Laryngeal narrowing measured with low frequency sound.

We measured laryngeal resistance with 800 Hz sound in human subjects. 800 Hz sound was forced into the mouth and the intensities of sound above and below vocal cord were detected by two microphones on the anterior neck. Respiratory resistance (Rrs) was simultaneously measured by an oscillation method. When the subject voluntarily controlled the glottis aperture at functional residual capacity, the percent increase of difference of intensity of sound between two microphones (Y %) was curvilinearly proportional to the increase of Rrs (X) from the control state; Y = 20.8 X X 1.24. Since Rrs below the larynx was constant at a given lung volume, we could estimate the increase of laryngeal resistance from the control state by knowing the percent increase of difference of intensity of sound. The present method would be clinically applicable to measure Rrs independent from laryngeal artifact.