The sound of silence

By Anne-Lise Aakervik
Photo: Rune Petter Ness

Almost everyone, at some time or another, has been kept awake by the throb of the bass from party music next door. No matter how thick the walls are or how many pillows you bury your head in there is no escape from the low- frequency sound.

With its low-frequency vibration, the diesel locomotive DI 4 is the noisiest in NSB's fleet. A Silent Zone headrest mounted on the driver's chair uses mirrored noise to neutralize the low-frequency throbbing. This makes the engineer, Arne Fodnes, less tired.

Similarly, the characteristic chug of a diesel locomotive pulses relentlessly through the body of the engineer. Listening to the monotonous beat for up to eight hours often results in fatigue.

Sound waves travel through the air in much the same way as ripples from a drop of rain spread over the surface of a lake. The traditional way of blocking out unwanted noise is by placing a sound-proofing wall between the listener and the source of the sound. However, low-frequency noise components travel far and are difficult to cut off.

Help is now at hand - at least for the train engineer. A headrest with loudspeakers, microphones and advanced electronics can be mounted on the back of an engineer's chair to create a quiet zone around the head.

At lightning speed
The microphones detect the low-frequency throbbing in the cockpit and transmit these to the box mounted on the headrest. Here, the electronic circuitry uses digital signal analysis to determine the precise mirror image of the unwanted noise. The speakers subsequently generate the reverse sound in the form of pressure waves in anti-phase with the unwanted noise. Close to the speakers, which are mounted at head level, a quiet zone will result where the noise is significantly reduced. The deep throbs that are so tiring are gone, while other sounds can still be heard. Since the sound is detected and transmitted at approximately the same location, the signal analysis must be extremely fast. At present, it takes about 60 microseconds; in other words, at lightning speed. In addition, there must be an exact correspondence between the generated sound and the unwanted noise for cancellation to be effective over a sufficiently large area.

- This is close to the limit of what is physically possible, says Odd K. Pettersen, head of research in the Department of Acoustics at SINTEF Telecom and Informatics. The quiet zone idea arose eight years ago when two SINTEF researchers were discussing new ideas. A graduate student, Eivind Bergsmyr, was given the task of determining whether it was technically possible to create such a "silent" zone.

- To our great surprise, Pettersen says, he managed to do it. The product was patented in several countries, including the US.

One out of three

The blue boxes to the right show the digital technology used in the microphones and loudspeakers mounted in the headrest.

In 1998 the company Silence International AS was established to prepare for the production of Silent Zone (Stille Sone) in cooperation with SINTEF. Silence International owns the patent and commercial rights to Silent Zone. - Dynamic counter-action as a means of minimizing low-frequency noise is the technology of the future, claims Peter Molthe manager of Silence International.

Silent Zone is one of three related products world-wide. While others have their sights on the aviation industry, Silent Zone is initially being designed for trains. Together with the Norwegian Regional and Industrial Development Fund (SND), the Norwegian State Railways (NSB) has contributed to funding the research and development. According to Asle Nordbotten, locomotive engineer supervisor at NSB, Silent Zone seems very promising. The Worker Protection and Working Environment Act in Norway requires the noise level in a locomotive to be below 70 decibels. This is not the case today. To compensate, drivers generally use ear cups which are tiring and awkward to wear.

- Even so, the noise stays with you for a long time after your shift is over, says Nordbotten. - We have great hopes for Silent Zone. Tests have shown that it is capable of removing the particularly annoying and exhausting low-frequency sounds.

Plans for the future include integrating the technology into operator seats in the processing industry as well as in passenger seats in jets and high-speed passenger ferries.

* Contact at SINTEF: Odd K.Pettersen
Tel: + 47 73 59 26 37
E-mail: Odd.K.Pettersen@informatics.sintef.no