Active Sound Attenuation Across a Double Wall Structure

In an approach different from established active noise control techniques, appreciable sound attenuation was achieved across double wall structures by placing control loudspeakers in the area between the walls. This technique was demonstrated in a transmission loss suite for a flat double wall structure radiating into an almost anechoic environment. The concept was also applied across the curved double wall of a plywood/Masonite® mock-up of an aircraft fuselage with a digital control system consisting of four error microphones and four control speakers. Substantial sound attenuation was obtained along the path of a rotating microphone in the semireverberant space of the fuselage for a 100-Hz sinusoidal signal. Less global attenuation was obtained when the frequency of the source signal was increased, or when fewer error microphones were used. The global attenuation increased as the distance between the error microphone and the source and control speakers was extended.