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Grades 7 and 9 Intermediate About 20 min Version 1 Awaiting review Premium

Pitch and loudness: the vibrating string

Every sound is made by something vibrating. A plucked string moves back and forth many times a second; the number of vibrations each second is its frequency, measured in hertz (Hz). A higher frequency sounds as a higher pitch. How far the string moves from its rest position is the amplitude; a bigger amplitude sounds louder. A shorter, tighter string vibrates faster and gives a higher note. An oscilloscope shows a sound as a wave: closer waves mean higher frequency, taller waves mean bigger amplitude.

What you will learn

  • Explain that sound is produced by vibrating objects.
  • Relate frequency to pitch and amplitude to loudness.
  • Describe how the length and tightness of a string change its pitch.

Before you start

  • Know that sounds can be high or low, loud or quiet.

Materials

  • Strong nylon or steel wire

Equipment

  • Sonometer (a box with a string, two bridges and a tension weight)
  • Microphone and oscilloscope (or a phone app)

Procedure

  1. Keep the tension medium. Pluck the string at 80 cm, 50 cm and 20 cm long, recording each note.
  2. Keep the length at 50 cm and pluck it loose and tight.
  3. Pluck the 50 cm string gently and then hard.

How this simulation is simplified

  • The string has a mass of 0.5 g per metre and vibrates in its simplest way (one loop).
  • Loudness is a rough guide for a listener 1 m away.