To measure the frequency of a tuning fork by using a sonometer
The sonometer is tuned first to unison with a standard fork of known frequency and the length of the wire noted. Keeping the tension constant, it is now tuned to the unknown fork by altering the length only. Suppose the frequencies, in Hz, of the standard and the unknown forks are 320 and f and the corresponding wire lengths for unison are 82.0 and 61.5 em respectively .
Since, at constant tension,
As a check, a second pair of lengths may be found using a different tension.
Alternatively, the length may be kept constant and the tension varied, in which case f may be found by equating the two values of frequency/ tension obtained.