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Breaking the sound barrier

8th February 1990
Page 23
Page 23, 8th February 1990 — Breaking the sound barrier
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Which of the following most accurately describes the problem?

• Improved fuel consumption could be one benefit of what is claimed to be the world's first electronically-silenced exhaust, which is being developed by the US parent of Birminghambased Walker UK.

In automotive applications a microphone is used to monitor exhaust noise and, through a microprocessor, generates the required anti-noise frequency through a speaker chamber surrounding the exhaust pipe.

By using the noise of the previous firing cycle, the characteristic of the following cycle can be predicted to cater for changes in engine speed.

Free of noise the exhaust system, constructed without internal baffles, reduces backpressure allowing improved engine efficiency and a reduction in fuel consumption. However, this is before either catalytic filters or particulate traps are considered.

One problem is that it takes as much power to generate the "anti-noise" as it does to produce the source noise; but advances in speaker technology, driven by low-power amplifiers, could provide a solution.

The concept of producing a synchronised noise wave image, 1800 out of phase, dates back to the 1930s. The peak of the original noise wave is eliminated by the trough of a generated noise wave and is most effective at low frequencies (below 680Hz).

Walker Manufacturing expects to have a system in production by 1994.