The active control of a sound wave in a finite waveguide can require a controller with a long weighting function (or impulse response) if the ANC system is of the feedforward type with an independent reference signal. In a real system that is constrained to act on-line, with FIR models of the transfer functions, the signals are sampled and the weighting function is truncated. This truncation can largely reduce the active attenuation of noise. The optimal truncated weighting function is therefore determined in order to predict the effect of the truncation in dependence on the loss coefficient in the waveguide. It is pointed out that the truncation limits the general level of attenuation and introduces furthermore minima of the attenuation in the frequency range. At these frequencies, the attenuation of an active system is actually limited by two factors: technological conditions that make the truncation necessary and physical conditions that make the evanescent modes largely excited. In this paper, the truncation is the main issue and the constraints from the physical conditions are removed. The presence of loss in the waveguide improves moreover the active control. Analytical, numerical and experimental results are presented for a waveguide with or without absorbent linings. In addition to their passive effect, absorbent linings favor active control by increasing the loss coefficient and by reducing the detrimental effect of the upstream reflective termination.
Document Type: Research Article
Publication date: May 1, 1998
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Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.