Earthquakes produce which kind of sound before the main shock wave begins?

$A$ source is producing $1500$ sound waves in $3$ seconds. If the distance covered by a compression and an adjacent rarefaction is $68 \, cm$,find $(a)$ frequency,$(b)$ wavelength,and $(c)$ velocity of the sound wave.

$(a)$ What is the cause of the sensation of sound due to the rolling of thunder?
$(b)$ Give the audible range of the human ear.
$(c)$ What are infrasonic and ultrasonic waves?

$(a)$ $A$ mobile ringing inside a vacuum chamber cannot be heard outside. Why?
$(b)$ Represent a transverse wave graphically.
$(c)$ What is meant by loudness of sound? On what factor does it depend?

$A$ 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.

If the density of a medium through which sound is propagating is minimum,then this point is called: