An echo is returned in $6$ seconds. What is the distance of the reflecting surface from the source (in $, m$)? [Given that the speed of sound is $342 \, m s^{-1}$]

Give two applications of ultrasound.

The stem of a tuning fork is pressed against a table top. Answer the following questions:
$(i)$ Would the above action produce any audible sound?
$(ii)$ Does the above action cause the table to set into vibrations?
$(iii)$ If the answer above is yes,what type of vibrations are they?
$(iv)$ Under what conditions does the above action lead to resonance?

$(a)$ The sound of an explosion on the surface of a lake is heard by a boatman $100 \ m$ away and a diver $100 \ m$ below the point of explosion. Of the two persons mentioned (boatman or diver),who would hear the sound first and why?
$(b)$ Calculate the wavelength of a sound wave whose frequency is $220 \ Hz$ and speed is $440 \ m \ s^{-1}$ in a given medium.

$(a)$ What is the audible range of the average human ear?
$(b)$ Explain how ultrasound is used to clean spiral tubes and electronic components?

For hearing the loudest ticking sound by the ear:
$(a)$ Find the angles $x$ and $y$ in the figure below.
$(b)$ Name the phenomenon observed here.
$(c)$ State the laws of the phenomenon.