Abstract: 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.

Abstract: In an automobile vehicle, fuel injection pulse outputted from an electronic control unit of an engine is transformed into a voltage value as a primary source. The voltage values is transformed by the convolution sum process and synthesized into a sound to cancel the engine noise. The synthesized sound is outputted through the speaker in the passenger compartment. On the other hand, the reduced noise sound is detected as an error signal by the microphone. The error signal is inputted to an LMS circuit where the filter coefficient is updated in accordance with both inputted error signal and the primary source. Since the fuel injection pulse before the combustion stroke is used, the noise sound can be precisely and immediately reduced even in the transient conditions of the vehicle operation.

Abstract: An ignition pulse shaped and demultiplied is used as a primary source and this primary source is converted into a canceling signal after being subjected to the sum of convolution products process with filter coefficients of an adaptive filter. Further the canceling signal is converted into a canceling sound by a speaker and outputted to the passenger compartment to cancel a vibration noise at a noise receiving point. The state of noise reduction is detected as an error signal by a microphone and the error signal is inputted to an exponential averaging circuit where the error signal is exponentially averaged with previous error signals by a trigger signal of the primary source from a trigger signal generating circuit. The error signal, as a result of this averaging, is compressed and then outputted to a LMS operational circuit. In the LMS operational circuit the filter coefficients are updated based on the primary source inputted via a C.sub.MNO circuit and the compressed error signal.