The walls of a hall built for musical concert should

Give two applications of ultrasound.

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 $342 \, m s^{-1}$]

Two musical notes of the same pitch and loudness are played on a violin and a piano. The waveforms are as shown in the figures below. Explain why the wave patterns are different.

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?

On a slinky,we can produce: