Adaptive Noise Cancellation for Speech Recorded During Magnetic Resonance Imaging: Formant Extraction and Analysis
Keywords:
speech recording, magnetic resonance imaging, noise reduction, adaptive comb filtering, formant extraction, vocal tract, acoustic artifacts.Abstract
Speech recordings obtained during Magnetic Resonance Imaging (MRI) of the upper airways often suffer from acoustic noise generated by the MRI scanner. This paper focuses on the post-processing of such speech samples using adaptive comb filtering to achieve accurate formant extraction. Two types of speech materials were used to validate the proposed algorithm: prolonged vowel productions recorded during MRI and comparison data recorded in an anechoic chamber. Spectral envelopes and vowel formants were computed from the post-processed speech and the comparison data. Additionally, numerical acoustic models and 3D printed vocal tract physical models were used for further analysis. The results reveal a significant frequency-dependent discrepancy between the vowel formant data obtained from recordings during MRI and the comparison data. This discrepancy is attributed to the acoustical changes caused by the surfaces of the MRI head coil, leading to "exterior formants" at frequencies around 1 kHz and 2 kHz. The observed discrepancy is too substantial to be disregarded when using MRI recordings for parameter estimation or validating numerical speech models based on MR images. However, the influence of test subject adaptation to noise and the effects of constrained space acoustics during an MRI examination cannot be ruled out.
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