2.8. What are some typical applications?
Description
This article is from the Active Noise Control FAQ, by Dr. Chris Ruckman
2.8. What are some typical applications?
The most successful demonstrations of active control have been for controlling noise in enclosed spaces such as ducts, vehicle cabins, exhaust pipes, and headphones. Note, however, that most demonstrations have not yet made the transition into successful commercial products.
One exception, active noise control headphones, has achieved widespread commercial success. Active headphones use destructive interference to cancel low-frequency noise while still allowing the wearer to hear mid- and high-frequency sounds such as conversation and warning sirens. The system comprises a pair of earmuffs containing speakers and one or more small circuit boards. Some include a built-in battery pack, and many allow exterior signal inputs such as music or voice communications. Used extensively by pilots, active headphones are considered indispensable in helicopters and noisy propeller-driven aircraft. Prices have dropped in recent years, and now start around US$650 for active pilots headsets. (See Section 2.11 for information about an active control conversion kit available for US$100.)
Another application that has seen some commercial success is active mufflers for industrial engine exhaust stacks. Active control mufflers have seen years of service on commercial compressors, generators, and so forth. As unit prices for active automobile mufflers have fallen in recent years, several automobile manufacturers are now considering active mufflers for future production cars. However, if you ask your local new car dealer about the active muffler option on their latest model, you will likely receive a blank stare: no production automobiles feature active mufflers as of this writing.
Large industrial fans have also benefited from active control. Speakers placed around the fan intake or outlet not only reduce low- frequency noise downstream and/or upstream, but they also improve efficiency to such an extent that they pay for themselves within a year or two.
The idea of canceling low-frequency noise inside vehicle cabins has received much attention. Most major aircraft manufacturers are developing such systems, especially for noisy propeller-driven aircraft. Speakers in the wall panels can reduce noise generated as the propeller tips pass by the aircraft fuselage. For instance, a system by Noise Cancellation Technologies (NCT) now comes as standard equipment on the new Saab 2000 and 340B+ aircraft. The key advantage is a dramatic weight savings compared to passive treatments alone.
Automobile manufacturers are considering active control for reducing low-frequency noise inside car interiors. The car stereo speakers superpose cancellation signals over the normal music signal to cancel muffler noise and other sounds. For example, Lotus produces such a system for sale to other automobile manufacturers. Unit cost is a major consideration for automobile use. While such systems are not at all common, at least one vehicle (currently offered only in Japan) includes such a system as a factory option.
The following list of applications for active control of noise and vibration was compiled by Colin Hansen and is used by permission; see IS&VD 1(2). The list includes topics which are currently being investigated by research groups throughout the world.
---------- begin quote from C. Hansen, IS&VD 1(2) ----------
1. Control of aircraft interior noise by use of lightweight vibration sources on the fuselage and acoustic sources inside the fuselage.
2. Reduction of helicopter cabin noise by active vibration isolation of the rotor and gearbox from the cabin.
3. Reduction of noise radiated by ships and submarines by active vibration isolation of interior mounted machinery (using active elements in parallel with passive elements) and active reduction of vibratory power transmission along the hull, using vibration actuators on the hull.
4. Reduction of internal combustion engine exhaust noise by use of acoustic control sources at the exhaust outlet or by use of high intensity acoustic sources mounted on the exhaust pipe and radiating into the pipe at some distance from the exhaust outlet.
5. Reduction of low frequency noise radiated by industrial noise sources such as vacuum pumps, forced air blowers, cooling towers and gas turbine exhausts, by use of acoustic control sources.
6. Lightweight machinery enclosures with active control for low frequency noise reduction.
7. Control of tonal noise radiated by turbo-machinery (including aircraft engines).
8. Reduction of low frequency noise propagating in air conditioning systems by use of acoustic sources radiating into the duct airway.
9. Reduction of electrical transformer noise either by using a secondary, perforated lightweight skin surrounding the transformer and driven by vibration sources or by attaching vibration sources directly to the transformer tank. Use of acoustic control sources for this purpose is also being investigated, but a large number of sources are required to obtain global control.
10. Reduction of noise inside automobiles using acoustic sources inside the cabin and lightweight vibration actuators on the body panels.
11. Active headsets and earmuffs.
---------- end quote from C. Hansen, IS&VD 1(2) ----------
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