Active control of low‐frequency turbine exhaust noise

Gas turbine generators radiate noise from their exhaust stacks that contains substantial acoustical energy at very low frequencies. This kind of noise can result in community annoyance due to window rattling and other structural vibration excitation as well as disturb residents. Because of the long wavelengths of sound at these frequencies, typical duct sound absorbing treatments are inefficient, large and expensive. A research program was conducted to demonstrate the ability of a Helmholtz‐resonator‐based active noise control system to attenuate low‐frequency noise in a heated exhaust stack. A unique thermal stratification concept was developed that successfully protected the sound sources from hot exhaust flue gases. The thermal stratification concept permitted the use of both conventional off‐the‐shelf JBL 18‐in. electrodynamic actuators and custom‐fabricated piezoceramic sound sources in the demonstration program. Test results showed that 14–22 dB of noise attenuation was achieved over the frequency r...