Vocal cord vibration
Voice is sound. Sound production is based on physics. All vocal impairments occur because of a physical change in vibration.
In the idealized situation, sound is made when:
- the back of the vocal cords leave the “breathing in”
- position, moving together until parallel,
- tension is applied to the vocal cords as they initially
- occlude the airway,
- air is propelled through them, usually from below,
- they start flexing open in the middle and
- increasing tension causes the cords to snap back closed.
- The last two steps repeat over and over, creating pulses
- of air – vibration.
Vocal cords vibratory cycle. They are completely closed on the left. They begin to open in the center and on the right they have reached maximal opening before they will begin to close again. The mucosal wave can be seen in the right photo as a curved line just lateral to the edge of the vocal cord (arrows). In a video, this line propagates from the margin and moves laterally.
The vocal cords oscillate quite rapidly, perhaps 100 to 200 times per second during casual speaking, with smaller cords tending toward faster oscillation. In a set of perfect cords, we could characterize them as:
- being open about half the time and closed about half the time,
- letting air out in measured puffs,
- not leaking air during the closed phase and
- vibrating regularly
- vibrating symmetrically.
This creates the sound that we hear and any single note can be visualized on an oscilloscope as a sine wave – a regular vibration and when we hear it, we hear a musical tone. We can talk about the tone in terms of frequency, often measured in Hertz or vibrations per second. Hertz is a common scientific measurement that requires the use of logarithms for calculations.
We can also use a musical scale such as the chromatic scale, composed of 12 equally spaced pitches, to label each tone produced (C3, C3#, D3, D3#, etc). Each succeeding note is one semi-tone higher than the previous. I like this “semi-tone method” for documenting the voice since the visual distribution of keys on the piano separates the sounds into audibly equal intervals without delving into the complexities of logarithms.
Measuring pitch and pitch notation.
When I evaluate a patients pitch by ear, I match the pitch to a note on a piano. Middle C on the piano is C4 – that is the fourth octave on the piano. One octave lower is C3. The numbering changes at C so the notes in an octave would be labeled
C3 C3# D3 D3# E3 F3 F3# G3 G3# A3 A3# B3 C4 C4#…
Though octave means eight and there are eight steps in the Western diatonic scale in music, there are 12 “half-steps” in this chromatic measuring system before returning to the same note (C4 has double the number of vibrations as C3). We describe these as “equal intervals”. To our ear, the distance between C3 and C3# is the same melodic interval as the distance between C4 and C4#, as well as the distance between any other half-step.
However, if one uses the Hertz scale for measurement, the distance of one semitone between C3 (130.81 Hz) and C3# (138.59 Hz) is 7.78 Hz. The distance of one semitone between C4 (261.63 Hz) and C4# (277.18 Hz) is 15.55 Hz. The Hertz scale is a nonlinear, logarithmic scale and not easily added and subtracted. Arithmetic manipulation can be done, but the simplicity of using “half-steps” far outweighs the precision of the Hertz scale for clinical diagnosis.