Sound waves are

What is an echo? When can we distinctly hear the echo of a sharp sound? Why cannot we hear an echo in small halls?

$20$ waves pass through a point in $2$ seconds. If the distance between one crest and adjacent trough is $1.5 \, m$,calculate: $(a)$ the frequency,$(b)$ the wavelength.

Flash of lightning reaches earlier than the sound of thunder. Why?

State whether the following statement is true or false:
The wavelength of $5\, MHz$ ultrasound in a medium where the velocity is $1540\, m s^{-1}$ is $0.3\, mm$.

What do you understand by the term ultrasonic vibrations?