The Helmholtz equation in heterogeneous media: a priori bounds, well-posedness, and resonances

TL;DR

This paper establishes explicit a priori bounds and well-posedness results for the heterogeneous Helmholtz equation in nontrapping media, and extends resonance analysis to less regular, star-shaped interfaces with discontinuous coefficients.

Contribution

It provides new explicit bounds and well-posedness results for heterogeneous Helmholtz problems, and advances resonance theory for acoustic transmission with minimal interface regularity.

Findings

Explicit a priori bounds depend on $k$, $A$, $n$, and geometry.

Well-posedness holds for $L^ty$ coefficients with monotonicity.

Resonance results extend to star-shaped, $C^0$ interfaces with discontinuities.

Abstract

We consider the exterior Dirichlet problem for the heterogeneous Helmholtz equation, i.e. the equation $\nabla\cdot(A \nabla u ) + k^2 n u =-f$ where both $A$ and $n$ are functions of position. We prove new a priori bounds on the solution under conditions on $A$, $n$, and the domain that ensure nontrapping of rays; the novelty is that these bounds are explicit in $k$, $A$, $n$, and geometric parameters of the domain. We then show that these a priori bounds hold when $A$ and $n$ are $L^\infty$ and satisfy certain monotonicity conditions, and thereby obtain new results both about the well-posedness of such problems and about the resonances of acoustic transmission problems (i.e. $A$ and $n$ discontinuous) where the transmission interfaces are only assumed to be $C^0$ and star-shaped; the novelty of this latter result is that until recently the only known results about resonances of acoustic transmission problems were for $C^\infty$ convex interfaces with strictly positive curvature.