| 摘要 |
<p>A method for active noise reduction based on destructive interference of sound waves in order to reduce the energy in a sound field employs two omnidirectional microphones (M1, M2) provided in connection with a loudspeaker. By means of the microphones the acoustic feedback is eliminated by a closed loop consisting of the microphone and the loudspeaker. The loudspeaker used is an open loudspeaker with a dipole characteristic, thus causing one of the microphones to be more sensitive to the far field and thereby to the noise which has to be suppressed. The method is implemented by means of a device which comprises a digital signal processor for processing the microphone signals and which transmits an output signal to the loudspeaker where the feedback component from the loudspeaker is substantially eliminated, while the output signal's phase and amplitude are adjusted in such a manner that an effective cancellation of the noise is obtained in a area around the loudspeaker's near field. The digital signal processor can preferably be implemented in the form of software modules on an integrated circuit. With the method and the device an integrated reduction in noise level of almost 20 dB is achieved depending on how the filtering in the digital signal processor is adapted. In practice a quiet zone can be obtained in the loudspeaker's near field with an attenuation band which extends from approximatively 100-500 Hz.</p> |
