Sound needs a medium in which to travel. Sound needs a medium in which to travel. It cannot travel through a vacuum. As the air is removed from the Bell jar the sound from the bell gets weaker and weaker until it cannot be heard anymore.
Ultrasound Bats make use of this to avoid obstacles. They emit ultrasound(a note of high frequency that humans cannot hear) and can determine distance by the time it takes for the sound to return. Ultrasound is also used in medicine to see inside the body and to shatter gallstones and kidney stones.
Sonar Ultrasound echoes are used by ships to determine the depth of sea below them.
Speed of Sound In air, sound travels at a speed of 340m/s. It travels faster in liquids and fastest of all through solids. Light travels much faster than sound. An example of this is thunder and Lightning.
Hearing protection Measuring the loudness of sound. The decibel is the unit used to compare the loudness of sounds. Regular exposure to noise above 85 decibels could be harmful if ear protection is not worn.