The loudness of a sound depends upon

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

$A$ ship sends out ultrasound waves that are detected after $4 \, s$ upon reflection from the seabed. If the speed of ultrasound through seawater is $1550 \, m \, s^{-1}$,find the distance of the seabed from the ship. (in $, m$)

Hertz is a unit of

What is $SONAR$?

Define the following terms and state their $SI$ units:
$(a)$ Wavelength $(b)$ Time period $(c)$ Amplitude.
Establish the relation $V = v \lambda$,where the symbols have their usual meanings.