The walls of a hall built for musical concert should

The audible range for a normal human being is

Why do astronauts talk to each other through radio telephones?

$(a)$ Why are sound waves called mechanical waves? List two practical applications of reflection of these waves.
$(b)$ $A$ stone is dropped from the top of a tower $125 \ m$ high into a pond of water at the base of the tower. When is the splash heard at the top? $(g = 10 \ m s^{-2}$ and speed of sound $= 340 \ m s^{-1})$

Why can we hear echoes in long galleries and big halls?

Which of the above two graphs $(a)$ and $(b)$ representing the human voice is likely to be the male voice? Give reason for your answer.