Gopal P. Mathur
McDonnell Douglas Aerospace, 1510 Hughes Way, Mail Code 71-35, Long Beach, CA 90810-1870
Aircraft cabin noise control in the past has relief heavily on improving
sidewall attenuation by passive ``add-on'' treatments. The conventional passive
methods, such as adding mass, damping, or acoustic absorption, etc., not only
impose a stiff weight penalty, they are also ineffective in improving the
low-frequency sound transmission loss of an aircraft fuselage sidewall. Active
control of sound inside aircraft cabins has been the focus of research in
recent years and has shown considerable promise. Laboratory and in-flight tests
of prototype active control systems for tonal noise reduction using secondary
speakers have demonstrated the feasibility of active noise control (ANC) in
aircraft cabins. In recent years active structural acoustic control (ASAC) has
also been applied to aircraft fuselage structures in controlling low- to
mid-frequency structural sound radiation. In the ASAC technique, control forces
are applied directly to the vibrating structure by actuators (such as
piezoelectric transducers) instead of using loudspeakers to minimize the
radiated sound field. The ASAC approach is also important in the design of
``smart structures,'' which incorporate both sensors and actuators in the
structure for noise and vibration control. This paper presents results of
investigations (conducted at McDonnell Douglas) of application of both ANC and
ASAC techniques to a full scale aircraft fuselage. Significant sound pressure
reductions were achieved throughout the cabin for multiple tonal frequencies of
excitation. The performance of the ASAC method is compared with that of the ANC
system using speakers. The flight test results with a prototype ANC system in
an MD-80 aircraft will also be presented.