Active noise control - theoretical basis

Classically, there are two main methods of noise protection: noise can be reduced at source, or its propagation to the area to be protected can be prevented. Noise reduction at source, where possible, should always be undertaken before any other form of noise control, as it is often the most effective. For example, low-frequency emissions from a machine can be reduced by placing it on a solid base which acts by inertia, then eliminating high frequencies by interposing a resilient material such as cork, rubber or fiberglass between the base and the machine. These actions at source can then be complemented by acoustic treatment inside the machine and enclosure around it. When reduction at source is impossible or difficult to implement, we interpose materials that oppose the propagation of sound waves between the noise source and the area to be protected. This can be achieved by using absorbent materials made of porous or fibrous substances, such as rock wool or polyurethane foam, or reflective materials, which reflect noise to areas where it is less disturbing.

Unfortunately, these two noise reduction techniques have one major drawback: they are difficult to implement when the noise to be reduced makes significant contributions in the low-frequency range. For example, the use of absorbent materials, which is highly effective at high and medium frequencies, leads to prohibitive dimensions and weight at low frequencies.

Absorption and reflection are not the only means of preventing the propagation of a sound wave. If we place a set of secondary sources in the path of this wave, emitting a second wave of the same amplitude, but of opposite sign, the addition of these two waves will, through destructive interference, give a zero resultant. Whereas methods of noise reduction by absorption or reflection are purely passive, noise reduction by interference is an active method, since it requires actuators (the secondary sources, also known as counter-noise sources). Various expressions have been used to designate this method, depending on whether attention is focused on a particular aspect of the noise reduction mechanism: active acoustic absorption, stimulated acoustic absorption, active anti-noise... These have now been abandoned in favor of the Anglicism "active noise control", which we will use in the remainder of this article.

The concept of active control is not new (we'll see that the first patents on the subject were filed in the 1930s), but for a long time attempts to implement and exploit it remained unheeded. There are two reasons for this long gestation period:

• if not combined with a method for reconstructing the sound field to be reduced, the use of interference can only achieve significant...