Giving noise a rest

A Norwegian company claims to have successfully tested an ‘anti-noise’ headrest designed to give train drivers, pilots and machine operators a quieter life.

Acoustic technology specialist Silence International says its active noise control system can respond to unwanted low-frequency sound in a fraction of a second, producing an acoustic mirror image that cancels out the original.

It has tested a headrest incorporating the system, called Silent Zone, in the driver’scabin of trains running on Norwegian State Railways.

According to the company, the system successfully cancelled out most of the rumble affecting diesel locomotive drivers on long journeys.

It will now commercialise the system for use in high-noise environments such as train and aircraft cabins, but eventually hopes to design versions at a low enough cost for general use on passenger seats.

Ultimately, it plans to develop a ‘silent pillow’ incorporating its technology, which can be used to aid a good night’s sleep anywhere.

The Silent Zone system monitors sound via microphones, digitally processes it and uses electro-acoustic transducers to produce an equivalent ‘anti-sound’.

This is a sound in direct counter-phase to the original, producing a mirror-image that cancels out low-frequency noise in the area immediately around the headrest.

Silence International claims its ultra-fast processing speed does away with the need for headphones, a regular feature of other active noise control systems designed for use by machine operators.

Because the system is designed to deal with random noise as well as ever-present low-frequency rumble, the anti-noise has to be produced almost instantaneously.

Silence said it has reduced to 50 microseconds the gap between noise being detected and anti-noise produced.

However, because only low-frequency noise is cancelled out, the user will still be able to hear spoken instructions from fellow drivers or over the radio.

Because the microphones and transducers are so close together, the company has also had to develop highly robust feedback cancellation.

Peter Molthe, Silence International’s managing director, said low-frequency noise is by far the most difficult to deal with.

‘Solid structures such as walls and glass can easily deal with high and medium frequencies, but a low frequency will just go right through them,’ said Molthe.

‘The only usual way to deal with it is headphones, but people don’t want to wear them for long periods. As well as keeping out unwanted noise they are uncomfortable, and stop people hearing the things they do want to hear.’

Molthe claimed the success of the railway test proved the system was practical for installation in individual cabins, but admitted it may be some time before it was in more general use.