Audio-Frequency Measurement by the Electrically-Excited Monochord

A flexible stretched string is resonant at an infinite number of frequencies, which are successive multiples of the lowest or fundamental frequency. It has been suggested that an electrically-excited stretched steel wire might therefore be used to measure audio frequencies by giving multiples and submultiples of the frequency of a tuning fork. In practice, however, the resonant frequencies are far from being exact multiples of the fundamental. In this paper it is shown that this is due to the imperfect flexibility of the steel wire. A simple expression is given for the resonant frequencies, together with experimental evidence of its accuracy. The expression involves the elastic modulus of the steel of the wire, and a convenient method of determining this quantity is described.