Aircraft Cabin Noise Control Research Articles

Aircraft cabin noise control: Challenges and opportunities.

Increasingly stringent limits on community noise from aircraft operations have been imposed over the past three decades. However, no comparable limits have been in place for cabin noise levels until recently. The introduction of regulations by the European Union on the allowable levels for noise exposure inside the aircraft cabin for pilots, flight attendants, and passengers has brought this problem to the forefront. Several noise sources contribute to high noise levels inside the cabin; the dominance of any particular source varies depending on the location. In addition to the noise due to the turbulent boundary layer, engine vibrations, and equipment, the noise from the engines plays an important role. The nozzles of a dual‐stream turbofan engine are operated invariably at super‐critical pressure ratios at cruise, thereby producing shocks in the jet plume. Consequently, broadband shock‐associated noise is generated in addition to the turbulent mixing noise component. The engine noise impinges on the fuselage and is transmitted into the interior of the aircraft cabin. Control strategies typically focus on appropriate designs for reducing source noise and optimizing acoustic treatment, as the added weight is undesirable. This talk provides an overview and concrete examples for noise control.

On heterogeneous blankets: Analytical solution for the interaction between masses and poro-elastic layers

There has been substantial research over the last five decades on control of aircraft cabin noise. One new passive approach is the heterogeneous (HG) blanket where a traditional acoustic blanket treatment is altered by adding mass inhomogeneities into the poro-elastic/viscoelastic layers. These masses act like distributed vibration absorbers and can be used to reduce vibration and sound transmission by targeting modes of the fuselage. The natural frequency of a mass inhomogeneity is determined by the mass itself and by the effective stiffness of the porous layer due to the mass/poro interaction. An experimental 1st order approach to predict the effective stiffness based on the shape of the mass inhomogeneities is reviewed. An analytical model for poro-elastic media proposed by Allard et al. was simplified for low frequencies and is used to validate and extend the 1st order approach. It is shown that the effective stiffness depends on mass shape, the foam thickness and material constants such as the modulus of elasticity and Poisson's ratio. Furthermore the model can be used to calculate the stress and displacements fields in the blanket in order to give further insight into the behaviour of the HG blankets.

Recent advances in active control of aircraft cabin noise

Active noise control techniques can provide significant reductions in aircraft interior noise levels without the structural modifications or weight penalties usually associated with passive techniques, particularly for low frequency noise. Our main objective in this presentation is to give a review of active control methods and their applications to aircraft cabin noise reduction with an emphasis on recent advances and challenges facing the noise control engineer in the practical application of these techniques. The active noise control method using secondary acoustic sources, e.g., loudspeakers, as control sources for tonal noise reduction is first discussed with results from an active noise control flight test demonstration. An innovative approach of applying control forces directly to the fuselage structure using piezoelectric actuators, known as active structural acoustic control (ASAC), to control cabin noise is then presented. Experimental results from laboratory ASAC tests conducted on a full-scale fuselage and from flight tests on a helicopter will be discussed. Finally, a hybrid active/passive noise control approach for achieving significant broadband noise reduction will be discussed. Experimental results of control of broadband noise transmission through an aircraft structure will be presented.

Evaluation of efficient passive damping treatments for aircraft cabin noise control

Visco-elastic damping has been used as one type of passive noise control treatment to reduce noise transmission through fuselage skin pan- els. In most conventional applications, uniform coverage of the damping layer is applied to skin panels. Recently, stand-off damping has also been used for aircraft noise reduction. Effects of various damping treatments have been well documented in the published literature. In this paper, some weight-efficient damping treatments, such as edge and wedge damping, are considered. The main objective of this research work is to evaluate passive noise control treatments for a stiffened sidewall section using analytical models. The goal is to achieve an improvement of 3–5 dB in the transmission loss of the sidewall over a conventional passive treatment, over the frequency range from 400 to 2000 Hz. The effect of various damping treatments on the vibration response of a stiffened panel to a point force input, acoustic pressure field and turbulent boundary layer excitation were investigated using a NASTRAN spectral model. The finite-element model of the stiffened panel included visco-elastic damping with and without constraining layer. The material properties of a commercially available constrained layer foam damping were used.

Active control of transmission of turbulent boundary layer noise using smart foam elements

Results concerning the active control of transmission of turbulent boundary layer noise using smart foam elements will be presented. Smart foam consists of a cylindrical shaped PVDF film embedded within partially reticulated polyurethane acoustic foam. A smart foam element is an active–passive control device utilizing the passive absorption characteristics of the foam (effective at high frequencies) and the electrically driven PVDF film as the active element (effective at lower frequencies). This lightweight, compact arrangement lends itself well to the control of aircraft cabin noise resulting from an exterior turbulent boundary layer. Six smart foam elements (each 2 in. thick×4 in.×5 in.) are used to control the sound transmission through a 0.05 in. thick×10 in.×20 in. aluminum plate mounted in a wind tunnel. The sound transmission is monitored by measuring the pressure inside an externally mounted anechoic box that encloses the plate. Various control strategies are tested and results compared.

Active Control of Aircraft Cabin Noise Using Collocated Structural Actuators and Sensors

This paper describes preliminary laboratory experiments conducted on a turboprop aircraft fuselage to reduce propeller-induced tonal cabin noise and vibration. Piezoelectric elements were grouped to construct a long one-dimensional array of actuators bonded to the fuselage in the main sound transmission path at the propeller footprint. Strain gauges and accelerometers were used as alternative sensor devices and were distributed along the actuator in a collocated arrangement. The array of actuators and sensors was designed to work in unison, generating a smart closed-loop array of control elements that possess wave-number filtering properties for the less critical acoustic modes of the cabin. The control system was tested in the laboratory using a simplified propeller pressure loading distribution. Promising results were obtained, as the closed-loop control system proved to be unconditionally stable and capable of significantly attenuating the fuselage vibration in the transmission path at the critical blade passage frequencies. Moreover, although only one array of control elements was used, interior noise reduction was also observed during the tests, proving the merit of the concept.

A comparison of actuation and sensing techniques for aircraft cabin noise control

A comparison of actuation and sensing techniques for aircraft cabin noise control

Active control of aircraft cabin noise

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.