A NOISE-reduction machine has been developed at the City Polytechnic that its inventor claims could cut 10 to 20 decibels off common sources of noise, such as construction work, vehicles and restaurant ventilation systems. The machine is pointed at the noise source and sends out ''anti-noise'' signals that counteract the loud sound waves and mute them. Dr Man Kim-fung, a senior lecturer in electronic engineering who developed the system, said the machine was of particular relevance to noisy Hongkong. ''The noise level in Hongkong is quite tremendous. We need to look at it desperately,'' he said. Noise pollution is a main source of complaints to the Environmental Protection Department, accounting for about 40 per cent of the total. The rest are to do with air, water and waste pollution. Dr Man began working on the noise-reduction machine two years ago, after spending 20 years in Britain, where he said the noise levels were considerably lower. The system he developed involves transmitting noise through a loudspeaker and picking it up with a microphone attached to a computer. The computer measures the decibel level and frequency of the noise and sends out anti-noise signals through other loudspeakers to counter the sound waves and reduce the noise. A demonstration in Dr Man's laboratory yesterday reduced the decibel levels from about 75 decibels to about 66 decibels. Every three decibel reduction cuts by half the sound heard by the human ear. ''Most of the noise in Hongkong constantly reaches the 80 decibel to 90 decibel range,'' Dr Man said - or the equivalent of a busy street or construction site. ''This is considered very noisy and what we're doing is not killing off the noise completely, as that would not be practical, but trying to reduce it to something more practical,'' he said. Dr Man said the attraction of the machine was that it acted on low-frequency sounds, such as those generated by ventilation systems, drills and other mechanical equipment, and it was adaptable. Other means of noise control usually were passive, such as putting barriers around the source, and only acted on high-frequency sounds, he said. Also, if a new noise source was added it might not be contained by barriers. Dr Man said his computer could adjust to the extra sound. Experiments have only been conducted in the laboratory to date, but Dr Man said they were now at a point where they could do field trials and were looking for a sponsor for a pilot scheme. The machine costs several hundred thousand dollars and Dr Man said it could be applied to ventilation systems, construction sites, roads or even inside vehicle engines, depending on feasibility studies.