Source-Filter Interaction in Speaking and Singing is Nonlinear

Many aspects of speech production have been successfully described by a linear sourcefilter theory. Linearity implies that the source-filter combination, which provides the acoustic output at the mouth, can be characterized by mathematical convolution in the time domain (or by multiplication in the frequency domain) of two independent functions representing the source and the filter. With this assumption, the filter cannot influence the source to produce new frequencies or change the overall energy level of the source. It has recently been shown, however, that this assumption is generally not valid, and only under certain conditions is at best an appropriate simplification. The linear theory is applicable to male speech, less to female and child speech, and much less to singing. As long as the dominant source frequencies lie well below the formant (resonance) frequencies of the vocal tract (which occurs in male speech), glottal airflow is influenced only in simple ways by the filter, mainly in terms of flow pulse skewing and pulse ripple. But when formants and harmonics cross, bifurcations in the glottal airflow and the dynamics of vocal fold vibration can occur that may generate secondary frequencies and even change the energy level at the source. It is hypothesized that humans (and perhaps many animals) have the ability to operate their source filter system with either linear or nonlinear coupling. One way to express the degree of coupling is through the relative impedances of the source and filter. For linear source-filter coupling, the source impedance is kept much higher than the input impedance to the vocal tract. This linear coupling is accomplished by adducting the vocal folds firmly and widening the epilarynx tube (a narrow region of the vocal tract above the vocal folds, also known as the laryngeal vestibule; see arrow in Figure 1). The glottal flow is then determined strictly by aerodynamics, while acoustic pressures above and below the glottis have little influence on either the transglottal pressure (which drives the glotVolume 17, Number 3 Summer 2007