Sound is a longitudinal wave made by vibrations. It needs a medium (a solid, liquid or gas) to travel through, so it cannot cross a vacuum. Sound travels at about 340 m/s in air, faster in water and faster still in solids like steel, because the particles are closer together.
Pitch depends on frequency: a high frequency gives a high pitch. Loudness depends on amplitude: a bigger amplitude gives a louder sound. Humans can hear from about 20 Hz to 20 000 Hz. Sound above that is ultrasound, used for scanning unborn babies and by bats to find prey. An echo is a reflected sound; timing an echo can measure a distance.
Worked example
An echo from a cliff returns after 2 s. Sound travels at 340 m/s. How far away is the cliff?
- Distance travelled = 340 × 2 = 680 m.
- That is there and back.
Answer:The cliff is 680 ÷ 2 = 340 m away.
Key words
- medium
- the material a wave travels through
- pitch
- how high or low a sound is; set by frequency
- ultrasound
- sound too high for humans to hear
- echo
- a reflected sound
New questions every time.
NGSS-referenced: MS-PS4-1