A Principle of Superposition

There are many cases in analytical acoustics in which the evaluation of the sound filed in an environment with absorbing boundaries is unknown or difficult, whereas the solutions for ideally reflecting boundary surfaces (hard or soft) are known. A principle of superposition will be derived and explained with some examples which permits to construct the solution with absorbing boundaries by a superposition of the solutions with hard and soft boundaries. On some conditions, the principle of superposition gives the exact solution for the sound field, on other conditions it is the base of good approximations.