Sound is a mechanical wave - it needs something to travel through. That's why there's no sound in space. The vibrations of molecules bump into each other, passing the energy along.

Different substances carry sound differently. Sound travels faster in water than air, faster in steel than water. Whales can communicate across oceans because sound travels so well in water.

Frequency and Pitch

The frequency of a sound wave determines its pitch. Higher frequency = higher pitch. Humans can hear from about 20 Hz to 20,000 Hz. Dogs hear higher frequencies, which is why dog whistles work.

Ultrasound uses frequencies above what we can hear - it's how doctors see inside bodies and how bats navigate.