A sonometer consists of a wooden soundbox with one or more metal wires anchored at one end. Slotted masses can be attached to the free end of the wires which can make them taught. When the wire is struck it will produce an audible frequency. The more masses added the higher the frequency of the note produced. If the sonometer does not include a pulley at one end, a bench mounted pulley will need to be used. In this case the sonometer needs to be attached to the desktop securely so it will not move when masses are added to the end of the wire.
Equipment typically required (per demo or investigation):
This practical can be conducted for a variety of ability groups. With higher ability groups, oscilloscopes and microphones may be required so that the frequency of the string can be shown in graphical format. Lower ability groups may simply refer to the frequencies as musical notes and decide by ear whether the note is higher or lower than a previous note.
- Slotted masses
- Metre rule
- Bridge piece
- Oscilloscope and microphone (optional)
- Bench pulley (if sonometer does not have one)
- Tuning forks (optional)
Bridge pieces are usually small wooden blocks with a metal edge which can be placed between the wire and the soundboard at any point along its length. They essentially adjust the length of the wire, creating a range of frequencies. Part of this investigation may involve changing the length of the wire using a bridge and seeing how this affects the frequency of the audible note produced when the wire is struck.
Some groups may want to produce a specific frequency with the aid of a tuning fork. Most tuning forks have a frequency printed on them which represents the frequency that can be produced when the fork is struck. Students may be able to reproduce the frequency generated by the fork on the sonometer simply by listening to the audible note produced and comparing the two, adjusting the masses or bridge until the correct note is produced. Again, using an oscilloscope and microphone the frequency produced can be checked a lot more accurately.