of beat frequency with time (Left plot)
$(D)$ The plot below represents schematically the variation of beat frequency with time (Right plot)

When a train approaches a stationary observer,the apparent frequency of the whistle is $n'$ and when the same train recedes away from the observer,the apparent frequency is $n''$. Then,the apparent frequency $n$ when the observer moves with the train is

$A$ source is stationary and the observer is in motion along a line joining the source and the observer. If the frequency heard by the observer is $1\%$ higher than the true frequency,the ratio of velocity of the observer and that of sound in air is:

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 $\%$)