On a unidirectional loudspeaker system for low frequency range reproduction using ANC technique

摘要

Directivity of an ordinary loudspeaker is far from unidirectional characteristics in low frequency range; a single loudspeaker works similar to an omni-directional sound source. When we would like to build an evacuation guidance system using multiple loudspeakers to guide the exit by speech in a tunnel or a long walkway, a loudspeaker system which does not gain in the back side is preferable. This paper describes the possibility to build a unidirectional loudspeaker system for low frequency range using two loudspeakers configured as back-to-back and an error microphone in front of one of loudspeaker which works as an ANC actuator to reduce the sound power at the error microphone position. According to the model experiments in anechoic chamber, more than 10dB reduction of power level against a single loudspeaker directivity behind the primary-side loudspeaker in the range of 100Hz-200Hz for the direction of+140 to +220 degree when the direction of a primary loudspeaker is defined as 0 degree. At 500Hz, the reduction is obtained for the range of +160 to +200 degree, however, positive gain, thus enhancement due to control, is observed for the range of +60 to +150 and +210 to +300 degree. Although the further improvement is necessary to utilize actual evacuation guidance system, the possibility to control the directivity in low frequency range is obtained.