Combustion roar is a “white” noise radiated from small regions in the flame which suddenly burn. Single frequency oscillations of the simple acoustic type in the combustion chamber (“screaming”) have a frequency 500 cps upwards; Helmholtz resonator oscillations have a frequency of the order of 10 times lower and occur fairly frequently in oil- and gas-fired boilers. The oscillation is sustained when a rise in combustion-chamber pressure causes the mean gas temperature to increase, often because it reduces the air-inflow rate. When the mechanism is understood the cure for an unwanted pulsation becomes relatively simple. Combustion instabilities can be used to increase combustion intensity, heat-transfer rate, and to give effectively constant volume combustion.