Ultrasonic waves are not used in which of the following?

In the bell jar experiment,as air is removed from the jar,

The given graph shows the displacement versus time relation for a disturbance travelling with a velocity of $1500\, m \,s^{-1}$. Calculate the wavelength of the disturbance.

Why do we hear the sound produced by the humming bees while the sound of vibrations of a pendulum is not heard?

$A$ 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.

$A$ vibrating tuning fork is placed over the mouth of a burette filled with water. The tap is opened and the water level gradually falls. It is observed that the sound becomes the loudest for a particular length of air column.
$(i)$ What is the name of the phenomenon taking place when this happens?
$(ii)$ Why does the sound become the loudest?
$(iii)$ How does the frequency of the loud sound compare with that of the tuning fork?
$(iv)$ State the unit for measuring loudness.