The close relationship between noise and vibration is reviewed and analyzed for the suppression of noise and vibration in structures. The suppression of noise and vibration by acoustic means are addressed. For the first, an analysis is carried out by representing noise by monopoles and higher harmonics, and to devise a straight-forward method to counter their influence by selective secondary acoustic source. The second problem is analyzed using a methodology developed earlier for the computational scheme for the calculation of the acoustic disturbance to the aeroelasticity of structures. The generic approach of the latter consists of three parts. The first is the formulation of the acoustic wave propagation governed by the Helmholtz equation by using boundary element approach, to allow the calculation of the acoustic pressure on the acoustic-structure boundaries. The structural dynamic problem is formulated using finite elements. The third part involves the calculation of the unsteady aerodynamics loading on the structure using generic unsteady aerodynamics computational method.
TopIntroduction
Noise and vibration occur in many engineering and domestic systems, are closely related and have received considerable attention. In industry, for example, engines, fans, transformers and compressors radiate noise. In cars, boats, trains and aircraft, for example, noise reduces comfort. With the progressive trends of the utilization of lightweight materials and more powerful and high performance systems, increased vibration and noise levels may prevail if these problems are not given due considerations. Structural-acoustic interaction problems have shown enhanced interest in the aerospace and automotive industries. Acoustic excitation of the fuselage has been one of the main concerns during certain flight operations (see Figure 1). Under such conditions, the coupling of the acoustic and the structure systems and determining their combined random responses become extremely important.
Figure 1. Dominant cabin noise due to engine noise; other source of noise in boundary layer noise on the outer surface of the fuselage
Associated with man-machine system interaction, the primary concern with noise in the low frequency range is not only the potential risk of damage to the hearing, since low frequency noise is annoying and during periods of long exposure causes fatigue, discomfort and loss of concentration. Reduced concentration may also lead to an increased risk for accidents. The masking effect which low frequency noise has on speech also reduces speech intelligibility (Johansson, 2000; Lagö, 1996; Lagö, Johansson & Hellström, 1997), which is considered to be disturbing. Active noise control is a significant demand for human comfort and environmental requirements, including by the aviation authorities.
Noise problems may be solved using traditional passive approaches and/or approaches based on the concept of active noise control (Kuo & Morgan, 1996; Juang, 1993; Johansson et al, 1997; Kempton, 1976; Germain, 2000; Johansson, 2000; Pabst et al, 2009; Griffin, 2006; Kusni & Soenarko, 2001). The choice of technique is determined by the characteristics of the noise as well as of the application. Conventional passive approaches consist of absorbers and/or reflectors/barriers. Active noise control can be carried out using secondary noise source (Johansson, 2000; Kusni, Soenarko & Djojodihardjo, 2014). Similarly, for vibration, active vibration control can also be carried out using noise source.