THEORY OF SYMMETRIC MULTILAYER STRUCTURES AS LAYER REPLACEMENTS FOR CHEBYSHEV ACOUSTIC ANTIREFLECTION COATINGS

Chebyshev filter theory can be applied to the design of wideband acoustic multilayer antireflection (ar) coatings. However, in many cases the acoustic impedance required for a particular layer is not readily available among common materials. In such cases, this study shows that the Chebyshev layer can be replaced by an acoustic symmetric multilayer (ASM) with the prescribed impedance. On the other hand, ASMs are dispersive, i.e., their reflection properties are a function of frequency. Consequently, the ASM replacement will somewhat degrade the performance of the resulting Chebyshev ar coating. In this study, theoretical results are obtained for a number of ASM replacement designs. For each design, normal-incidence reflection coefficient versus frequency is calculated. It is concluded that the performance of a Chebyshev ar coating is only slightly degraded by the use of ASM replacements, provided that the ASM dispersion is kept as small as possible. Accordingly, the ASM technique offers a useful means for the practical design of acoustic Chebyshev ar coatings. © 1990, Acoustical Society of America. All rights reserved.