When a structure is subjected to an excitation, it vibrates and subsequently radiates noise. Sound pressure level at a given location around the structure is somehow correlated with the structural vibrations and hence it is possible to identify major sources of the noise and do treatments on the structure by observing its operational deflection shapes. If sources of a non-stationary noise such as a one due to an impact excitation is to be identified, analysis of time-varying deflection shapes is desirable. Also, sound pressure level of noise from a structure of high modal density can be more meaningfully interpreted by frequency domain analysis over proportional bandwidth than over constant bandwidth. In order to satisfy these two requirements, harmonic wavelet transform technique is proposed. Since noise is usually analyzed over 1/3 octave bands, use of 1/3 octave wavelet transform is studied in this thesis.
Basic backgrounds to generate transient deflection shapes using the 1/3 octave wavelet transform are presented. Characteristics of the 1/3 octave wavelet transform as a tool for estimating the distribution of power of a signal with respect to time and frequency is investigated. Since it is basically an off-line digital filter, its performances as a digital filter such as roll-off slope and conservation of phase information are compared with other filters. Usefulness of the 1/3 octave wavelet transform is illustrated by its application to vibration and noise reduction of a stainless-steel plate of high modal density under impact excitation.
