Frequency of vibrations and pitch

The frequency of a vibration, measured in Hertz (Hz), is simply the number of to and fro movements made in each second. A tuning fork or piano string vibrating at 256 Hz will produce a pitch of middle C. A greater frequency than this will produce a higher-pitched note and so on. Children will often mix up pitch and loudness believing that a higher pitched sound is a louder one. Higher pitched sounds produce waves which are closer together than for lower pitched sounds. (Think of the slinky - if you produce waves rapidly they will travel quite close to each other, thus demonstrating a higher pitched sound.)

The pitch of a note will depend on a number of factors. One of these is the size of the vibrating object. On a glockenspiel or xylophone the high notes are made by the smaller bars. A smaller triangle or cymbal will make a relatively higher pitch note. On a stringed instrument such as a guitar or violin a thinner string will generally make a higher note, but also shortening the string by stopping it with the finger will produce a similar effect. On a set of pan pipes or a church organ it is the shorter pipes which make the higher notes when the air inside them vibrates.

Another factor which produces higher pitched notes is the tension within the vibrating object. A guitar string can be tuned to a higher pitch by adjusting the string tensioner. An elastic band can be stretched tighter and a drum skin can be tensioned to increase the pitch of the sound it produces.

 

Fig 5. Diagram to show an object making a high pitched sound together with a graphical interpretation.

 

Contents

What is sound, Vibration
Sounds travel
The speed of sound in air, Sound travels in solids and liquids too, Hearing sounds through solids
Receiving vibrations at the ear drum, Amplitude of vibrations and loudness
Amplification of sounds
Self Assessment