Interpreting Lx waveforms

• The normal Lx waveform can be related to vocal fold vibration:
  
  
  Vocal fold closure sequence.
  
  
  
  Laryngograph waveform.
  

  Notes:

  1. The closure/opening sequences are regular. Normal vocal folds have similar mass, shape and stiffness.
  2. The folds close faster than they open (the Bernoulli effect). The rising edges on the Waveform are steeper than the falling edges.
  3. Each cycle exhibits a similar shape for the closure-opening sequences.
  
  
  
• Lx waveforms provide the basis for a physical interpretation of pathological voice conditions. The following are some of the features which can be investigated: vibration REGULARITY; CLOSURE definition, OPENING definition; OPEN/CLOSE phase ratio; CLOSURE/OPENING sequence shape, and these in turn can indicate vocal fold: MASS asymmetry SHAPE asymmetry; STIFFNESS asymmetry; the POSITION of nodules or polyps (in a vertical plane); and whether treatment/therapy is affecting a return towards normality.

Calibration technique for the Laryngograph

The Laryngograph monitors vocal fold closure by measuring variations In the conductance between a transmitting electrode delivering a high frequency signal to the neck on one side of the larynx and a receiving electrode on the other side of the larynx. The opening and closing of the vocal folds, by varying the conductivity of the path between the electrodes causes an amplitude modulated version of the transmitter signal to reach the receiving electrode. The Laryngograph demodulates this signal and thus derives a waveform representing the variations in conductance. The relationship between this waveform the vocal fold opening and closure cycle is thoroughly documented elsewhere.

Calibration of the Laryngograph involves the use of a dummy neck; an electrical circuit designed to model the conductivity of the neck and the modulation of this conductivity by the vibration of the vocal folds. In the dummy neck the conductivity of the circuit is modulated directly by an electrical signal. Comparison of the modulating signal with the consequent Laryngograph output makes possible all the standard test procedures for deriving the response of a circuit in the frequency or time domains. See attached frequency, phase and time responses for the Laryngograph with normal, fast and Gx time constants.

A further calibration and correction technique is available when the laryngograph waveform can be digitised and stored and processed by a computer. The laryngograph "Waveforms" software and A-D unit include a program for low frequency phase distortion calibration and correction. This program can be used to correct low frequency phase distortions of the time domain waveform whether introduced by the Laryngograph or by any recording apparatus. Essentially this technique involves comparison of the waveform output by the system being calibrated with a rectangular waveform used as the reference input waveform. Fourier series expansions are made of the distorted and ideal waveforms, and the magnitude and phase ( and hence phase delay ) of each of the first sixty four harmonics found. By comparing the two sets of phase delays, an algorithm determines the delays suffered by the components of the distorted waveforms. A model of the phase distortion is produced in the form of two filter parameters, which, when converted to digital filter coefficients can be used for correction purposes. The result is a Laryngograph waveform which can be used to determine with certainty features such as the exact point of vocal fold closure.