All materials reflect sound waves equally.

Bats can produce and detect infrasonic.

Snakes hiss by forcing air out of their mouths.

The unit of amplitude is second.

Waves of sound produced by a body move in all directions.

An echo can be heard by us only if the reflecting surface is at least _____ $\mathrm{m}$ away from us (22.6/ 11.3/ 45.2)

Sound requires a _____ to travel.

Birds have a ring of cartilage called _____.

Fill in the blanks.

The SI unit of frequency is _____. (meter/ hertz/ second/ kilogram)