ACTIVE NOISE-CONTROL FOR TRACTOR OPERATOR

Active noise control (ANC), which is one of the active control methods applied to reduce noise using the interference of sonic waves, aims at suppressing the noise around the operator of an agricultural tractor. By several simulations of adaptive system using an adaptive digital filter (ADF), the possibility of noise reduction by ANC was confirmed. Results of simulations showed that the amount of noise reduction for sound pressure level (SPL) would be about 15 dB, which indicates that only one-sixth of the primary sound pressure may remain under ANC. An evaluation system of ANC was developed. 'Filtered-x LMS (least mean square)' algorithm was applied to this system to carry out the noise reduction. Its ability to reduce noise was confirmed by experiments with two tractors. Amount of noise reduction was about 10 dB at control point. Three target systems were developed to reduce the noise around the tractor operator: one based on the same method of feedforward control as that of the evaluation system, the others based on the method of feedback control. The effect on the reduction of noise was also confirmed. Amount of noise reduction was 4-7 dB at operator's ear for SPL, which was equivalent to 1-2 dB (A) for A-weighted SPL because the effective frequency range of ANC was less than 500 Hz.