An investigation of the physical characteristics of a mass element resonator.

Abstract

This thesis investigates the characteristics of an acoustic resonator with a passive mass element. The mass element is substituted as an impedance matched replacement for a section of gas-filled half wavelength resonator tube. The mass element consists of a disk bonded to a Servometer electroformed single convolution nickel bellows. The bellows can be modelled as a circular plate with a spring supported boundary placed in the acoustic center of a resonator between two equivalent gas springs driven by a loudspeaker. Loading changes were used to characterize the bellows' motional response to different drive conditions for the frequency range near the first resonance of the system. The response of the system was highly nonlinear for nearly all the configurations despite minimal loudspeaker drive levels. The nonlinear response was a consequence of the coincidence of the resonant frequencies for several modes of the bellows, particularly two prominent modes - an expected piston-like mode, and an unexpected rocking mode. Attempts were made with varying success to achieve high amplitudes for the piston mode, which is important for its use in a new generation Thermoacoustic Refrigerator. The new refrigerator design will utilize the mass element to reduce those acoustic losses that are a parasitic heat load on the cold end of the refrigerator, as well as make the resonator more compact.