Active control of low-frequency, pure-tone noise
Schematic anti-noise layout inside a passenger compartment
Sound pressure can be reduced in some cases by active noise control, sometimes known as 'anti-noise'. This technique uses the principle of sound cancellation. The characteristics of a sound field in a given environment are measured and then reproduced by means of an artificial source such as an array of loudspeakers. The reproduced sound is generated in such a way that the new sound field produces a wave of, for example, positive pressure in exactly the same position and at exactly the same time as a negative pressure wave produced in the original sound field. The original sound wave is thus cancelled out or at least significantly reduced.
In practice, effective noise reduction by this technique is possible only for low-frequency pure tones occurring within well-defined enclosed spaces. Most attention has so far been devoted to the active control of noise in ducted systems carrying noise from such sources as fans and engine exhausts, resulting in some successful solutions.
More recently, attention has turned to the possibility of active noise control in larger spaces subjected to low-frequency, pure-tone noise. One example was the suppression of engine-induced 'boom' inside cars. Since this type of noise is primarily low frequency, conventional methods of acoustic insulation, besides adding both cost and weight to the vehicle, are not very effective. However, active noise control can provide a viable solution. By sampling the engine-generated sound field at a number of positions inside the vehicle, and by reproducing the field with the appropriate phase shifts using the vehicle's own loudspeaker system, noise reductions can be achieved at some positions in the vehicle.
£2000 for equipment, plus project-specific development and programming costs. (1995)
Reductions of up to 10 dB in individual frequency bands and 5 dB overall. There may be similar benefits for operators of commercial vehicles and industrial site and construction vehicles.
Low-frequency, pure-tone noise generated in small production areas by, for example, vibratory machinery or reciprocating air compressors may have characteristics appropriate for active noise reduction measures.
Report reproduced courtesy of Dr P Nelson and Dr S Elliott, ISVR, University of Southampton.