Acoustic enclosure with active sound transmission control (Investigation of sensing method of reference signals)

Abstract

This paper deals with an active acoustic enclosure for preventing vibration of precision measuring equipment, e. g. Scanning Probe Microscope (SPM) and Scanning Electron Microscope (SEM) etc., due to surrounding sound noise. In our previous work, an effective actuation method in an active sound transmission control was proposed and the high performance of the method was demonstrated both theoretically and experimentally. Moreover, the acoustic enclosure using the proposed actuation method and piezoelectric film sensors has been developed. In this study, the practical acquisition method of the reference signal for the acoustic enclosure is investigated experimentally. In order to get reference signal of the sound waves with every incident angles so that the causality of the control system can be satisfied, the reference microphone is placed above the center of the each panel and the signals from a number of reference microphones are used for controlling the vibration of one panel, which constitutes the Multi Input Single Output feedfoward control system. The relation between the distance from the reference microphone to the panel and the angle of the incident sound wave which can be sensed by the reference microphones is clarified. The experiments in the case that multiple noise sources existed around the enclosure were performed. A control effect of about 10 dB was obtained up to 122 Hz even when the three noise sources existed, which suggests that in this case the weight of the panel of the enclosure can be reduced about one-third according to the mass law.