Keeping noise under control

A J Salloway and C E Millar from GEC-Marconi Materials Technology (GMMT) look at how design engineers can minimise vibration and noise

Using active vibration control (AVC) and active noise control (ANC), designers can now eliminate unwanted noise in numerous applications in both military and commercial projects. The principle is simple. Vibration or noise is detected using a sensor and the signal is inverted, amplified and fed back to an actuator which acts in an opposite sense, reducing vibration or noise (Figure 1).

In practice, however, there are many issues to overcome, such as the 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 in size. 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 and so on) are generally very effective at higher frequencies (>500Hz) 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 (<500Hz) where the wavelengths of the vibration/noise are long. Thus combining active and passive methods allows vibration/noise to be reduced over a wide frequency range (Figure 2).

There are various different approaches to take when implementing a passive or active noise or vibration reduction system. 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 airplane, to detect the noise and produce noise in antiphase, all of which can be controlled by a central digital control unit. GEC-Marconi Avionics have 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 benefits for both turbo-prop 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. All of these approaches have advantages and disadvantages and, depending on the application, one approach will be more appropriate or cost effective than another.

The approach at GMMT 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.

Active Noise Reduction Headsets

An active noise reduction headset (Figure 3) detects 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 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 (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 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 40dB reduction in the noise level at the high frequencies and 20dB(A) reduction in typical AFV noise. The active system achieves 15dB of reduction at 250Hz and a further 10dB(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.

In certain applications, passive reduction is not required and a purely active system can be used. This has been demonstrated in a lightweight headset for use in commercial jet aircraft. A pair of `walkman’ type headphones was used in a broadband active system, and noise reduction of up to 10dB was achieved over the frequency range in which the jet engine noise dominated (100Hz to 2.5kHz). Such headsets could be given to crew and passengers for use in noisy propeller planes.

The military type of headset has since been adapted for use in MRI equipment. Patients undergoing a scan are exposed to high levels of noise that most patients find uncomfortable, and some are unable to endure. The active noise reduction headset, combined with a noise-reducing microphone, makes communication with the radiographer possible without machine noise and allows music or stories to be played to the patient during the scan.

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 13dB reduction (<1/4 of the noise level) for three typical noise spectra produced by routine scans. These headsets are being developed into a commercial product in collaboration with Picker.

GMMT have also designed an active noise-reducing telephone and have demonstrated that the background noise level could be reduced by 10dB up to frequencies of 1kHz. 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.

Active vibration control of PCBs

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 – both undesirable in any airborne vehicle. Sensitive components are often packaged separately making a component normally a few millimetres in size occupy tens of centimetres of space and weigh ten to twenty times as much.

For constant amplitude acceleration, displacement is inversely proportional to frequency squared; a reduction in vibration at the lower resonant modes is therefore particularly important in order to reduce fatigue and increase reliability. An active system can achieve a reduction at the lower frequency modes, which would be extremely difficult, if not impossible, to achieve by passive means. The active system developed by GMMT comprises simply sensors and actuators, and an analogue control circuit. Thus the use of an active control system eliminates the need for bulky passive damping, significantly reducing the weight and size of a vibration reduction system, whilst giving improved control at low frequencies.

GMMT are currently carrying out a programme sponsored by DERA on the active vibration control of printed circuit boards too. The aim of the programme has been to develop a system that can be implemented easily on a wide range of boards. The AVC system for PCBs is shown in Figure 4.

The work has included an investigation of actuator and sensor devices, the positioning and bonding of the devices (based on the vibration modes in the boards), and control systems. In this application, space is of prime importance, as PCB `real estate’ is not cheap.

The work has been carried out by combining modelling, experimental examination of the board’s behaviour and the effect of the active vibration-control system. Modelling work has investigated vibration modes under different clamping conditions, allowing recommendations to be made concerning the positioning and type of actuators and sensors. An ANSYS finite element package is being developed to allow analysis of boards with different mechanical properties, to study the effect of populating the boards, and to predict the effect of vibration control.

From the modelling and experimental work, recommendations can now be given for the positioning and type of actuators required for a particular board and clamping condition. A feedback circuit has been designed for the active damping technique and is now being miniaturised using surface mount technology. The results have shown that for most clamping conditions the actuators can be mounted in the centre of the board. Multilayer piezoelectric actuators are used which allow lower driving voltages (< 50V). The sensors are either small accelerometers or piezoelectric ceramics.

Three types of vibration control have been developed. The first is Passive – Active Vibration Control. This method has the advantage of requiring no power, and therefore can be used as a `fail safe’ or in situations, such as a space launch, where there is no power. The system uses a piezoelectric material to convert the mechanical vibration energy into electrical energy, which can then be dissipated as heat. This method gives up to 10dB reduction of a resonant mode.

The second method is called Tuneable Passive – Active Vibration Control. This employs a similar principle to the passive – active system, but uses a tuneable circuit so that the system can be set to the resonant frequency of the system. The advantage of this method is that the system can track a resonance and thus achieve a high level of reduction, even when variations in temperature or board mounting cause large changes in the resonant frequency of vibration modes. The circuit can be controlled by an adaptive analogue or digital control system.

Thirdly, there is Active Vibration Control. This active system combines sensors and actuators with a feedback system, providing the highest vibration control, with up to 20dB reduction of the vibrations up to 4g acceleration. This corresponds to a reduction to about one tenth of the vibration level of the board. The effect of the performance of an oscillator mounted on a PCB with and without this active vibration control system, has been successfully demonstrated.

Subjecting the oscillator to vibrations produced side-lobes in the frequency response and demonstrates why such devices are generally packaged in separate bulky packages. By applying AVC to the board, the side-lobes can be reduced to insignificant levels, thus reducing the requirements for the packages.

The feedback circuit is currently being miniaturised so that it can be mounted onto the board. This eliminates the need for a stack of instrumentation required by more complicated active vibration control systems.

The next stage will be to develop an `AVC package’; this incorporates the sensor, actuator and miniature control circuit, and can be applied to PCBs and similar structures to reduce the vibration level. The package will be mounted onto PCBs as an additional small component. The effect of the reduction in vibration levels will be demonstrated by comparing the life-time and reliability of PCBs, with and without the AVC package. Other applications for the AVC package include: aircraft, car and machine panels, space satellites and antennas, vibration-sensitive components, noisy equipment (such as coolers and compressors), bridges, and buildings.

The technology is now becoming mature and is being considered for a wide range of applications. Although relatively expensive, ANR is already commercially exploited in headsets for civil aircraft and military vehicles, and noise-control systems on aircraft. At present, the high cost of AVC systems inhibits exploitation in volume markets.

The path to commercial realisation is beginning via high performance requirements in military and space applications, where vibration levels are extreme, or where weight is of prime importance. The development of a low cost AVC package will be a step towards making active vibration control accessible to a wider range of applications.

Figure 1: 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

Figure 2: Typical frequency response of passive and active noise control for an active noise control system

Figure 3: Active noise reduction headsets can be used in commercial applications such as MRI systems

Figure 4: Active vibration control for PCBs could help increase their lifetime in service