Hertz is a unit of

Explain why echoes cannot be heard in a small room.

Name the two types of mechanical waves.

$A$ girl is sitting in the middle of a park of dimension $12 \,m \times 12 \,m$. On the left side of it there is a building adjoining the park and on the right side of the park,there is a road adjoining the park. $A$ sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.

An astronaut cannot hear his companion on the surface of the Moon because

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.