Ultrasound and Infrasound

- Humans can hear from 20 Hz to 20,000 Hz. 

- Sound above 20,000 Hz is ultrasound, which we cannot hear. 

Animals such as dolphins and bats use ultrasound for communication or echolocation.

Humans use ultrasound for foetal scanning, so we can safely see inside the body. The different tissues in the body reflect ultrasound at different amounts, and the echoes are used to create an image on a monitor. 

-Sound below 20Hz is infrasound, which we cannot hear, but can feel the slow vibrations.

Animals such as whales use infrasound for communication.

The speed of sound varies in different materials. The closer the patricles in the material, the faster sound travels. This means sound travels faster in solids and slower in gases.

Sonar is a technique used by ships to determine the depth of water. Pulses are sent to the seabed. The time delay between sending the pulse and recieving the echo is used to calculate the distance travelled.