The minimum distance required between the source and the reflector so as to hear the echo in air is:

1. 10 m
2. 17 m
3. 34 m
4. 50 m

Wavelength is measured in:

1. kg
2. second
3. litre
4. metre

Sound travels the fastest in:

1. liquids
2. solids
3. gases
4. vacuum

What do you mean by a vibratory motion?