Noise reduction of an acoustical enclosure --- Mechanisms and prediction accuracy
Noise reduction (NR) of an acoustical enclosure with flexible boundary walls has been predicted using t he Statistical Energy Analysis (SEA) method by several authors. Although it is useful for a rough NR estimation, a lar ge discrepancy often exists between the predicted and measured NR levels. Moreover, some physical mechanisms which may affect NR prediction were not addressed in the e xisting SEA models. The sources of the discrepancy were identified by investigating the limitation of SEA f or system energy transfer in the entire frequency r ange of noise transmission, and the effect of enclosure wall coup ling and sound-structural coupling on the NR and its prediction accuracy. This paper presents a modified SEA model, which includes the non-resonant response and more accurate transmission coefficient of finite panels, and comp ares the model prediction with experimental results . A reasonable agreement between the prediction and experiment was observed.
Insertion loss of an acoustic enclosure.
Acoustical enclosures are the common arrangements in reducing airborne noise from shipboard machinery such as engines and generators. In this paper the theoretical models, established based on statistical energy analysis, are presented for predicting the insertion loss of acoustical enclosures in different frequency ranges. In addition to the consideration of resonant modal coupling between internal sound field and enclosure structural vibration, the nonresonant transmission though and the interaction between enclosure walls in the models are also included. It is shown that the insertion loss of enclosures is mainly controlled by the nonresonant modes in the intermediate frequency range. At high frequencies, the insertion loss of enclosures can be improved by increasing the sound absorption at the internal boundaries of enclosures. Experiments were carried out on two enclosures made of different materials. The measured results are compared with the predicted values and the good agreement between them is the initial demonstration of the validity and feasibility of the theoretical models.
Kinemamtic analysis of flying-saw vehicle
The paper analyzes the kinematic motion of the flying-saw vehicle.After the analysis,it is concluded that the ac- celerating periods and decelerating periods in both forward and backward directions will follow sin~2αcurve and,in order to increase the sawing dimensional accuracy,flying-saw vehicle must come back as soon as possible while acceleration remains constant.Based on this premise,the paper gives out the equations of kinematic motion of the flying-saw vehicle in every sta- ges and one example result in a common situation.By now this method has been used in the real situation.