ACTIVE NOISE CONTROL

The ISVR has been involved in active control research for many years. Early work was  carried out on active headsets, and further studies were undertaken on the active  control of enclosed soundfields.

Why Active Noise Control ?

There are several ways to reduce the noise levels emitted by a sound source in a room. One standard approach consists of passive noise absorption, i.e. covering the walls with absorbing materials like glassfibre coatings. These materials dissipate acoustic energy into heat and so contribute to the noise reduction, but they work best at high frequencies (e.g. above 500 Hz). Active Noise Control systems have been developed to overcome the limitations of passive strategies at low-frequencies (e.g. below 100 Hz).

How does it work ?

Active noise control tries to eliminate sound components by adding the exact opposite sound. As we saw earlier when we studied superposition of waves, two waves with equal amplitude and identical phase add together constructively, resulting in a doubling of overall amplitude. On the other hand, two waves with equal amplitude and opposite phase add together destructively, resulting in a cancelling of overall amplitude. The phase describes the relative position of the wave in its rising and falling cycle. If two waves are in phase, they rise and fall together, whilst if they are exactly out of phase, one is rises as the other falls, and so they cancel out.

By detecting unwanted sounds with microphones the active noise control system can automatically generate the correct signal to send to loudspeakers which will produce anti-sound to cancel out the original sound.  The size of the zone of silence created depends on the wavelength of the sound.  The effectiveness of the system will depend on the size and shape of the room in which it is applied, the size of the unwanted source of sound and the number and positions of loudspeakers used to generate the anti-sound.

• Active noise control in a room with swept frequency

The animations below show how the acoustic pressure distribution generated by one primary loudspeaker located at the lower left corner of the room (mid figure, primary field) is modified by a secondary loudspeaker, or 'anti-sound' source, located next to it and suitably driven in order to achieve attenuation of sound throughout the entire enclosure (bottom figure, primary and secondary field). This represents the best possible situation for the application of active noise control, where the primary source is small and the secondary source can be placed close to it. The animations illustrate how the resonance patterns (due to standing waves in the room) are changed as frequency increases. You can see how the resonance mode shapes in the room are 'redistributed' when the active noise control system is turned on.

Research into the active control of enclosed sound fields has produced practical demonstrations of control inside cars (1986), aircraft (1988) and helicopters (1995). Further illustrations are shown in section 2.