Vocal cord vibration
Voice is sound. Sound production is based on physics. All impairment of voice comes down to a physical change in vibration.
In the idealized situation, to make a sound:
- 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
- increasing tension causes the cords to snap back closed
Steps four and five repeat over and over, creating pulses of air or vibration.
They oscillate quite rapidly, perhaps 100 to 200 times per second during casual speaking, with smaller vocal cords equaling faster oscillation. In these 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 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 or vibrations per second. Hertz is a common scienfic measurement, though we can also use a scale such as the musical, western chromatic scale to label each tone produced (C3, C3#, D3, D3#, etc) as one semi-tone higher than the previous. This “semi-tone method” corresponds to the visual distribution of keys on the piano that separates the sounds into audibly equal intervals without delving into the complexities of logarithms.
Voice can be thought of as the underlying signal on which the speech or “information” is carried. The signal has two predominant characteristics that can be altered by the larynx. They are pitch and volume.
The vocal cords are quite good at putting out a strong signal if desired. A well-produced sound can carry information quite a distance, such as crying baby in church, a young girl screaming, or an opera singer still heard clearly in the cheap, upper balcony seats.