1,721,149 research outputs found
Noise generated by flow over perforated surfaces
No full textThis work deals with the prediction of the noise generated by unsteady flow over the surface of acoustically absorbent duct linings. Ffowcs William's analysis of the acoustics of turbulence near sound absorbent linings is used as a theoretical basis. An equivalent source model is used to describe the radiated sound in terms of the mass flow driven through the circular perforations of a liner surface by hydrodynamic pressure fluctuations. The assumption of a realistic form for the turbulent velocity fluctuations in the liner perforations makes clear the inherent dependence of the radiated noise on the Strouhal number based on hole diameter. The predictions of the equivalent source model are in excellent agreement with measurements made of the noise radiated by liner surfaces in a free jet test rig. The model and the experimental data are further used to predict successfully the level of surface generated noise in attenuator sections of low speed flow ducts.</p
Active control of acoustic fields and the reproduction of sound
No full textIn recent studies of the active control of acoustic fields analytical methods and multi-channel signal processing techniques have been used that can be usefully applied to problems in sound reproduction. In this paper several aspects of the reproduction of sound are considered. First, the possibility is considered of the perfect reproduction of an acoustic field in both space and time. Results from classical acoustics suggest a means by which this could be achieved, but it is soon concluded that this is an unrealistic objective in practice. The reproduction of a sound field over a restricted spatial region is also considered. Some new results are presented which demonstrate that a field can be reproduced that closely approximates the original by first recording the acoustic signals at a finite number of positions in the original sound field. The signals are processed via a matrix of linear filters in order to produce the inputs to a number of sources used for reproduction. An analysis in the frequency domain shows that such a strategy could be useful, but its practicability at high frequencies appears to be limited by the need to provide adequate spatial samping of the original field. Another approach that is considered is to concentrate on ensuring that the direction of propagation of the waves in the original field are well approximated in the reproduced field. This approach appears to be a more practicable alternative, and offers the promise of successful operation over a wide frequency bandwidth. Some discussion is presented of the realizability of the optimal filter matrix and a practical, adaptive, filter design technique is presented which has already provded successful in some limited experiments. Finally, some further possibilities are suggested in which the same principles are used to improve the quality of existing stereophonic sound reproduction systems.</p
Active techniques and their potential for application in aeroacoustics
No full textA brief review is presented of the application of active techniques to problems in acroacoustics. Emphasis is given to the basic physical limitations of active techniques in controlling both noise radiated into the interior of aircraft and to noise radiated by aircraft to the surrounding atmosphere. The possibilities for the control of fan noise generated at harmonics of blade passage frequency are addressed, together with the use of optimally designed in-duct sensor arrays for the prediction of far field radiation from in-duct measurements. A discussion is also presented of techniques for the control of boundary layer flows and the possible consequences for the control of broadband noise generated by flow over lifting surfaces. Finally, a preliminary theoretical analysis is outlined which enables an estimate to be made of the potential for the use of flow control techniques in contributing to the reduction of fan tonal noise, the latter often being associated with inlet flow distortions.</p
Filter update techniques for adaptive virtual acoustic imaging
No full textThis paper deals with filter updates for adaptive virtual acoustic imaging systems using binaural technology and loudspeakers. The problem is to update the inverse filters without creating any audible changes for the listener. The problem can be overcome by using either a very fine mesh for the inverse filters or by using commutation techniques.</p
A model based approach for estimating the strength of acoustic sources within a circular duct
No full textThis paper describes a possible experimental technique for the investigation of the characteristics of acoustic sources within a circular duct. It would be extremely beneficial for noise control design purposes to have information regarding the separate amplitude and phase of each source component which constitutes a complex radiating volume. Unfortunately this is not possible to achieve by measurement of the radiated pressure alone, since this will only yield information about the acoustic field radiated by the whole source volume, which itself is a convolution of the strength of each source element with the response of the surrounding duct. A technique is presented here which uses a model based approach to determine the best estimate of the complex source strength using measurements of the acoustic pressure within the duct. Results are presented which show that the technique has some potential as a measurement technique. As a simple means of examining some important parameters which influence the quality of the source strength estimate, the technique is applied to the case of a point monopole source within a hard walled duct.</p
Algorithm For multichannel Lms adaptive filtering
No full textA modification of the LMS algorithm is presented in which the coefficients of an array of FIR filters, whose outputs are linearly coupled to an array of sensors, are adapted to minimise the mean-square sum of the error signals from these sensors. The application of the algorithm to the control of distributed parameter systems is discussed.</p
Active noise control in finite length ducts
No full textA simple technique for the active control of sound in ducts, initially suggested by Olson and May [1], is investigated in detail. A simple, "virtual earth" principle, feedback loop is used to drive the sound pressure to a minimum at a microphone placed close to a loudspeaker in the duct wall. This produces a reflection of downstream travelling plane waves. A detailed investigation of the loudspeaker near field has enabled the optimum position of the microphone to be identified. The system is shown to be especially effective at the frequencies of the longitudinal duct resonances, where the acoustic response of the duct produces a high loop gain. Results are presented which show a reduction of up to 20 dB in the amplitude of low frequency broadband noise at a position downstream of the cancelling source.</p
Multichannel active sound control using adaptive filtering
No full textThe active control of harmonic sound using multiple secondary sources and multiple error microphones is formulated in a adaptive digital filtering framework. A stochastic gradient algorithm is presented to minimize the sum of the squares of the error microphone outputs by adapting the coefficients of an array of FIR (finite-impulse response) filters driving the secondary sources. The properties and implementation of the algorithm are discussed and some results are presented which demonstrate that the noise at the engine firing frequency inside an automobile can be reduced by 10-15 dB using a practical implementation of such an active sound control system, which had a response time of 0. 1 s in this application.</p
Models for describing active noise control in ducts.
No full textA number of methods were proposed for controlling the sound propagating in a duct by introducing 'secondary' sources to cancel the original sound wave produced by a 'primary source'. The physics of such active noise control (a.n.c.) systems is fairly straightforward and well understood. It is the purpose of this memorandum to examine some of the most widely used models for active noise control in ducts and in so doing to elucidate the assumptions inherent in the models so that each model can be wisely applied. (A)</p
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