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

Why are the ceiling and the wall behind the stage of good conference halls or concert halls made curved?

How does a stretched string,on being set into vibration,produce an audible sound?

$A$ 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.

What change,if any,would you expect in the characteristics of musical sound when we increase $(i)$ its frequency and (ii) its amplitude?

$(i)$ Which property of sound wave determines:
$(a)$ loudness
$(b)$ pitch?
$(ii)$ $A$ $SONAR$ station picks up a return signal after $3 \ s$. How far away is the object? (Speed of sound in water is $1440 \ m \ s^{-1}$)
$(iii)$ How can reverberations in a big hall or auditorium be reduced?