Friday, 17 February 2012
Cars that cancelled out their own noise by playing it back in stereo was not sci-fi fantasy. I heard it working. Yet little has been done with Adaptive Noise Control developed 20 years ago at Lotus. Cars have become so quiet, it seems, there is no need.
Quietude by technical mastery. Crewe. Assembling a 12-cylinder Bentley engine (above).
Lotus's invention consisted of a computer, four loudspeakers some microphones, and sound sensors for tyre and exhaust noise. Development engineers chose a small Citroën AX for experiments because it was lightweight and noisy, and demonstrated it to me on the runways at Hethel.
Unitary small saloons tend to be boisterous inside. Layers of sound-damping materials would only cancel out the advantages of weight-saving for economy. The engineers decided the most annoying noises were low-frequency booming, which reverberated through the body shell from what they described as, “an acoustically complex mix of tyre swish, suspension rumble, engine vibration, and exhaust resonance.”
They had worked with Southampton University's Institute of Sound and Vibration Research since 1986 on a system they claimed was ready for a production car. There were four tiny microphones in the headlining, costing about 35p each, connected to a microprocessor control unit linked into the ignition to sense engine speed.
Detecting sound pressure levels inside the car through the microphones, the control unit matched them with changes in engine speed and played them back through an amplifier with 40 Watts RMS per channel. The effect was astonishing. You could switch the system in and out, making it easy to hear a 20dB reduction in noise in the lower-frequency sounds below about 100HZ.
It didn’t make the car silent. Tackling higher frequencies, the sort of buzz that comes from engine valvegear, or whine from gears, demanded more microphones and loudspeakers, as well as sensors in each seat to localise noise levels to each occupant. It could have been incorporated into a stereo system relatively cheaply for about the cost of the microprocessor Lotus used, under £100 then and probably a lot less now.
Anti-noise would have permitted softer engine mountings which, it was claimed, could make cars almost vibrationless. What high hopes. Adaptive noise control turned out difficult to engineer, much like active suspension Lotus and Volvo tried out in the 1990s. Noisy and heavy, it was overtaken by simpler computer controlled oleo and air springing.
Gaydon, cutaway MGB GT