Physics
Sound
Class 7
Intermediate
About 20 min
Premium
Pitch and loudness: the vibrating string
Pluck a string on a sonometer, change its length, tightness and how hard you pluck it, and watch the sound wave on an oscilloscope.
Core concept
Sound is made by vibrations; frequency sets the pitch and amplitude sets the loudness
Topic: Sound
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
- Keep the tension medium. Pluck the string at 80 cm, 50 cm and 20 cm long, recording each note.
- Keep the length at 50 cm and pluck it loose and tight.
- 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.