TechArchive

This article was originally written in the period 1995-2000

The basic principle behind active vibration control (AVC) and active noise control (ANC) is simple. The vibration or noise is detected using a sensor (or microphone) and the signal is inverted, amplified and fed back to an actuator (or loudspeaker) which then acts in an opposite sense, reducing the vibration or noise.

In practice, there are many issues to overcome, such as selection of sensor and actuator devices and their positions relative to each other, as well as to the system. The feedback circuit is also very important as this determines the effectiveness of the vibration/noise control and its frequency range. Circuits can either be analogue or digital. Digital circuits have the advantage that the system can be predictive if the type of noise/vibration is well-known, as is the case for deterministic fields (for example, a fixed vibrating motor), but the control system is often large. Analogue control circuits are generally more simple and are thus more suitable for local control applications.

Active noise reduction (ANR) can be used either to replace bulky, passive damping methods or combined with passive methods to enhance the noise or vibration reduction over a wider frequency range. Passive damping methods (foams, springs etc.) are generally very effective at higher frequencies (> 500 Hz) where the dimensions of the devices are comparable with the wavelength of the vibration/noise. In contrast, active reduction is usually more effective at lower frequencies.

There are various different approaches to take when implementing a noise or vibration reduction system (either passive or active). These can be demonstrated by considering a severe vibration source, such as an engine on an aeroplane or ship. The engine will produce vibration and noise which is transmitted throughout the structure. One approach to reduce the noise and vibration levels is to isolate the engine by placing it on an active vibration-reducing platform without any mechanical connections to the rest of the structure. The GEC-Marconi Research Centre (MRC) at Great Baddow has taken this approach and has developed a digitally-controlled, active vibration-reduction raft for ships. This approach is effective as the vibration is reduced at source, but it is complex and expensive.

A second approach is to reduce the effects of vibration by using individual dispersed vibration-control units in critical places, such as pipes, mechanical connections and panels within the structure. This approach is less complex than the first, often costs less, and works well regardless of the vibration source, but the overall performance can be poorer compared to isolation of the vibration source.

A third approach is to use an ANR system to reduce the engine noise, and noise caused by transmitted vibrations. This is achieved by placing a series of microphones and loudspeakers, for example, in the cabin of an aeroplane, to detect the noise and produce noise in anti-phase, all of which is controlled by a central digital control unit. GEC-Marconi Avionics has successfully developed such an ANR system which has been selected by BAe for the Jetstream 41 and by Lockheed for the C-130 aircraft. On-flight trials showed that the system provides significant benefits for both turboprop aircraft and large, four-engine aircraft.

Noise reduction can also be achieved by using small, independent units, for example, by providing each crew member or passenger with noise-reducing headsets, or by using active noise control in the head-rests of seats.

The approach at GEC-Marconi Material Technology has been to develop small independent AVC and ANC units. Using analogue control, they have developed several systems for both active noise and vibration control, concentrating on the selection of sensor and actuator devices, their positioning and the development of relatively simple and small size analogue control systems.

An active noise reduction headset detects the acoustic noise using a miniature microphone. The signal is then filtered, phase-inverted and fed back to the earphone drive unit as the inverse of the original signal. A speech-shaping filter (a pre-emphasis filter) compensates the speech signal for the effect of ANR, so that speech can be heard without modification.

One application of ANR headsets is in military vehicles (such as armoured fighting vehicles Ð AFVs) where the noise levels are very high and even short term exposure can be damaging to an operator’s hearing. Protective headsets are used by all AFV operators, and until recently, most were based on passive methods. GMMT has developed headsets that combine passive noise control with active noise control, in collaboration with Plessey Military Communications. The headsets comprise a pair of ear shells containing sealed foam cushions for passive attenuation, which achieve 40 dB reduction in the noise level at the high frequencies and 20 dB(A) reduction in typical AFV noise. The active system achieves 15 dB of reduction at 250 Hz and a further 10 dB(A) reduction in AFV noise. A system was developed which comprised a central control unit and slave units: two headsets could be connected to each slave/control unit and six slave units connected to the central control unit. This system was put into full-scale production to supply all crew members of the Warrior AFV tanks.

The military type of headset has since been adapted for use in MRI equipment. For this application, not only does the noise control system have to be effective, but the headset has to be invisible on the scan. This was achieved by the use of plastic components, where possible, and the use of non-ferromagnetic materials where conductors were essential. The coupling of the headset to the RF fields was reduced, as much as possible, by providing a high impedance at the headset. The headsets achieved a 13 dB reduction (<1/4 of the noise level) for three typical noise spectra produced by routine scans. These headsets are currently being developed into a commercial product in collaboration with Picker.

GMMT has also designed an active noise-reducing telephone and have demonstrated that the background noise level could be reduced by 10 dB up to frequencies of 1 kHz. Such telephones could be used in noisy environments, such as motorway phones, train phones and mobile phones. Similar systems could be used in active ear inserts for noise protection and hearing aids.

Printed circuit boards (PCBs) are subjected to large levels of vibration in many applications, such as helicopters, aircraft and space structures (particularly during launch). These vibration levels degrade the performance of components such as oscillators and resonators causing shifts in their operating frequencies and generation of side bands and, in some cases, destruction of the circuits through fatigue of component legs. At present, vibrations in the boards are reduced by passive damping, which is effective only at high frequencies and is bulky and heavy Ð both undesirable in any airborne vehicle. Sensitive components are often packaged separately making a component normally a few millimetres in size, for example, occupy tens of centimetres of space and weight ten to twenty times as much.

GMMT is currently carrying out a programme sponsored by DRA on the active vibration control of printed circuit boards. The aim of the programme has been to develop a system that can be implemented easily on a wide range of boards.