Name the waves of frequency greater than $20,000 \,Hz$. Give one use of these waves.

$(a)$ Mention two practical applications of reflection of sound waves.
$(b)$ How is the pressure variation in a sound wave amplified in the human ear?
$(c)$ In a ripple tank,$10$ ripples are produced per second. If the distance between a trough and a neighbouring crest is $12 \, cm$,calculate the frequency,wavelength,and velocity of the wave.

$(i)$ How do bats make use of ultrasonic waves to catch their prey? Explain.
$(ii)$ $A$ radar signal is reflected by an aeroplane and is received $2 \times 10^{-5} \text{ s}$ after it was sent. If the speed of these waves is $3 \times 10^{8} \text{ m s}^{-1}$,how far is the aeroplane?

$(a)$ Why does the stage of an auditorium have a curved background,curtains,carpets,and a false ceiling?
$(b)$ The sound of a ringing bell inside a vacuum chamber cannot be heard. Why?

$A$ stretched wire $0.5\, m$ long is made to vibrate in two different modes as shown in diagrams $(A)$ and $(B)$ given below:
$(i)$ If the wavelength of the wave produced in mode $(A)$ is $1\, m$,what is the wavelength of the wave produced in mode $(B)$ of the following diagram?
$(ii)$ In which case is the note produced louder? Give a reason for your answer.
$(iii)$ In which case is the pitch of the note produced higher? Give a reason for your answer.

How does a stretched string,on being set into vibration,produce an audible sound?