Index of content:
Volume 112, Issue 5, November 2002
- NOISE: ITS EFFECTS AND CONTROL 
112(2002); http://dx.doi.org/10.1121/1.1508778View Description Hide Description
Low-frequency duct noise is difficult to deal with by passive methods such as porous duct lining. Reactive methods like expansion chamber are rather bulky, while compact resonators are too narrow banded. This study shows that a suitably stretched thin membrane backed by a slender cavity can achieve a satisfactory performance from low to medium frequencies over an octave band. The present paper focuses on the details of the modal behavior of the fully coupled membrane-cavity system, and examples are given with parameters set in a practical range. Typically, the membrane has a structure to air mass ratio of unity, and is stretched towards the elastic stress limit for a material like aluminum. The backing cavity has a depth equal to the duct height and a length five times the duct height. Three resonant peaks are found in the low to medium frequency range while the transmission loss between adjacent peaks remain above 10 dB. For the first peak, almost complete sound reflection occurs as a result of an out-of-phase combination of the first and second in vacuo modes of simply supported membranes. The second peak is solely contributed to by the first mode, while the third peak features mainly the second mode vibration.
112(2002); http://dx.doi.org/10.1121/1.1508779View Description Hide Description
The theoretical finding of the broadband performance of a reactive silencer is validated experimentally. The silencer consists of two highly stretched membranes lining part of the duct and backed by two long and shallow cavities. The test rig was built with a small square duct of 5 cm in dimension, and each cavity is 5 cm deep and 25 cm long. Two types of metal foils, stainless steel and copper, were used, and the lowest membrane-to-air mass ratio was 1.3. A transmission loss in excess of 10 dB was achieved over more than one octave band. For one configuration close to the optimal parameters, the predicted ratio of the frequency band limits is 2.47, while the experiment gave 2.35. Three spectral peaks were found in the stopband, as predicted, but the peaks were broader than prediction, indicating the presence of significant sound energy dissipation mechanisms. Comparison with theoretical simulation shows that the cavity damping dominates over membrane friction. Tests using heavier membranes and membrane with different levels of tension also agree with predictions. Issues of practical implementation of the concept as a flow-through silencer are also addressed.