experiments

calculators

 

 

 

 

 

The leaving certificate syllabus includes the mandatory experiment of investigating the natural frequencies of a stretched string at various lengths. At higher level, the effect of tension is also to be explored. A sonometer is specified in the syllabus. A sonometer has a stretched metal string held over two bridges by tension measured typically with a force meter, spring balance or weights. One of the bridges is mobile so that the length may be changed and the tension can be increased by turning a screw. The string is fixed to a hollow wooden sound box. Some references include 1 2 3 

The usual method of determining resonance suggested in text books is to place a small paper "rider" on the string. Tuning forks are struck in turn and placed on the fixed bridge on which the string rests. On resonance, the paper rider falls off as the string vibrates strongly.

Many students (and teachers) have had difficulty with this method. Many have resorted to plucking the string and comparing the note by ear with that of various tuning forks. A simple alternative is to use Audacity or any other sound editor with a microphone held close to the string. Pluck the string and measure the frequency. Measure the length then move one bridge and repeat.

The same simple method can be used in an experiment to measure the frequencies for different tensions. Even tone deaf students (and teachers) can perform this experiment but the sonometer must be correctly tensioned or spurious harmonics interfere with the frequency determination.

Moreover, this is not the ideal way to perform the experiment.

Probably the best method uses a signal generator attached to the wire on which the paper rider sits. The frequency is adjusted until resonance is detected. There are variations possible here including using coils to induce resonance rather than have physical contact. Similarly, a coil close to the wire could be connected to an oscilloscope to detect resonance. The oscilloscope could be replaced with suitable software.

A reference here describes the process well and could be adapted for any such experimental setup. Some teachers have devised their own modifications on this experiment.

Modification of tension can also be used to explore the production of harmonics. Use of Audacity could be used to explore experimentally which harmonics are produced by different types of instrument and the derivation of general rules for the production of harmonics.

Measurement of the speed of sound in air is also specified in the syllabus and this could be achieved by the methods described on the speed pages and by using the resonance tube. Both should be used.

The experiment with the resonance tube is wonderfully evocative and may best done in a traditional manner given the wonderful physicality of the experience. this is one way to measure the speed of sound. A plot of wavelength against 1 / f will give a slope of the speed. Wavelength is best measured as length for 3/4 wave - length for 1/4 wave.

Standing waves or interference provide the acid tests for wave behaviour.

With simple modern equipment much more practical investigation becomes possible and desirable even within the bounds of the traditional syllabus.