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 $60\, km / hr$ 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 location $P , Q$ and $R$, respectively. The speed of sound in air is $330\, ms ^{-1} .$ Which of the following statement(s) is(are) true regarding the sound heard by the person?
