Active noise control

Ferroelectric—polymer composite devices have been developed for large-area transducers, active noise control, and medical imaging appHcations. North American Philips, Hewlett-Packard, and Toshiba make composite medical imaging probes for in-house use. Krautkramer Branson Co. produces the same purpose composite transducer for the open market. NTK Technical Ceramics and Mitsubishi Petrochemical market ferroelectric—polymer composite materials (108) for various device appHcations, such as a towed array hydrophone and robotic use. Whereas the composite market is growing with the invention of new devices, total unit volume and doUar amounts are small compared to the ferroelectric capacitor and ferroelectric—piezoelectric ceramic markets (see Medical imaging technology). 

Noise. One system for abating unwanted sound is active noise control. The first successful application of active noise control was noise-canceling headphones,... 

Active noise control earphones use a technology whereby external sounds are recorded, phase inverted, and reemitted with the original sound to effectively cancel it by destructive interference. This ensures a quiet background for hearing recorded music in noisy surroundings. 

Another space related topic is the reduction of noise impacts on pay-loads through active noise control using technologies such as an acoustic foam with embedded PVDF material, distributed active vibration absorbers (DAVA) made from acoustic foam hnked to a metallic plate and in a latest version even added by a very small electrodjmamic shaker that allows to cover lower frequencies [103]. This sandwich of acoustic foam, PVDF and electrodynamic shakers is then used as an active coating on the fairings of space vehicles. 

Smart actuators for active shape control, active vibration control, active noise control... 

Some common methods used to control plant noise are barriers, enclosures, absorption, mufflers, lagging or wrapping, damping, isolation, equipment selection, and improved maintenance. An additional, newer method that is available to reduce plant noise is active noise control. Each of these methods is discussed subsequently. Further information about noise control techniqnes and costs is available from noise control material manufacturers and publications. Some manufacturers have simple computerized selection programs available. 

Considerable research efforts have been undertaken to develop reliable and cost-effective means to actively (or electronically) control noise produced by equipment. Though it is not a new concept, modem theory and hardware for adaptive digital signal processing have recently made the commercial use of active noise control worth considering for certain appU-cations with special requirements. 

The use of active noise control should be considered when passive noise control methods are expensive or difficult to install. The control of very-low-frequency noise in a duct is an example where passive noise control may be difficult. In low-frequency applications where space is limited and no additional pressure is available, active noise control becomes even more attractive. However, it is important to recognize that the microphones, electronic circuits, and loudspeakers will require at least some ongoing maintenance, particularly when installed in hot or corrosive environments. 

Active noise control has also been applied inside earmuffs and coiiununication headsets to reduce noise exposure and improve speech intelligibility. Earmuffs and headsets with built-in... 

For special applications requiring significant attenuation of low-, mid-, and high-frequency noise, it might be most practical to consider the use of both active and passive noise control methods. A passive noise control treatment, such as a small muffler or duct lining, can be installed to attenuate the mid- and high-frequency noise and active noise control might be employed to attenuate the low-frequency noise. 

Additional information on active noise control and its applications can be found in sources listed at the end of this chapter. 

Power Supply Cookbook, Second Edition has been updated with the latest advances in the field of efficient power conversion. Efficiencies of between 80 to 95 percent are now possible using these new techniques. The major losses within the switching power supply and the modern techniques to reduce them are discussed at length. These include synchronous rectification, lossless snubbers, and active clamps. The information on methods of control, noise control, and optimum printed circuit board layout has also been updated. 

Another instructive example of the relevance of ambient stimuli to the nature of the effect of serotonergic manipulations on locomotor activity is provided by Brody s (27) study of PCPA. He monitored the locomotion of vehicle- and PCPA-treated rats in an open field to which the animals had been previously familiarized. Animals were tested either with or without additional stimulation in the form of flashing lights and 90 dB(A) noise bursts. Without stimulation, PCPA-treated rats were less active than controls with stimulation, PCPA-treated rats were more active. The results of these and other such studies indicate that the central serotonergic systems may not directly modulate the level of locomotor activity per se, but they may profoundly influence locomotor activity by virtue of their effects on the sensory responsivity of the animal to a wide variety of environmental stimuli. 

Wilson, K. J., E. Gutmark, and K. C. Schadow. 1992. Flame kernel pulse actuator for active combustion control. In Active control of noise and vibration. ASME DSC-38 75-81. 

These limitations do not usually prevent convergence in practice, provided noise is controlled as discussed above. However, they may lead either to slow progress due to constraints that flip repeatedly from active to inactive or to the need for stringent noise control as the constraint and its gradients go to zero and the Lagrange multiplier grows. 

Coal preparation plants invariably contain various types of active machinery, in addition to large throughput tonnages of free-falling solids, and have long been recognized as presenting a serious noise problem. Most of the noise control measures may be classified broadly into measures that... 

Residential annoyance by aircraft noise is best controlled by zoning restrictions prohibiting housing developments near airports. It may also be possible to relocate runways to reduce flights over nearby homes. Noise-reflecting screens and active noise-sup>-pression procedures, although expensive, are under serious deliberation as possible means of airport noise abatement. 

Ver, Istvan L., and Leo L. Beranek, eds. Noise and Vibration Control En neering Principles and AppUcor tions. 2d ed. Hoboken, N.J. John Wiley Sons, 2006. Details sound absorption, passive silencers, enclosures, vibration isolation and damping, machinery and heating, ventilation, and air-conditioning (HVAC) noise control, and active noise suppression. 

Most automotive R D projects within adaptronics are concerned with active vibration control (AVC) and active structural acoustic control (ASAC) for optimization of NVH (noise, vibration and harshness) characteristics and even soimd design. Furthermore, active measmes to increase the passive safety of vehicles are being developed. Shape and position control as well as structmal health monitoring (SHM) are stiU of secondary importance. 

Sensitivity to Other Non-Social Exteroceptive Stimuli - Several studies indicate that PCPA increases various behavioral or neurological effects of electrical stimulation. Thresholds of current required to reinforce selfstimulation behavior are lowered by PCPA in rats,46 and PCPA has been reported to cause strong excitatory effects on intracranial self-stimulation rates in rats 3 days after treatment.47 5-HT-depleted animals have been reported to be more active than controls under conditions of extra environmental stimulation, particularly "novel" stimulation.18b,19c Cats "overreact to slight noises,"48 sensory thresholds are lowered,29 the responsiveness to a startle stimulus of rats with low basal responsiveness Is Increased,49 and habituation to an audltory-ellcited startle response, as measured by cortical electrical responses. Is reduced.50... 

Although the PFA was designed to run at 20K, operation of the FPA has been demonstrated all the way to liquid helium temperatures (4K). This means that the array can be used in a liquid helium dewar with no active temperature control, greatly simplifying dewar construction. Of course, the detector dark current does not get significantly better at temperatures below 30K, and the noise of the multiplexer will be somewhat higher due to the increase in MOSFET noise. Nonetheless, some applications will benefit from this extra capability. 

Proceedings of the Sixth International Symposium on Active Noise and Vibration Control. Adelaide, Australia Sept. 2006. 

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