Primary Sound Field Research Articles

ACTIVE CONTROL OF A MOVING NOISE SOURCE—EFFECT OF OFF-AXIS SOURCE POSITION

An optimally arranged multiple-channel active-control system is known to be able to create a large quiet zone in free space for a stationary primary noise source. When the primary noise source moves, the active control of the noise becomes much more difficult, as the primary noise field changes with time in space. In this case, the controller of the control system must respond fast enough to compensate for the change; much research has been focused on this issue. In this paper, it is shown that a moving source also causes difficulties from an acoustical perspective. A moving source not only changes continuously the strengths and phases of the sound field in the space, but also changes the wavefront of the primary sound field continuously. It is known that the efficiency of active noise control is determined mainly by the wavefront matching between the primary and control fields. To keep the control system effective in the case of a moving source, the wavefront of the control field needs to change, in order to continuously match the primary-wavefront change. This paper shows that there are limitations to the control-wavefront change. An optimally pre-arranged, multiple-channel control system is not able to construct a matching wavefront when the primary source moves outside a certain range. In other words, the control system is still able to create a large quiet zone only when the primary source moves within a range around the central axis of the control system. Both the location and the size of the quiet zone change with the location of the primary source.

Active control of fan noise from high-bypass ratio aeroengines: a numerical study

AbstractUnder the national research project, dubbed Turbotech II, in which MTU Aero Engines, DLR Institute of Propulsion Technology and EADS Corporate Research Centre participate, active noise control (ANC) has been tested with a scale model fan of one metre diameter for a high bypass ratio aeroengine. MTU’s task in this project was to develop a computer code to predict the sound field in the intake duct of the fan-rig by the use of active control. The primary objective of the numerical study was to specify numbers of actuators (loudspeakers) and error sensors (microphones) and their positioning to control the harmonic sound power, radiated upstream to the duct intake. The computer model is based on the geometry of an annular or circular duct of rigid walls and infinite length, containing a subsonic axial uniform flow. The modal amplitudes of the primary sound field are input data. The actuators are modelled by acoustic monopoles. Two control algorithms have been used for achieving the control objective. The first consists simply in the reduction of the in-duct mean squared pressures. The second, so called modal control, is designed to cancel dominant modes selectively. Numerical results are presented using a typical configuration of wall mounted actuators and error sensors in the form of a number of rings uniformly distributed along the length of the intake duct. Guidelines have also been derived to design a favourable configuration of actuators and sensors. The findings of the numerical study are compared with the results of the ANC tests.

SENSOR CONFIGURATION EFFICIENCY AND ROBUSTNESS AGAINST SPATIAL ERROR IN THE PRIMARY FIELD FOR ACTIVE SOUND CONTROL

The efficiency and the robustness of active noise control against fluctuations depend, amongst other things, on the secondary source and control microphone configurations. Within the context of harmonic sound attenuation, the performance (efficiency and robustness) according to the source configurations has recently been dealt with. The present study is focused on the performance according to microphone, or sensor, configurations. It is demonstrated that all sensor configurations may become equally efficient due to the fact that they may be made to control equally well a domain defined by a certain number of observation points. The paper also gives a definition of robustness starting from the result of previous work on source configurations. Robustness is understood here in relation to the spatial fluctuations of the primary sound field. Given this definition, indications are provided for choosing the most robust sensor configuration or how to improve the robustness of any one of the sensor configurations.

Active Control of Low-Frequency Sound Radiation From Vibrating Panel Using Planar Sound Sources

Active control of low frequency sound radiation using planar secondary sources is theoretically investigated in this paper. The primary sound field originates from a vibrating panel and the planar sources are modeled as simply supported rectangular panels in an infinite baffle. The sound power of the primary and secondary panels are calculated using a near field approach, and then a series of formulas are derived to obtain the optimum reduction in sound power based on minimization of the total radiate sound power. Finally, active reduction for a number of secondary panel arrangements is examined and it is concluded that when the modal distribution of the secondary panel does not coincide with that of the primary panel, one secondary panel is sufficient. Otherwise four secondary panels can guarantee considerable reduction in sound power over entire frequency range of interest.

Control performance and robustness of an active noise control system for uncertain primary sound fields

In this work, the control performance and robustness of an active noise control system subjected to uncertain primary sound fields are investigated. For this purpose, the performance index, residual potential energy in a desired quiet zone, is derived as a function of sound field variables, quiet zone variables, and control system variables. In the presence of uncertainty, typical measures of the robustness and performance of a control system, maximum, minimum, mean, and variance of the performance index are derived theoretically. In addition, based on the least-squares orthogonality principle, the condition for implementing the best-oriented control system, which is robust and can maximize the control performance by using a given number of control sources and sensors, is investigated. Finally, numerical simulations are carried out to illustrate and verify the proposed theory.

A modal concept for active noise control in circular or annular ducts

A new active noise control strategy is presented which is aimed at reducing circumferential as well as radial acoustic modes in circular and/or annular ducts as used, for example, in aircraft engines. The azimuthal mode structure of the duct sound field is measured by using circumferentially spaced sensors in several axial planes which are mounted flush with the inner duct wall. No attempt is made to control the entire primary sound field, but only the dominant azimuthal modes which are propagational in the duct. With this procedure, the number of control variables is greatly reduced. The radial modes are reduced in an indirect way, i.e., the exact radial mode structure of the primary sound field in the duct need not be known for this purpose. a)Now with LUK GmbH & Co., Industriestrasse 3, 77815 Buehl, Germany.

ELECTROPNEUMATIC TRANSDUCERS AS SECONDARY ACTUATORS FOR ACTIVE NOISE CONTROL PART III: EXPERIMENTAL CONTROL IN DUCTS WITH THE SUBSONIC SOURCE

Abstract The possibility of performing active control of periodic noise propagating in ducts using a subsonic electropneumatic acoustic generator as secondary source is investigated. The subsonic generator has been studied both theoretically and experimentally in two companion papers, and this sound source was shown to be highly efficient, but non-linear. The non-linear behaviour of the source decreases as the acoustic pressure at its output is reduced, however, as in the case when the source is used as a secondary actuator in an efficient active control system and thus the source is well suited to such applications. Residual non-linearities of the subsonic source are shown to be due to the mechanical design of the actuator. An harmonic controller is discussed which accounts for the residual non-linear behaviour of the subsonic source. Experiments carried out with a manual version of this controller, that controls the first five harmonic components of the signal driving the subsonic source, reveal that it is efficient in controlling periodic primary sound fields. The implementation of a fully coupled harmonic controller is, however, shown to require large processing capacities. A theoretical analysis of a simplified version of the harmonic controller—the decentralized harmonic controller—is carried out, and simple conditions are established under which the decentralized controller offers similar performances to the fully coupled harmonic controller. These conditions are shown to be satisfied when using the subsonic source as a secondary actuator. The non-linear behaviour of the subsonic source can also cause a further problem, since the error surface experienced by the control system may exhibit local minima. It was found that the likelihood of this happening was much reduced is only the fundamental component of the harmonic controller was adapted initially, and then the other harmonics were changed in a second phase of adaptation. The implementation of a decentralized harmonic controller is considered, using a dual channel signal processing board. A purely linear model for the plant under control is shown to be accurate enough for modelling the system under control and to ensure convergence of the controller. Experiments with the automatic controller reveal that attenuations measured at the monitor microphone are around 25 dB, for sinusoidal primary sound fields.

Generation of zones of quiet using a virtual microphone arrangement

A local active noise control system is described which uses a virtual microphone arrangement. This arrangement is based on the assumption that the primary pressure at the physical and at the virtual microphone locations are similar. The implication of this assumption on the acoustic performance of a local system in a diffracted primary sound field is theoretically studied. The results show that the error at the virtual microphone position is lower when the virtual microphone arrangement is in the vicinity of a diffracting surface. A practical local active noise control system in a headrest has been built and used to measure the zone of quiet produced by a single and a dual channel system when the total pressure is canceled at one or two virtual microphone positions. It is shown that this type of arrangement is capable of projecting the zones of quiet further away from the secondary source than the position of the physical microphone. The measured zones of quiet produced by a single-channel system have been compared with the results produced with a theoretical model which includes the diffraction between two spheres, one representing the secondary source and the other the listener’s head. It has been found that, in the frequency range of applicability of a local active noise control system, the two-sphere model predicts the experimental results well. The effect that the diffracting head has on the performance of an adaptive feedforward controller using a physical and a virtual microphone is also explored and the results show that this effect is small provided the initial system identification is performed in the presence of the head.

OPTIMIZATION OF AN ACTIVE NOISE CONTROL SYSTEM USING SPHERICAL HARMONICS EXPANSION OF THE PRIMARY FIELD

The design of an active noise control system involves several steps. One of the most important is to find the best locations of the control sources and error sensors according to the primary source distribution and its spectrum. Thus the first step in this optimization problem is to obtain an analytical expression for the primary sound field. In this work this was done by using a spherical harmonics expansion, determined on the basis of measurements of the primary field. It is shown how, by using such an expansion, a method can be developed for optimizing the transducer locations for the case of free field radiation of a period general primary source. Finally it is shown how this measurement database can be used to choose a good compromise between the number and the locations of the error sensors.

A genetic algorithm for active noise control actuator positioning

Abstract Active noise control systems (ANCS) have been used extensively in several diverse applications in order to reduce the sound levels within a confined space. Several control schemes have been proposed, all of which rely on the same basic idea, namely that of introducing an artificial secondary sound field in order to partially cancel the primary sound field. This secondary field is created by using a number of actuators (usually speakers), placed in several locations within the space of interest. In our particular case of interest, the ANCS under consideration is an interior one. The overall ANCS comprises a set of sensors, a control system, and the actuators. Clearly, the optimal design of all three components needs to be addressed when designing an ANCS. In this paper, the problem of deciding to which specific locations (out of several possible ones) the actuators will be located for optimal performance of the ANCS is addressed. Specifically, a genetic algorithm is designed for this problem. The algorithm was implemented and tested against a simple greedy approach. Results indicate the efficiency of the Genetic Algorithm for the problem at hand.

Boundary-layer microphone

First Page

Near Field Zones of Quiet

This paper examines the consequences of driving a single secondary loudspeaker to cancel the pressure due to some primary source at a point in its near field. This simple technique has been applied to the sound field in a highly reverberant room to produce zones of quiet in the vicinity of the loudspeaker, which have diameters that are typically equal to one-tenth of the acoustic wavelength, within which the sound pressure level is attenuated by at least 10 dB. The principal advantage gained with this strategy over other active techniques for controlling the sound field in rooms is that the sound pressure level well away from the control point is largely unaffected, an increase of only a small fraction of one dB being typical. Such a loudspeaker-microphone configuration could be located, for example, in the head rests of cars or aeroplanes, or indeed anywhere where the listener is seated for significant lengths of time and subjected to high ambient noise levels such that auditory comfort may be disturbed. Measurements are presented of the near field quiet zone in a reverberant room at frequencies well above the Schroeder frequency. These experimental findings, which represent a space averaged result over source position, indicate good agreement with the simple theory developed in this paper. It is demonstrated theoretically that the diameter of the quiet zone formed at any arbitrary point and direction in the near field of the loudspeaker is numerically proportional to the specific acoustic impedance at that point and direction. In general terms, the 10 dB quiet zone is observed to increase as the diameter of the secondary loudspeaker increases and the microphone is moved increasingly further from the loudspeaker. Close to the loudspeaker the pressure due to the secondary source is predominantly governed by the directly radiated near field and so the quiet zone formed in this region is therefore insensitive to the nature of the primary sound field. This important feature makes it highly suitable for producing reductions in the sound pressure level within enclosed spaces at frequencies where the modal density is high such that global control strategies are rendered ineffective.

Numerical studies of actively generated quiet zones

Abstract A local active control system has been investigated, in which the pressure at a point in front of a piston secondary source is driven to zero. The spatial extent of the zone of quiet created around the point of cancellation has been determined numerically, for both a uniform and diffuse primary sound field. The 10 dB zone of quiet generally becomes larger the further the cancellation point is from the secondary source, up to a limit of about one tenth of an acoustic wavelength. At low frequencies, the zone of quiet forms a shell round the secondary source, although this is less pronounced for diffuse primary fields. This finding may be used to reduce the pressure in front of the piston, at low frequencies, by cancelling the pressure in the plane of the piston.

The active minimization of harmonic enclosed sound fields, part II: A computer simulation

This paper is Part II in a series of three papers on the active minimization of harmonic enclosed sound fields. In Part I it was shown that in order to achieve appreciable reductions in the total time averaged acoustic potential energy, Ep, in an enclosed sound field of high modal density then the primary and secondary sources must be separated by less than one half wavelength, even when a relatively large number of secondary sources are used. In this report the same theoretical basis is used to investigate the application of active control to sound fields of low modal density. By the use of a computer model of a shallow rectangular enclosure it is demonstrated that whilst the reductions in Ep which can be achieved are still critically dependent on the source locations, the criteria governing the levels of reduction are somewhat different. In particular it is shown that for a lightly damped sound field of low modal density substantial reductions in Ep can be achieved by using a single secondary source placed greater than half a wavelength from the primary source, provided that the source is placed at a maximum of the primary sound field. The problems of applying this idealized form of active noise control are then discussed, and a more practical method is presented. This involves the sampling of the sound field at a number of discrete sensor locations, and then minimizing the sum of the squared pressures at these locations. Again by use of the computer model of a shallow rectangular enclosure, the effects of the number of sensors and of the locations of these sensors are investigated. It is demonstrated that when a single mode dominates the response near optimal reductions in Ep can be achieved by minimizing the pressure at a single sensor, provided the sensor is at a maximum of the primary sound field. When two or three modes dominate the response it is found that if only a limited number of sensors are available then minimizing the sum of the squared pressures in the corners of the enclosure gives the best reductions in Ep. The reasons for this behaviour are discussed.

Active acoustic absorption—Where does the energy go?

Active attenuation of noise in three dimensions around a complex source is an accomplished fact [M. J. M. Jessel and O. L. Angevine, in Inter‐Noise 80 Proceedings, pp. 689–694]. But this author continues to receive questions regarding the theory such as “Where does the energy go?” When cancellation sources are added, the existence of a primary sound field changes the acoustic impedance seen by these cancellation sources, as pointed out by Levine [R. C. Levine, Am. J. Phys. 48, 22–31 (1980)]. The use of cancellation sources having a cardioid radiation pattern makes it possible to avoid influencing the acoustic impedance seen by the primary source, however, thus the secondary sources become acoustic absorbers. [M. J. M. Jessel, Acoust. Lett. 4(9) (1981)]. Further consequences of this approach are discussed.

Experiments with cavitating parametric sources

A cavitating parametric source utilizes a field of collapsing cavitation bubbles created by an intense two‐frequency primary sound field. Since the cavitation zone is modulated at the difference frequency, this frequency (and its harmonics) is generated. The beam pattern is determined by the shape of the cavitation zone while the source level depends on the size of the zone. Experimentation with a number of sources utilizing face cavitation and remote cavitation has resulted in a better understanding of the performances of these devices. Cavitation thresholds for pure‐tone and dual‐frequency primary excitation, have been found to be nearly identical. [Sponsored by Naval Material Command.]

The Active Control of Higher-Order Modes in Cylindrical Ducts

Higher-order modes in a cylindrical duct were generated by an planar array of electro-mechanical sound drivers (Spinning Mode Synthesizer) by appropriate phase and amplitude control of the array. Predictions of the sound field generated show excellent agreement with measurements. Active control of the sound field was achieved by the introduction of a properly phased second coplanar array of sound drivers resulting in substantial reduction in the primary sound field. The analytical models agree well with experiment and furthermore clearly delineate the limitation on the complexity of the higher-order modes that can be controlled as a function of the number of drivers in the arrays and the driver frequency. Although substantial reduction of the primary modes was achieved, both analysis and experiment show that secondary higher-order modes are generated, again, depending on the number of drivers in the array and the frequency of excitation.

Difference Frequency Parametric Array Using an Exact Description of the Primary Sound Field

In parametric arrays, the nonlinear interaction between two, harmonic time-dependent waves creates sum and difference frequency radiations. A solution for the pressure field of the secondary radiations has been obtained numerically for interaction in the farfield of a circular piston source [Papers CC4, 80th meeting, and FF7, 81st meeting of the Acoust. Soc. Amer.]. In the present analysis, this solution is extended to include interaction in the nearfield as well as the farfield of the circular piston. A single intergral expression, which describes the piston nearfield exactly, is derived from the impulse response of a circular piston. The results are compared to experiments involving the amplitude and azimuthal response of a difference frequency radiation. The theoretical results are also compared to other models using (1) plane-wave and (2) spherical-wave approximations of the primary sound field. [This work was sponsored by the U. S. Navy Office of Naval Research.]

An Approximate Model for Parametric Acoustic Source Design

A parametric acoustic source is comprised of a high-amplitude radiated sound field of two frequencies. Because of fluid nonlinearities, this primary field generates a secondary sound field at the difference frequency. Using a simplified mode[ for the primary sound field of a piston transducer, the secondary source levels and directivities generated by such a parametric source have been predicted. The extent of the primary field is assumed to be limited by linear and nonlinear absorption, and this limit is considered to occur in either the collimated-beam or the spherically spreading portions of the primary field. Numerical values have been computed for a variety of conditions and compare favorably with the experimental data obtained to date.