Effects of reference microphone position on active sound reduction across an acoustic plenum window

• Effects of reference microphone location on active sound transmission reduction across a plenum window was studied. • Effects of regular error microphone array settings and acoustic modes were examined. • Maximum 6.8 dB improvement in sound transmission loss between 100 Hz and 1 kHz was recorded. • Selection of reference microphone location could be done coherence-based. • Experimental results are inline with transfer function analysis. An investigation on the possible effects of reference microphone position on the improvement of sound reduction across a practical acoustic window by active noise control method was carried out in the present study. Error microphones were located over the indoor window opening, while the cancelling sound sources along the periphery of the window void started from the lower corner of the outdoor window opening to the overlapping region of the window. The analysis was done using experimentally measured transfer functions of all related sound transmission paths. In general, the active control performance is better when more error microphones are adopted. The best sound reduction improvement reaches 6.80 dB over the frequency range from 100 Hz to 1 kHz when the reference microphone was mounted midway on the external window vertical frame. The control performance is weakened at a reduced number of error microphones in general. However, it is found that a simple system with three error microphones and two cancelling sources can achieve an average 5.2 dB broadband sound reduction improvement, when the reference microphone was located at the top of the outdoor window opening. While the former reference microphone position performs better when more error microphones and cancelling sources are adopted in a control system, the latter works better with simpler systems. Also, the present results suggest that it is better to keep the number of microphones larger than that of the cancelling sources and indicate the possibility of a coherence-based approach for locating reference microphone.