Acoustic attenuation mechanisms

Acoustic-absorbing material measurement of acoustic properties, 248 selection for underwater application, 248 Acoustic attenuation mechanisms mode conversion, 182-187 redirection of sound, 182 scattering of sound, 185-194... 

While the operating principles are relatively simple, the analysis of the attenuation data to obtain particle size distributions does involve a degree of complexity in fitting experimental results to theoretical models based on various acoustic loss mechanisms. The advent of high-speed computers and the refinement of these theoretical models has made the inherent complexity of this analysis of little consequence. In comparison, many other particle sizing techniques such as photoncorrelation spectroscopy also rely on similar levels of complexity in analyzing experimental results. 

By focusing on customer-driven development, American Aerogel has successfully commercialized a form of aerogel materials. Aerocore, a black organic monohthic open cellular rigid foam, can be produced with a wide range of properties and sizes. It can be used as a thermal insulator, acoustic attenuator, chemical support media, electrical conductor, and mechanical shock absorber. 

The temperature locations of attenuation maxima such as those in Fig. 18-17 have been determined for many polymers by Thurn and Wolf, and these and other results have been discussed by Woodward and Sauer. They can tentatively be identified as corresponding to the shear maxima discussed in Chapter 15 at 2 X 10 Hz, the bulk longitudinal maxima in attenuation occur at temperatures 20 to 70 higher than the respective shear maxima in tan d at much lower frequencies. In measurements of acoustic attenuation in the megahertz range in a series of poly(methacrylate)s, North and collaborators observed one relaxation in the methyl and ethyl polymers and two in the -butyl and /-butyl the one at lower temperatures was identified as the 7-mechanism. 

The deconvolution is the numerical solution of this convolution integral. The theory of the inverse problem that we exposed in the previous paragraph shows an idealistic character because it doesn t integrate the frequency restrictions introduced by the electro-acoustic set-up and the mechanical system. To attenuate the effect of filtering, we must deconvolve the emitted signal and received signal. 

The oscillating resonator surface may be considered as a source for shear waves that are radiated into the contacting film. The upper film surface reflects these radiated shear waves downward, so that the mechanical impedance seen at the quartz surface is dependent upon the phase shift and attenuation undergone by the wave in propagating across the film. When the film is rubbery, significant phase shift across the film occurs. Consequently, the coupling of acoustic energy into the film depends upon thin-fllm interference. 

The results presented here demonstrate that thin films can be characterized based on acoustical monitoring of changes in film mass density, conductivity, and viscoelasticity. Additional sensing mechanisms are available to probe film properties. Some examples are thin-film dielectric constant, stress, and structure (e.g., roughness). Some of these sensing mechanisms will be hard to quantify since they involve a complex interaction (e.g., wave attenuation based on wave scattering due to film roughness) however, they may still be useful to provide a qualitative monitor based on empirical data. 

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