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Science

Sound and light

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Sound waves are longitudinal waves that must pass through a medium. Echoes are reflections of sounds. Light and other forms of electromagnetic radiation travel as transverse waves. These waves can travel through a vacuum, and they all travel at the same speed in a vacuum.

Sound

Sound waves are longitudinal waves. Their vibrations occur in the same direction as the direction of travel. Sound waves can only travel through a solid, liquid or gas.

Vibrations

When an object or substance vibrates, it produces sound:

  • the greater the amplitude, the louder the sound
  • the greater the frequency, the higher the pitch.

These diagrams show snapshots from oscilloscope traces of three sounds.

Sounds 1 and 2:

  • the sound waves have the same frequency, so the sounds have the same pitch
  • sound 2 has a greater amplitude than sound 1, so sound 2 is louder.

Sounds 2 and 3:

  • the sound waves have the same amplitude, so the sounds have the same loudness
  • sound 3 has a greater frequency than sound 2, so sound 3 is higher pitched.

The range of human hearing

For the exam, you only need to know about sounds in the normal range of human hearing. This is between about 20 Hz and 20 kHz, but the range becomes less as we get older. Sounds with frequencies above about 20 kHz are called ultrasound.

Range of human hearing and ultrasound

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