ACTIVE CONTROL OF NOISE AND VIBRATION PDF
Active isolators cancel the incoming vibration by generating a dynamic force of The objective is to improve the insulation of vibration in the range 10 to After some general comments and assessments with respect to the current status of active noise and vibration control measures in practice, the basic structure of. regulation of noise and vibration, however it is not intended to be a Active noise control, the latest hi-tech noise control technique, is described separately.
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Request PDF | On Jan 1, , C. Hansen and others published Active Control of Noise and Vibration. Since the publication of the first edition, considerable progress has been made in the development and application of active noise control. In a narrow sense, coherent active control of sound and vibration is the 2 Active noise control (ANC) Early investigations The first experiments on the.
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Omer Khalid. Festschrift DPI, 1—32 Herausgeber ed. In a narrow sense, coherent active control of sound and vibration is the cancel- lation or less often enhancement by superposition of an antiphase or in-phase additional signal, usually from an external source of sound or vibration.
The historical development of the technologies are outlined, the fundamentals under aspects of physics, signal processing and algorithms are treated, and the current states of research and applications are reviewed, more or less systematically sorted.
Related fields such as adaptive optics, active flow control, and control of nonlinear dynamic systems are also included. Active control of sound and vibration in a wider sense, the incoherent superposition, aimed at sound power enhancement etc. In acoustics, most of the early publications in this field are patent applications, showing that technical applications have been considered possible.
However, for a long time experiments were nothing more than laboratory demonstrations which were smiled at as curiosities, far from reality. Only modern electronics made technical applications feasible. The situation was different with the compensation of low-frequency mechanical vibrations; these techniques were used in practise at a very early stage.
In the fol- lowing, an overview is given of the historical development, technical realisations, and present research activities. Lueg already proposed the usage of electroacoustic components, but the first labo- ratory experiments were documented by Olson  and , who also listed far-sighted prospective applications.
Technical applications were not possible at that time because of the clumsy electronic vacuum tube equipment, lacking sufficient ver- satility. Also, our ears present a problem, namely the nearly logarithmic dependence of the perceived loudness on the sound pressure.
For example, a sound level reduc- tion by 20 dB requires an amplitude precision of the compensation signal within 1 dB and a phase precision within 6 degrees of the nominal values — for all frequency components of the noise signal. These demands, together with the requirement of temporal stability, have impeded for a long time the technical use of coherent-active compensation systems also termed anti-sound until in recent years digital adaptive filters proved to be the appropriate tool.
The objection is correct if the cancellation is achieved by interference only; a local cancellation leads to doubling of the sound pressure elsewhere.
But a more detailed consideration reveals that the secondary sources can, properly placed and driven, absorb the primary energy. In other situations, the sources interact such that the radiation impedance is influenced and thereby the sound production reduced.
This will be elucidated in the following sections. Jessel and his coworkers G. Mangiante and G.
They have treated the problem sketched in Fig. Sound sources Q are located within a volume V with surface S. Along S, secondary sources shall be arranged such that they compensate the sound field radiated to the outside, but do not alter the field within V. With reversed poling, they produce a field which is in antiphase to the original one.
Assuming that such reversed and acoustically transparent substitute sources operate together with Q, the sound fields in the outside cancel each other. The inward radiation can be prevented by combining monopoles q0 along S with dipoles q1 so that the primary field in V is not altered.
As to the energy, the tripoles formed by the q0 and q1 directional radiators with cardioid characteristic absorb, along S, the sound coming from Q. They serve as perfectly matched absorbers with an acoustic input impedance equal to the characteristic impedance of the medium. With the same argument, it follows that a source-free region V can be shielded actively against sound influx from the outside by arranging appropriate compensation sources along the surface S of V.
Monopole distributions along S reflect, tripoles absorb the incident sound.
Active Control of Noise and Vibration | Taylor & Francis Group
For a given surface S and primary source distribution Q r , where r is the position vector, the substitution sources q0 r and q1 r can be calculated from the Helmholtz- Huygens integral equation which links the sound field in a region to the sound pressure and its gradient along the surface . For practical applications, the theoretically required continuous source distribution has to be replaced by discrete sources.
This concept has been verified in computer simulations  and experimentally in an anechoic room . A practical application is noise shielding of large open-air power transformers by an array of loudspeakers to save the people living in the surroundings from the annoying hum .
It was also reported that cattle grazing near a large power transformer gave less milk.
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A few researchers are further developing the JMC theory [15, 16]. It is therefore easier to cancel plane, guided waves in ducts below the onset frequency of the first lateral mode than, for example, three-dimensional sound fields in rooms with omnidirectional propagation.
In a set-up as sketched in Fig. Guicking principle already been proposed by P. Lueg  in the sound incident from the left is picked up by the microphone and, after some processing, fed to the loudspeaker such that to the right side the primary and the additional signal cancel each other. Principle of active feedforward cancellation of sound in a duct.
However, an active noise control system is not practically applicable in this simple form. First, the acoustic feedback from the loudspeaker to the microphone has to be avoided and, second, in most cases it is necessary to follow up the transfer function adaptively since the time delay and the sound spectrum can change as a result of temperature drift, superimposed flow, and other environmental conditions.
It is therefore common practise today to apply adaptive digital filters which are implemented on fast signal processors to enable online updating.
ANC in a duct by adaptive feedforward control with feedback cancellation and error path identification for the filtered-x LMS algorithm. Active Control of Sound and Vibration 5 The transfer function of the acoustic feedback path from the loudspeaker L to the reference microphone R is modeled by the feedback compensation filter F CF so that the input signal x t to the main filter A does not contain contributions from L.
The error microphone E receives, in the case of incomplete cancellation, an error signal e t which serves for the adaptation of the filter A. This filter adapts such that it models the acoustic transfer function from R to L, including the complex frequency responses of R and L.
The algorithm is controlled by the product e t x t and adjusts the fil- ter coefficients by a stochastic gradient method so that x t and e t are decorrelated as far as possible. If the primary sound is broadband, the propagation delay from L to E decorrelates x t and e t to a certain degree which impairs the performance of the ANC system.
The necessary error path identification is performed with an auxiliary broadband signal of the noise generator N G in the adaptation unit shown at the lower right of Fig. After adaptation, the loudspeaker acts as a sound-soft reflector for the wave in- cident from the left which is, hence, not absorbed but reflected to the left. The reason is that it is not possible to achieve perfect impedance matching with a single loudspeaker mounted at the duct wall. But a loudspeaker at the end of a duct can be driven to perfectly absorb the incident sound .
If the standing waves or the stronger sound propagation to the left in arrangements as those in Figs. A series of commercial ANC systems working on the principle of sound-soft reflec- tion have been developed by the US company Digisonix and successfully installed mainly in industrial exhaust stacks since . The signal processors allow on-line operation at least up to Hz, suppress tonal noise by up to 40 dB and broadband noise typically by 15 dB.
Similar systems have been installed also in Germany [23, 24, 25] and elsewhere . The lower frequency limit is given by pressure fluctuations of the turbulent flow, the upper limit by the computational speed of the signal processor and the lateral dimensions of the duct.
The higher modes occurring at higher frequencies can also be cancelled, requiring, however, a greater amount of hardware ; therefore, only few such systems with multi-mode cancellation have been installed so far. Guicking cessing power requirement the numerical complexity is O 2N if N is the filter length , but its convergence is very slow for spectrally coloured random noise.
Fan noise spectra have typically a steep roll-off with increasing frequency so that the convergence behaviour of the algorithm is often insufficient. Efforts have therefore been made to develop algorithms the convergence behaviour of which is independent of the signal statistics, but which can still be updated in real time. Since it furthermore calculates the optimal filter coefficients in one single cycle, it is particularly useful for nonstationary signals and nonstationary transfer functions.
Stability problems in the initialisation period could be solved by the FASPIS configuration which stands for fast adaptive secondary path integration scheme [29, 30]. More on algorithms can be found in the books  and . Schirmacher . Overviews are presented, e. An important concept in many fields of ANC is adaptive noise cancelling which became widely known since by B.
Widrow et al. Then, sr is adaptively filtered and subtracted from sp to obtain a signal estimate with improved signal-to-noise ratio SNR since the adaptive filter decorrelates the output and sr. This concept, realised by a linear predictive filter employing the least mean squares LMS algorithm, has been patented  and found wide applications: The interference by the overwhelmingly stronger signals from terrestric radio transmitters shall be eliminated adaptively .
In adaptive feedforward control systems as shown in Fig. The limiting factor is usually not the computation time in the signal processor but the group delay in the antialiasing lowpass filters which are necessary in digital signal processing. Problems in technical ANC applications are often posed by the loudspeakers.
Very high low-frequency noise levels are typically encountered in exhaust stacks or pipes, demanding for high membrane excursions without nonlinear distortion and, often, robustness against aggressive gases and high temperatures.
On the other hand, a smooth frequency response function as for Hi-Fi boxes is not an issue because fre- quency irregularities can be accounted for by the adaptive filter. Special loudspeakers for ANC systems have been developed —.
Adaptive noise cancelling. However, if it is possible to reduce the primary sound production by the operation of the secondary sources, this will be a particularly effective method of noise reduction. The product v 2 Rs determines the ra- diated active power.
The particle velocity v of the primary source depends on its source impedance and the radiation resistance. These relationships can be utilised, e. Guicking haust pipes ships, industrial plants, automobiles with internal combustion engines. A large demonstration project was implemented as early as Each loudspeaker was fed from one microphone pair through amplifiers with fixed gain and phase settings.
Such a simple open-loop control was sufficient in this case due to the highly stationary noise and its narrow frequency band.
Furthermore, active mufflers have to compete with the highly efficient and comparatively cheap conventional mufflers from sheet metal. Researchers in the muffler industry are, however, still developing and improving active systems, testing prototypes, and they are optimistic that active mufflers might go into production because they combine noise cancellation with backpressure reduction; perspectives to include sound quality design are seen, too .
It assumes quasi periodic noise, the source of which is accessible for ob- taining synchronisation pulses e. The principle is explained in Fig. A loudspeaker is mounted next to the exhaust pipe end, and is fed from a exhaust noise error microphone 1 0 0 1 exhaust pipe 0 1 0 1 engine 0 1 compensation sound waveform synchronisation pulse synthesiser loudspeaker Figure 5.
Active cancellation of quasi periodic noise by tracking control with sync input and waveform synthesis after . An error microphone is placed in the superposition zone and yields a control signal by which the loudspeaker output is optimised. The sync pulses obtained, e. The waveform is adapted using a trial-and-error strategy either in the time domain or, faster, in the frequency domain.
The prominent feature of this active system is that no microphone is required to receive the primary noise because the signal processor performs the waveform synthesis by itself. The loudspeaker must only provide the necessary acoustic power; resonances, nonlinearities and ageing are automatically accounted for.
An example for a technical application of ANC with waveform synthesis in medicine is a noise canceller for patients undergoing a magnetic resonance imaging MRI inspection. Because no ferromagnetics and preferably no metal at all must be brought into the MRI tube, pneumatic headsets with long plastic tubes as sound guides have been developed for this purpose which are fed from a signal processor with a simple feedforward control and fixed filters .
Since, however, the compensation is not very good, an improvement with a metal-free optical microphone for controlling an adaptive filter has been developed [55, 56, 57].
A different approach is aimed at controlling the structural vibrations of the MRI tube walls [58, 59]. The sound pressure is then spatially almost constant, and the cancelling source can be placed anywhere in the enclosure.
Correctly fed, it acts as an active absorber. One such small enclosure is the space between a headphone and the ear drum. Independent developments by the US company Bose  and Sennheiser in Germany  resulted in active headsets for aircraft pilots; active headsets are mean- while also produced by other companies, being offered as pure hearing protectors in open or closed construction, with feedforward and feedback control in analog elec- tronics, and also with a signal input for telecommunication.
The initially very costly active headsets have become so much cheaper that a wider application in vehicles and noisy working places appears realistic. Quite recently, also adaptive digital signal processing is being applied to active headsets and hearing protectors . Guicking for acoustic laboratory experiments, such as head-related stereophony when dummy head recordings are reproduced by two loudspeakers . As the sound radiated from the left loudspeaker should be received by the left ear only, a compensation signal is superimposed onto the right channel which compensates the sound coming from the left loudspeaker to the right ear, and vice versa, see Fig.
As compared to the familiar source localisation between the loudspeakers of a conventional stereo set, this procedure provides true three-dimensional sound field reproduction with source localisation in any direction, including elevation, and also gives a reliable depth impression.
Crosstalk cancellation in head-related stereophonic sound field reproduction with two loudspeakers by prefiltering. Of great practical relevance is local active sound field cancellation for telecon- ferencing and hands-free telephones speakerphones in order to eliminate, at the microphone location, acoustic room echoes which degrade the speech quality and tend to cause howling by self-excitation; the active system causes dereverberation of the room response [65, 66].
Echo cancellation and a speech enhancement system for in-car communication has been described in . Echo cancellation for stereophonic sound field reproduction is more involved than single channel applications. Solutions are presented, e. The psychoacoustic aspect of masking has been introduced in acoustic echo cancellation combined with perceptual noise reduction .
For echo cancellation in fast changing environments, a special algorithm has been developed . A hot topic in speech transmission with multiple not precisely known sound sources is blind source separation, using microphone arrays and algorithms such as spatial gradient estimation, independent component analysis ICA , statistical source dis- crimination, maximum likelihood, and Kalman filters;  presents a comprehensive survey.
A related older problem is the removal of electric line echoes in long-distance telephony with satellite communication links where the long transmission path leads to audible echoes which greatly disturb speech communication . The signals are reflected from an impedance mismatch at the so-called hybrid where the two-wire line branches into the four-wire local subscriber cable. All satellite telephone links are therefore equipped with transmission line echo compensators see, e.
Olson  as early as ; they absorb low-frequency sound in a narrow space around the microphone and have been proposed for aircraft passengers and machine workers .
Because of the very restricted spatial field of efficiency, such systems did not receive general attention. In more recent experiments the test persons disliked also the strong sound level fluctuations when they moved their head. The application of acoustic echo cancellation has also been proposed for ultrasonic testing where flaw echoes can be masked by strong surface echoes.
It is possible to subtract the latter from the received signal and so improve the detectability of flaws [76, 77].
Similarly, the ANC technique can be applied to cancel the reflection of the ultrasonic echo from the receiver . More realistic is the concept of reducing room reverbera- tion by placing active absorbers along the walls.
The incident sound is picked up by microphones which feed the loudspeakers so that their acoustic input impedance is matched to the sound field.
The situation is the same as in Fig. The loudspeakers can also be driven such that their reflectivity takes arbitrary values in a wide frequency range experimentally, reflection coefficients between 0. This would facilitate the construction of a room with adjustable reverberation time , but at present still with a prohibitive amount of hardware. Intensive research has been devoted to the active cancellation of sound in small enclosures such as vehicle, aircraft and helicopter cabins.
Since this noise is strongly synchronised with the engine speed its active cancellation is possible with a relatively small amount of hardware and software . It has, however, only been offered in a production car for some time by Nissan for their model Bluebird in Japan.
Many other car manufacturers develop their own systems, and some of them have success- fully built prototypes, but all of them are hesitating for several reasons to install the ANC systems in series production e. One argument is that customers would complain if they pay for noise reduction, and there still remains some disturbing noise.
Laboratory experiments and driving tests have led to preliminary solutions; the nonstationarity of the noise input and of the acous- tic transfer functions demand for fast adapting algorithms, also for the error path identification [88, 89, 29, 33]. The noise and vibration problems are becoming more severe with small low-consumption cars now under development; they will possibly be equipped with both active noise control for the interior space and active vibration control for the engine and wheel suspensions.
For economical reasons, the aircraft industry has replaced jet engines by propeller or turboprop aircraft for short and medium distances which are, however, much louder in the cabin.
Relatively little effort is necessary to employ a technology known as synchrophasing. The eddy strings separating from the propeller blade tips hit the fuselage and excite flexural vibrations of the hull which radiate sound into the cabin.
Better results, however with more involved installations, are obtained with multichannel adaptive systems. An important issue in ANC applications to three-dimensional sound fields is the placement of microphones and loudspeakers.
Attention has to be paid not only to causality, but also to observability and controllability, in particular in rooms with distinct resonances and standing waves modal control.
If, for some frequency, the error microphone of an adaptive system is positioned in a sound pressure node, it does not receive the respective frequency component or room mode so that no cancelling signal will be generated and no adaptation is possible. If the loudspeaker is placed in a node, then a compensation signal calculated by the processor cannot effectively be radiated into the room, which usually forces the adaptive processor to produce higher and higher signal amplitudes, finally leading to an overload error of the digital electronics.
The problems with active mufflers for cars with internal combustion engines have been discussed in Section 2. A technically similar problem is the fly-over noise of propeller aircraft which mainly consists of two components: If the exhaust tail pipe is shifted to a position near to the propeller plane, and if the angular position of the propeller on its shaft is adjusted so that in downward direction the pressure nodes of one source coincide with the antinodes of the other one, then the destructive interference reduces the fly-over noise by several dB .
Active Control of Sound and Vibration 13 A method for reducing traffic noise by cancelling the tyre vibrations of an auto- mobile is disclosed in a patent , proposing electromagnetic actuation of the steel reinforcement embedded into the tires.
A frequently investigated problem is the cancellation of power transformer noise, the annoying hum of which consists of multiples of the power line frequency 50 Hz, in USA 60 Hz. It is a seemingly simple problem because of the strong periodicity and the readily accessible reference signal. Several methods have been proposed, either by loudspeakers arranged around the site , by force input to the oil in which the transformer is immersed  or to the surrounding tank walls , or by sound insulating active panels enclosing the transformer .
Experimental results are discussed in . Problems are posed, however, first, by the weather-dependent sound propagation — wind and temperature gradients tilt the wave front —and second, because the hum spectrum depends on the electrical load of the transformer .
It has also been tried to actively improve sound shielding noise barriers along roads, in particular to cancel the low frequency noise diffracted around the barrier top. The idea is to place loudspeakers along the upper edge and to drive them with adaptive feedforward control, the reference microphones being placed on the roadside and the error microphones in the shadow zone .
Improvements are concerned with multiple loudspeaker arrays also along the side walls of the noise barrier , or multiple reference control and virtual error microphones . The latter technology is still practised today. Active damping of aircraft skin vibrations has been proposed by , providing multichannel feedback control with displacement sensors and electromagnetic actua- tors, mainly in order to prevent fatigue damage.
Early publications can also be found on the active control of vibrations in beams, plates and composite structures. In mechanical wave filters where a desired longitu- dinal wave mode in a bar is superimposed by an interfering detrimental flexural wave mode, the latter can be damped by pairs of piezoelectric patches on either side of the bar which are connected through an electrical resistor . In special environments, e. In the s, longitudinal vibrations of the ship superstructure caused by nonuni- form propulsion have been reduced with a type of dynamic absorber, realised by a centrifugal pendulum.
This is a pendulum swinging along the length direction of the ship and rotating about an axis pointing also in lengthwise direction. Other than the above-mentioned whole-body vibrations of ships which are comparatively easy to control due to their very low frequencies, one is here confronted with elastic structures, i. First, there are the different wave types in solids of which longitudinal, torsional and transversal waves are the most important , their control demands various types of actuators and sensors.
Furthermore, the propagation speed is generally higher in solids than in air so that causality problems occur with broadband adaptive feedfor- ward controllers. As a consequence, many problems are treated with modal control where, especially in case of overlapping modes, the control spillover problem has to be considered: In Fig. Owing to the phase slope around a resonance, the neighbouring modes are usually enhanced rather than damped when the N th mode is suppressed.
While control spillover leaves the system stable, observation spillover can produce instability . Active Control of Sound and Vibration 15 For satellites, the damping of modal vibrations is important after pointing position manoeuvres etc.
The optimisation of number and placement of sensors and actuators for the mostly applied adaptive feedback controllers requires precise knowledge of the structural dynamics so that reliable modelling in state-space coordinates and a realistic estimation of discretisation errors are possible.
An introduction to this field is given by Meirovitch . Damping and stiffness control in mechanical junctions can also be achieved by dry friction control where the pressing force is controlled by a piezoelectric actuator, in feedforward or feedback control, typically by a nonlinear algorithm, e.
In aircraft technology, active controllers have been developed for manoeuvre  and gust load alleviation , as well as for wing flutter control , and for noise and vibration reduction in helicopters , in particular by individual blade control IBC  and higher harmonic control HHC .
Initially, technical problems were encountered, among others, by the fact that sen- sor and actuator materials such as piezoceramics, piezopolymers, electro- and mag- netostrictive materials, shape memory alloys, electro- and magnetorheological fluids are no constructional materials with a mechanical strength sufficient for load-bearing structures; some of them are also too brittle or too weak for fail-safe operation.
This led to a new research field since the end of the s: Active vibration control has found a popular application in digital cameras with image stabilisation.
The image blur by camera shake during the exposure is avoided by actively shifting the position of the CCD chip with a piezo-actuator, in response to a motion sensor signal . The predominant sources of interior noise are engine and wheel vibrations which propagate as structure-borne sound through the car body and finally radiate as air- borne sound into the cabin.
It is therefore reasonable to develop active engine mounts and active shock absorbers which are stiff enough to carry the static load, but dy- namically resilient so that vibrations are not transmitted.
Guicking forces at frequencies of a few Hertz, hydraulic and pneumatic actuators are available . Compact and robust combinations of conventional rubber mounts with elec- trodynamically driven hydraulics have been constructed as active hydromounts for a wide frequency range . In helicopter cabins, the principal noise source is the gear box, the vibrations of which are transmitted through typically 7 struts to the cabin roof as structure-borne sound , and then radiated into the cabin as airborne sound.
Particularly annoying are tonal components between Hz and 4 kHz. The vibration transmission has been reduced by piezoelectric actuators at the struts so that the noise level in the cabin became much lower, as was verified in ground tests.
The development towards a technical product is a current research topic . Active control technology has been applied for improved vibration isolation of ta- bles for optical experiments, scanning microscopes, vibration sensitive semiconductor manufacturing stages, etc. Commercial products are offered by several companies, e. For satellite missions, sophisticated controllers have been designed to actively isolate facilities for microgravity experiments from structural vibrations which are caused by position controllers and other on-board machinery [, ].
The performance of hydraulic shock absorbers can be improved by applying elec- trorheological fluids ERF . ERF are fine suspensions of polarisable small di- electric particles in an unpolar basic fluid, e.
Also suitable are magnetorheological fluids MRF , suspensions of small ferromag- netic particles in a basic fluid, requiring a magnetic field for the viscosity to be changed. The field is usually applied by electromagnets which require high electric current instead of high voltage . In order to provide a wide range of viscosity control, the viscosities of the basic fluids selected for ERF and MRF are as low as possible, which leads to sedimentation problems, in particular with MRF because of its specifically heavier particles than in ERF.
Nevertheless, much research is focusing on MRF applications: This low-frequency sway can be reduced by tuned mass dampers TMD acting as resonance absorbers: Their performance is raised by actively enhancing the relative motion. Less additional mass is required for aero- dynamic appendages, protruding flaps that can be swivelled and utilise wind forces like sails to exert cancelling forces on the building .
Many research activities in the USA, Canada and in particular Japan are aimed at the development of active earthquake protection for buildings where, however, severe technical problems have still to be solved [, ].
For slim structures such as antenna masts, bridges etc. Modern swivelling large telescopes suffer from deformation under their own weight which is compensated more efficiently by active shape control than by additional stiffeners which inevitably enhance the mass of the structure.
This technology is called active optics [, ]. While the telescope motions are very slow time constants above 0. The large primary mirror is fixed, but the smaller secondary mirror surface rests on a matrix of piezoceramic actuators which are adjusted by an adaptive multichannel controller so that a reference star is op- timally focused.
If no reference star exists in the vicinity of the observed object an artificial guide star can be created by resonance scattering of an intense laser beam from sodium atoms at about km height [, ]. Adaptive optics have improved the optical resolution of the best telescopes by a factor of 10 to 50, to almost the diffraction limit.
This technology was developed in the USA during the s for the military SDI project and has been declassified not before when civil research had reached almost the same state . Meanwhile, this technology is applied to nearly all mod- ern large optical infrared telescopes such as the Gemini North Telescope on top of the Mauna Kea on Hawaii  and the Very Large Telescope VLT in Chile, and will be applied to even larger telescopes planned for the future [, ].
Adaptive optical mirrors have also found applications in industrial production for laser cutting and welding , and generally for optimising the quality of high- intensity laser beams [, ]. Other non-astronomical fields of adaptive optics application are confocal microscopy , spatial light modulators SLM for optical telecommunication , and ophthalmology .
Most of the small deformable mir- rors are manufactured as micromechanical systems MEMS e. A survey of industrial and medical applications of adaptive optics is presented in . Guicking adaptive optics have been published e. But since some time the two fields have become connected. Many noise problems result from radiation of structure-borne sound, e.
Here comes into action a concept known under the acronym ASAC Active Structural Acoustic Control  where noise reduction is not at- tained by superimposing airborne sound to the disturbing noise field but by control- ling the vibrating structure itself.
This is possible by suitably placed and controlled actuators to suppress the structural vibration, although this is not necessarily the optimal solution. Much work has been done to investigate how, e. This is often achieved by ASAC see preceding chapter , but in some instances also by different means.
Experiments have shown that sound transmission through double-glazed windows can be reduced by actively controlled loudspeakers in the gap between the glass panes . Actively controlled double wall partitions are also reported  and , the latter one for insulating floor impulsive noise.
The favourite actuators for active structural damping are piezoceramics, bonded to the structure to form adaptive smart structures . Semi-active approaches apply passive sometimes actively controlled shunts across the piezoactuators to save energy [, ], or even to gain electrical energy from the vibrated piezos, a rather new technology labelled energy harvesting or energy scavenging [, ]. Active Control of Sound and Vibration 19 3.
Of practical importance is the control of magnetic bearings to stabilise a rotor in its unstable equilibrium by feedback control.
Being frictionless and free from lubricants, magnetic bearings are often applied in vacuum apparatus also in spacecraft such as high-speed centrifuges e. A particular realm of research is chaos control, forcing a chaotic oscillation into a stable periodic orbit [, ]. Major control concepts are 1 feedforward control ; 2 feedback control by applying small perturbations to an accessible system parameter when the trajectory comes close to the unstable periodic orbit where it is desired to stabilise the system, the so-called OGY control named after the protagonists of this method  ; 3 Time Delay Autosynchronisation or Delayed Feedback Control, where the feedback signal is the difference of the actual and a previous output signal of the chaotic system ; and 4 sliding mode control .
An early form of the delayed feedback control concept has been formulated in the theory of balancing rods by humans and bicycle riding . A medical application is the stabilisation of atrial fibrillation, a chaotic rapid os- cillation of blood flow in the heart vestibules . A potential application of chaos control is secure communication by masking the message with a broadband chaotic carrier at the transmitter site and demasking it at the receiver site by synchronising the chaotic transmitter and receiver oscillators .
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