A recently-developed experimental equipment for optical measurements with acoustical proposes of vibrating parts of musical instruments is described in this thesis. Its methodology consists of performing optical measurements of a string using a high-frame-rate line-scan camera complemented with an optical lens system and a mirror arrangement and applying a straightforward image-processing method to extract the acoustical signal. The optical tube and lens are designed to capture the proper amount of light and to provide sufficient magnification. Measurements in two space dimensions are possible by using a mirror at a 45o angle with respect to the camera plane. This set-up has been calibrated and successfully used to capture the motion of some instruments’ strings such as piano, guitar and bass guitar. An experimental monochord has been built and it is used to analyse strings individually. The captured signals are presented and two image-processing methods for extracting the vibration signals from two dimensional measurements are described and compared. This thesis presents some results in piano and guitar strings. The three main string excitation techniques –plucking, striking, and bowing– are analysed. The effects of string torsion on the exchange of energy between polarizations during the oscillation decay are discussed, and it is shown that the rotation of the string does not result in higher polarization shifting. This work concludes that high-accuracy results can be obtained with the described system which can be used for numerous acoustical applications requiring string measurements.

Outgoing