A NOVEL INSTRUMENT TO MEASURE ACOUSTIC RESONANCES OF THE VOCAL-TRACT DURING PHONATION

Acoustic resonances of the vocal tract give rise to formants (broad ba nds of acoustic power) in the speech signal when the vocal tract is ex cited by a periodic signal from the vocal folds, This paper reports a novel instrument which uses a real-time, non-invasive technique to mea sure these resonances accurately during phonation, A broadband acousti c current source is located just outside the mouth of the subject and the resulting acoustic pressure is measured near the lips. The contrib ution of the speech signal to the pressure spectrum is then digitally suppressed and the resonances are calculated from the input impedance of the vocal tract as a function of the frequency. The external excita tion signal has a much smaller harmonic spacing than does the periodic signal from the vocal folds and consequently the resonances are deter mined much more accurately due to the closer sampling. This is particu larly important for higher pitched voices and we demonstrate that this technique can be markedly superior to the curve-fitting technique of linear prediction. The superior frequency resolution of this instrumen t which results from external vocal tract excitation can provide the p recise, stable, effective, articulatory feedback considered essential for some language-learning and speech-therapy applications.