ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

112.1.2 The glottal impedanceThe glottal impedance differs from traditional acoustical or mechanical impedanceexpressions in that it is defined as the ratio between the transglottal pressure andglottal airflow. It is a time-varying quantity with both steady (DC) and unsteady(AC) components. Many studies have helped to quantify each of these componentsusing two main approaches: experiments with either steady or pulsatile flow. Theformer has been more successful in describing glottal impedance by directly estimatingits steady behavior and making use of the quasi-steady assumption to describe itsunsteady component. The quasi-steady assumption states that unsteady representationscan be described as a sequence of steady scenarios. Attempts to estimate theunsteady component directly using pulsating flow, normally obtained by an externalacoustic excitation and a fixed glottal orifice, are not able to reproduce importanteffects that are a product of the periodic interruption of the flow at the glottis. Thus,pressure buildups and other important effects in the system are not taken into account,which typically makes these investigations less physiologically relevant.The first experiment to describe the transglottal pressure drop using steadyflow [76] was known to misrepresent the geometry of the larynx and was later found tocontradict results from more detailed and adequate experiments [77]. The early studiesfrom van den Berg [78] provided the foundations for many future investigations,as the study linked its experimental results to the Bernoulli equation to describe thetransglottal pressure relations. In these experiments, the properties of the glottalimpedance were investigated using a model casted from a normal human cadavericlarynx. The glottis was constructed to form a rectangular orifice that was maintainedstatic during the experiments. The resulting glottal impedance from this experimentconsisted of a nonlinear glottal resistance (flow dependent) with viscous and kineticcomponents. The equation describing these results is given by:Z g = 12µhl g d 3 + k ρU g2(l g d) 2 = 12µhl2 gA 3 g+ k ρU g, (2.1)2A 2 g