Read the clues across and clues downwards and fill up the blank squares.

Across :

1. Sound travels in air in the form of …………… waves.
2. Hertz is a unit of …………… .
3. The sound heard after the reflection.
4. Higher the ……………, shriller the sound.

Down :

2. Bat can produce …………… waves.
3. Sound is produced when a body creates …………… .

Can two persons hear each other on moon's surface? Give reason to support your answer.

The outer case of the bell in a temple is made big. Give a reason.

During a drama rehearsal, in an auditorium, Aftab shouted and heard a faint echo. On the day of the final event carpets, chairs with cushions and curtains were placed in the auditorium. The auditorium now had clear sound with no echo.

Answer the following :

(i) Why was an echo heard in the auditorium initially?

(ii) Why did echo disappear after adding of carpets, chairs with cushions and curtains ?

(iii) When is an echo heard ?

(iv) What is the minimum distance required for sound to travel to generate echo ?