ACTIVE NOISE REDUCTION SYSTEM FOR AUTOMOBILE COMPARTMENT

In an automobile compartment noise reduction system, an ignition signal transforming circuit processes an ignition pulse signal to obtain a vibration noise source signal with a frequency spectrum composed of 0.5.times.n (integers) order components of the engine r.p.m. as the primary source signal. The signal is applied to an adaptive filter and an LMS calculating circuit via a speaker-microphone transmission characteristic correcting circuit. The primary source signal is synthesized by the filter into a cancel signal and then outputted through a speaker as canceling sound. The canceling sound is received by at least one error microphone at a noise receiving point as an error signal. The error signal is applied to the LMS calculating circuit. The LMS circuit updates the filter coefficients of the adaptive filter on the basis of the primary source signal and the error signal so that the error signal can be minimized. The noise reduction system has high reliability with low cost, and is easy to mount.