Design of Apparatus for Studying Aerodynamics of Voice Production

Abstract

The design and testing of an experimental apparatus for in vitro study of phonatory aerodynamics (voice production) in humans is presented. The presentation includes not only the details of apparatus design, but flow visualization and Digital Particle Image Velocimetry (DPIV) measurements of the developing flow that occurs during the opening of the constriction from complete closure. The main features of the phonation process have long been understood. A proper combination of air flow from the lungs and of vocal fold tension initiates a vibration of the vocal folds, which in turn valves the airflow. The resulting periodic acceleration of the airstream through the glottis excites the acoustic modes of the vocal tract. It is further understood that the pressure gradient driving glottal flow is related to flow separation on the downstream side of the vocal folds. However, the details of this process and how it may contribute to effects such as aperiodicity of the voice and energy losses in voiced sound production are still not fully grasped. The experimental apparatus described in this paper is designed to address these issues. The apparatus itself consists of a scaled-up duct in which water flows through a constriction whose width is modulated by motion of the duct wall in a manner mimicking vocal fold vibration. Scaling the duct up 10 times and using water as the working fluid allows temporally and spatially resolved measurements of the dynamically similar flow velocity field using DPIV at video standard framing rates (15Hz). Dynamic similarity is ensured by matching the Reynolds number (based on glottal flow speed and glottis width) of 8000, and by varying the Strouhal number (based on vocal fold length, glottal flow speed, and a time scale characterizing the motion of the vocal folds) ranging from 0.01 to 0.1. The walls of the 28 cm × 28 cm test section and the vocal fold pieces are made of clear cast acrylic to allow optical access. The vocal fold pieces are 12.7 cm × 14 cm × 28 cm and are rectangular in shape, except for the surfaces which form the glottis, which are 6.35 cm radius half-circles. Dye injection slots are placed on the upstream side of both vocal field pieces to allow flow visualization. Prescribed motion of the vocal folds is provided by two linear stages. Linear bearings ensure smooth execution of the motion prescribed using a computer interface. Measurements described here use the Laser-Induced Fluorescence (LIF) flow visualization and DPIV techniques and are performed for two Strouhal numbers to assess the effect of opening time on the development of the glottal jet. These measurements are conducted on a plane oriented perpendicular to the glottis, at the duct midplane. LIF measurements use a 5W Argon ion laser to produce a light sheet, which illuminates the dye injected through a slot in each vocal fold piece. Two dye colors are used, one for each side. Quantitative information about the velocity and vorticity fields are obtained through DPIV measurements at the same location as the LIF measurements.