Innovation: More noise equals sound of silence: Using one sound to cancel another promises quieter rides

AN IDEA for reducing noise first patented more than 80 years ago is now being applied to give plane, car and train passengers a quieter ride.

The technique of cancelling out one noise by superimposing another, known as active noise control, has become feasible with the development of small, cheap and powerful electronics.

Its successful application depends on determining exactly the nature of the noise that is bothering the listener. This is done with a microphone located as close to the ear as possible. After the frequency and amplititude of the noise is identified, the next step is to create a noise of exactly the same frequency and amplitude but exactly out of phase with it. This is done with a loudspeaker whose noise output can be varied as required. The process is controlled by a computer, which analyses the signals from the microphone and sends appropriate anti-noise instructions to the loudspeaker.

The car manufacturer Nissan recently launched a version of its Bluebird saloon car in Japan fitted with active noise control. Next year a similarly equipped plane will be launched by Avions de Transport Regional (ATR), the joint venture between the French Aerospatiale and the Italian Alenia aerospace companies.

Other interested manufacturers include Dowty, which makes the systems to be fitted by ATR; the French automotive engineering group Bertin; the Swedish aircraft maker Saab; Lucas; Philips; and Noise Cancellation Technologies of the US.

ATR hopes that installing active noise control will improve the image of its short-haul turbo-prop planes. Customers think the plane is old- fashioned because it has propellers, a perception heightened by the high noise levels.

The system on the ATR 42-500, a 50-seater aircraft to be launched next May, will take into the account the speed of the propellers, which is directly related to the noise they make. Specially fitted tachometers will measure the speed of rotation of the propellers, and pass this information to the computer. The computer will use this to calculate a first approximation of the cancellation noise to be emitted by the 38 loudspeakers built into the cabin.

The 76 microphones distributed throughout the cabin will check the effectiveness of this first signal and pass the error level to the computer to optimise its calculations.

ATR's vice-president of engineering, Alain Fontaine, said trials are still in progress and it is not clear what the noise reduction will be, but he is optimistic. 'Our aim is to achieve sound levels in the ATR 42-500 equivalent to those in a jet aircraft.' This calls for a reduction from 75 to 63 decibels.

Noise control is only part of a wider noise reduction programme. 'But it has to provide a significant improvement because it adds 40kg (88lb) to the plane's weight and that costs money,' Mr Fontaine said.

Nissan says that noise control has reduced noise inside its Bluebird by 10 decibels. The average noise level in a car travelling at 75mph is 75 decibels. The system, set up to reduce noise from the engine, consists of four microphones located in the car roof and two loudspeakers hidden under the front seats. Measurements of engine speed again enable the computer, the size of a portable CD player and housed in the shelf behind the rear seats, to make a first approximation of the cancellation signal.

The engine is only one source of noise in cars, and further work on road noise is being carried out by Lotus Engineering in partnership with the Institute of Sound and Vibration Research at the University of Southampton. A system that uses accelerometers fixed under the car, close to the wheel hubs, to help calculate the noise cancellation signal has been tested, although no manufacturer has come forward to integrate it in a production model.

Engine noise is more important at lower speeds, while at higher speeds road noise is more intrusive. The eventual aim is to have a system that responds to both noise sources.

Noise control is also being integrated into such products as earmuffs and double-glazing by Technofirst, a French company set up five years ago to commercialise noise control technology developed by France's public research institute, the Centre National de Recherche et Science. A standard earmuff reduces high-frequency sounds to acceptable levels, but is ineffectual against lower frequencies, says Christian Carme, a director of Technofirst. 'It is these lower frequencies that noise control is best suited to tackling,' he said.

The Technofirst earmuff has a tiny microphone, loudspeaker and electronic circuit in both earpieces. The result is a sound level reduction of 40dB, compared to having no protection. Double-glazing is also ineffectual against low-frequency noise. But by placing a microphone outside the window and a microphone and loudspeaker inside the sealed gap to control the noise, reductions of 20dB are possible. Discussions are now going on between Technofirst, the glass manufacturer St Gobain and SNCF, the French national railways company, to use the system on trains.

Both Mrs Carme and Mr Fontaine emphasise that noise control cannot cure all problems. It is no help to those with noisy neighbours or snoring partners, because the changes of sound level are too sudden and too great.

(Photograph and graphic omitted)