The analysis of a two-dimensional drum brake shows that there are four mechanisms present in generating drum brake squeal. The analysis shows that the unstable modes from the mechanisms contain waves. These waves move in different directions. In a drum brake, all four mechanisms are present and four waves travelling in different directions are superposed. If waves with different wave directions are superposed, the solution is forced towards a standing wave, which is always stable. Every step towards a standing wave is a step towards stability. If the waves could form a standing wave, the noise would be eliminated at the source.