A miniature, broadband acoustic spectrometer : design of a unified attenuation model, device development, and experimental performance

Abstract

The attenuation of sound occurs in polyatomic gases due to both classical and nonclassical physics. Classical attenuation is dominated by viscous dissipation and irreversible heat conduction. Nonclassical attenuation arises from the thermal relaxation between internal and external degrees of freedom for each constituent molecule. Currently, we are not aware of any commercial gas sensors that leverage classical attenuation. Existing methods to detect gas composition using nonclassical attenuation are bulky, heavy, and slow at resolving measurements, as the instruments utilize highly resonant, single frequency transducers mounted within rigid containment vessels that are pressurized sequentially over a wide range of pressures. We leverage both classical and nonclassical attenuation to develop a miniature, broadband acoustic spectrometer capable of detecting gas composition. This thesis covers modeling, design, and experimental efforts.

The first part focuses on the design of an acoustic attenuation simulation software package for gases. This package unifies classical and nonclassical models presented in the literature for the attenuation of plane waves within straight or curved tubes. With this simulation tool, attenuation components can be readily compared and optimized. We also tabulate a parameter library for twenty four unique gases which details the required properties to model a wide range of pure samples and mixtures. Second, we deploy this modeling capability to develop a functional sensor. Through four generations of instruments, we construct a unique approach that pairs broadband transducers with stochastic system identification techniques at constant pressure. This paradigm shift enables the fourth-generation device to function with approximately 5% of the total volume and 0.2% of the total mass of the smallest acoustic spectrometers previously described in the literature.

We produce an attenuation spectrum (derived from a bode plot) in less than 30 seconds, which is two orders of magnitude faster than existing methods. Third, we present experimental results captured using the fourth-generation device. The spectrometer produces measurement results that characterize classical and nonclassical attenuation. These results are unique to each gas, allowing for different samples to be distinguished. The experimental results from the fourth generation device agree with the expected theoretical predictions from the simulation package. Our methods and hardware hold promise in a number of application areas requiring medium sensitivity (± 1%), wide specificity sensing in a miniaturized, rugged, and inexpensive package.