How do our ears permit us to receive sound?

$A$ sound wave with frequency higher than $20 \,kHz$ is not audible for human ear. Answer the following in respect of this statement:
$(i)$ What is the term used for such a sound?
$(ii)$ Name two organisms producing sound in this range.
$(iii)$ Write an application of such a wave.

If the frequency of a tuning fork is $400 \, Hz$ and the speed of sound in air is $340 \, m/s$. Find how far sound travels when the tuning fork makes $16$ vibrations. (in $m$)

The rear view mirror of a motorbike starts vibrating violently at a particular speed of the motorbike.
$(a)$ Why does this happen?
$(b)$ What is the name of the phenomenon taking place?
$(c)$ What could be done to stop the violent vibration?

$A$ key of a mechanical piano is struck gently and then struck again but much harder this time. In the second case,

Two friends were playing on identical guitars whose strings were adjusted to give notes of the same pitch. Will the quality of the two notes be the same? Give a reason for your answer.