Effective position of reference microphone of active noise control for acoustic noise of magnetic resonance imaging

Acoustic noise produced by magnetic resonance imaging (MRI) is over 100 dB. Ear protectors, such as earmuffs or earplugs, are insufficient for reducing the subjects' discomfort. The reference microphone position is the outside of the ear pad in case of general active noise control (ANC) headphones. We propose an effective position for the reference microphone for ANC for the subjects undergoing lower limb examination via MRI device. The MRI noise source is shown to be near the center in the hole of the MRI device. Here, we discuss the effect of the reference microphone position in ANC of the feedforward-type filtered-x normalized LMS algorithm. The normalization of the input signal is needed because this signal comprises unsteady pulsed MRI noise. The noise was simulated in a laboratory with the reverberation to evaluate the effects of the proposed method. The simulated result showed that controlling the sound-source position is more effective than the controlling the conventional ANC headphone position. The experiment showed that the proposed setup reduced the noise level by 3 dB more than that of the conventional one, just as predicted by computer simulation.