Abstract:
|
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. |