(1) Field of the Invention
The present invention relates generally to a system for reducing noise sounding in a closed space due to a propagation of a vibrating noise in the closed space from a vibration source of a power unit (such as a vehicular engine) mounted in the vehicle (hereinafter referred to as a closed sound). Particularly, the present invention relates to the apparatus and system for reducing the closed sound sounded in the closed space due to the propagation in the closed space from the vibrations of the power unit which is easy in adjustment during an initial power supply of the apparatus.
(2) Description of the Background Art
Noises on secondary harmonics of the engine revolution speed, i.e., frequency components of an ignition frequency of a vehicular engine are propagated in the vehicular compartment of the vehicle, an unpleasant feeling is given to a vehicular driver.
Therefore, at least one speaker is mounted in the vehicular compartment to output a sound having a 180.degree. opposite (out of phase) to the closed noise so as to cancel the closed noise.
A U.S. patent application Ser. No. 07/629, 637 exemplifies a previously proposed active noise cancelling system.
A PCT international patent application international publication W088/02912 exemplifies a previously proposed closed sound cancelling system. In the case disclosed in the PCT international patent application, since, at first, a white noise is output in the vehicular compartment and is detected through the microphone so that a spatial acoustic transfer characteristic in the vehicular compartment including an electric/acoustic conversion characteristic (function) between the microphone and speaker is identified. A value of the spatial acoustic transfer characteristic is stored into a memory. Using the stored content values, a plurality of filter coefficients to provide the 180-degree opposite sound by means of a steepest descent method in an adaptive digital filter are adaptively changed to reduce the closed sound. A volatile memory such as RAM is always used to update the stored values as the memory storing the spatial accoustic transfer characteristic.
In a case where the closed noise in the limited space is reduced using the previously proposed noise cancelling system disclosed in the above-identified PCT international publication (under PCT Article 20), the identification of the values on the spatial acoustic transfer characteristic using again, the white noise needs to be carried out when the power supply to the volatile memory is turned off and the content of the memory is accordingly erased or destroyed. If this previously proposed noise cancelling system is applied to an automotive vehicle, the content of the memory is destroyed when a battery of the vehicle is disconnected to repair or replace the battery. This requires a new setting operation for the previously proposed noise cancelling system by a user. Consequently, the reduction of the noise cancelling system into practice to the automotive vehicle becomes difficult.