It is now known that there are two types of combustion noise: direct combustion noise and indirect combustion noise.
When fuel is burned in a combustor, the chemical reaction generates both sound and hot spots. The sound generated is
referred to as direct combustion noise. The hot spots or entropy waves are temperature and density fluctuations. They have
no accompanying pressure fluctuations. However,when these hot spots/entropy waves are convected by the mean flow
through a constriction in the engine ducting system,such as a turbine stage,or through a region with non-uniform mean