Colin H. Hansen

Flow resistance information for acoustical design

Abstract The measurement and use of flow resistance information for the calculation of the acoustical properties of porous materials has been examined. It is demonstrated that flow resistance information provides a description of a porous material which is sufficient to characterise its acoustical performance for all common applications. Design charts are presented which facilitate the use of flow resistance information for the three most common applications: (1) the control of the reverberant sound field in an enclosure, (2) the improvement of the transmission loss through pipe wrappings and enclosure walls and (3) the attenuation of sound propagating in ducts. Measured values of flow resistance for fibrous and foamed products available in Australia are presented. Information is also presented for estimating the flow resistance of fibrous products of generally uniform fibre diameter. Alternatively, a means for the measurement of flow resistance is described.

The effect of transfer function estimation errors on the filtered-x LMS algorithm

The filtered-x LMS algorithm, which is commonly implemented in active noise and vibration control systems, requires an estimate of the cancellation path transfer function to maintain algorithm stability. This correspondence considers the effect of errors in this estimate on the stability of the algorithm implemented in the time domain, concluding that while a maximum phase error of /spl plusmn/90/spl deg/ is a bound for stability, there is no simple relationship between error and stability within this region. >

Active control of sound radiation from a vibrating rectangular panel by sound sources and vibration inputs: An experimental comparison

Abstract Active control of sound radiation from a rectangular panel by two different methods has been experimentally studied and compared. In the first method a single control force applied directly to the structure is used with a single error microphone located in the radiated acoustic field. Global attenuation of radiated sound was observed to occur by two main mechanisms. For “on-resonance” excitation, the control force had the effect of increasing the total panel input impedance presented to the noise source, thus reducing all radiated sound. For “off-resonance” excitation, the control force tends not significantly to modify the panel total response amplitude but rather to restructure the relative phases of the modes leading to a more complex vibration pattern and a decrease in radiation efficiency. For acoustic control, the second method, the number of acoustic sources required for global reduction was seen to increase with panel modal order. The mechanism in this case was that the acoustic sources tended to create an inverse pressure distribution at the panel surface and thus “unload” the panel by reducing the panel radiation impedance. In general, control by structural inputs appears more effective than control by acoustic sources for structurally radiated noise.

Active control of sound transmission using structural error sensing

The active minimization of harmonic sound transmission into an arbitrarily shaped enclosure using error signals derived from structural vibration sensors is investigated numerically. It is shown that by considering the dynamics of the coupled system, it is possible to derive a set of “structural radiation” modes which are orthogonal with respect to the global potential energy of the coupled acoustic space and which can be sensed by structural vibration sensors. Minimization of the amplitudes of the “radiation modes” is thus guaranteed to minimize the interior acoustic potential energy. The coupled vibro-acoustic system under investigation is modelled using finite element analysis which allows systems with complex geometries to be investigated rather than limiting the analysis to simple analytically tractable systems. Issues regarding the practical implementation of sensing the orthonormal sets of structural radiation modes are discussed. Specific examples relating to the minimization of the total acoustic...

A study of the response of a simply supported beam to excitation by a piezoelectric actuator

This article presents a dynamic model of the response of a simply sup ported beam to excitation by actuators made using piezoelectric ceramics glued to the beam surface. The dynamic extensional strain on the beam surface is calculated by includ ing the free stress conditions at the piezoelectric actuator boundaries and by considering the dynamic coupling between the actuator and the structure. The results obtained from this analysis are compared with those from past studies, which were based on the assump tion of static coupling between the structure and the actuator. The analysis shows that the strain field (and so the beam displacement) close to the actuator is significantly different from that obtained by using static stress analysis results, and that to obtain an accurate description of the response, the coupled structural and piezoceramic actuator system must be modelled dynamically.

The influence of transducer transfer functions and acoustic time delays on the implementation of the LMS algorithm in active noise control systems

Abstract Forms of the LMS algorithm have been implemented recently in active noise control systems. Parallel to this, many studies of the convergence properties of the LMS algorithm have been conducted in the field of adaptive signal processing. However, these results cannot be applied directly to active noise control systems as the acoustic time delay and electro/acoustic transfer functions, inherent in an an active noise control system, require modification of the results. In this paper, the effect of the time delay and transfer functions on the convergence properties of the LMS algorithm for a single actuator, single error sensor active noise control system is examined. The results obtained provide some insights into efficient implementation of the algorithm.

Active control of far‐field sound radiated by a rectangular panel—A general analysis

In this paper a general analysis is presented for the active control of the far‐field harmonic sound radiated by a rectangular panel that is built into an infinite baffle. In this analysis, the panel vibration may be generated by either airborne sound (incident sound field) or by structure borne vibrations. The far‐field radiated sound is controlled either by acoustical sources or vibration sources. Minimization of both the local sound pressure and the total power output is considered. Analytical results for the particular case involving minimization of the sound pressure at a single point are compared with experimental data. The physical mechanisms involved for different control sources (vibration or acoustic) are demonstrated analytically. For the case of vibration control sources, the panel modal velocity components are adjusted to produce far‐field sound control. This can be done either by decreasing their amplitudes, and/or by changing the temporal phases of the panel modes. However, for acoustic con...

Active control of noise transmission through a panel into a cavity. II: Experimental study

The active control of the noise transmission through a panel into a cavity was investigated experimentally. A sound field generated by external loudspeakers was transmitted into a cavity through a simply supported panel. To control the sound transmission, an electromagnetic driver was used to adjust the panel vibration, so that the sound pressure sensed by a microphone in the cavity was minimized. Distributions of the sound pressure in the cavity and the panel vibration were measured and analyzed under controlled and uncontrolled conditions. Two different control mechanisms were identified and were associated with different types of acoustical mode. For a cavity‐controlled mode, which had its resonance frequency close to an uncoupled cavity mode, the panel response increased for the controlled case even though the sound pressure in the cavity was reduced. However, for a panel‐controlled mode, where the interior sound field was generated by the radiation of a resonant panel mode, the panel response was sup...

Experiments on active control of sound radiation from a panel using a piezoceramic actuator

Abstract The use of a piezoceramic element as an actuator to actively control sound radiation from a thin rectangular panel is experimentally studied. The piezoceramic element is bonded directly to the panel surface and provides control inputs directly to the structure, while the error sensor is a single microphone located in the radiated field. Two cases, corresponding to the resonance of the (1, 1) and (3, 1) panel modes, are considered. The results demonstrate that excellent global attenuation of sound levels of the order of 45 dB is achieved in both cases. Mechanisms of control are discussed.

Total power flow from a vibrating rigid body to a thin panel through multiple elastic mounts

When active control is used to reduce the vibration transmission through machine mounts into supporting structures, the vibratory power flow through the mounts is often used as the cost function to be minimized. This is because the mounts behave as vibration ‘‘sources’’ for the supporting structure and they have a finite number of degrees of freedom. To apply active control effectively to suspension systems, it is necessary to investigate the nature of the power flow from the vibrating machine to its supporting structure through the mounts. In this paper, a multi‐mount suspension system is modeled analytically. The transmission of vibratory power flow from a vibrating rigid body into a thin supported panel through two elastic mounts is evaluated numerically. This power flow consists of the contributions from normal and shear stress forces, and moments. The magnitude of each power flow component is examined and the conditions under which the contribution of each component to the total power flow is signifi...