## Problem description

This example solves adaptively the pressure field in a 3D axisymmetric model
of a harmonic acoustic horn. The geometry and initial mesh are shown below.

Equation solved:

Boundary conditions are Dirichlet (prescribed pressure) on the bottom edge, zero Neumann (symmetry)
on the left edge, Newton (matched boundary)

on the outlet arc and zero Neumann (wall) on the rest of the boundary. Here is pressure,
density of air, angular frequency, and speed of sound. See
the main.cpp file for concrete values.