An echo is heard after $0.8 \, s$,when a person fires a cracker $132.8 \, m$ from a high building. Calculate the speed of sound.

What is 'Ultrasound'? Explain how defects in a metal block can be detected using ultrasound.

Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also,define wavelengths and time-period using this curve.

$(a)$ What is meant by intensity of sound?
$(b)$ Mention the conditions for an echo to be heard clearly.
$(c)$ $A$ ball is dropped into a pond from a height of $44.1 \, m$. The splash of sound is heard $3.13 \, s$ after the ball is dropped. Determine the velocity of sound in air.

$(i)$ What does the acronym $SONAR$ stand for?
$(ii)$ What is the audible range of sound for human beings?
$(iii)$ How do the bats search and catch their prey in dark night?

What do you understand by the term ultrasonic vibrations?