T. This can be expressed simply as the percentage of time in each cycle during which the folds are open. This is the portion of time in each cycle during which the folds are moving outward , divided by the time in which they are moving inward. The shape of the glottal waveform helps to determine the loudness of the sound generated, and also its timbre. In addition to the desired vocal sound produced by the larynx, other sounds are also often produced. Turbulence , a 'non-smooth' flow in the glottal airstream, can create a 'hissing' sound, which is called aspiration if it is combined with sound from vocal fold vibration, and whisper if it is not.
Aspiration is a contributing factor to 'breathy' voice.
Other sources of extra sounds in the larynx include glottal 'clicks' or 'stops', which are produced by an especially effortful and sudden opening or closing of the vocal folds. Also, any extra mucus or other fluids which are on or near the vocal folds can produce a rough or 'gurgly' noise as they move around during phonation. Audible Frequencies in Sound Sources Generally, human detection of sound is limited to waves having frequencies from 20 Hz to 20,000 Hz. The precise limits of hearing vary on an individual basis, and tend to degrade with age and other causes of hearing loss. Audiophiles enhance these less discernable frequencies with various graphic equalizers, woofers, and tweeters.