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Unvoiced sounds are produced when the vocal cords are open and air passes through the mouth, but the sound remains muted until it strikes the teeth and lips. Surrounded sounds can affect the movement of the vocal cords, and a dull sound can occur when the listener perceives a loud sound. Linguists must be careful to distinguish between unaspirated and voiced stops. Spectrograms can visually display the airflow produced in speech.
An unvoiced sound is one in which the vocal folds, or vocal cords, are open, allowing air to pass from the lungs without restriction until it strikes articulators such as the teeth and lips. The vocal cords are two pieces of tissue found in the larynx in the throat. The two folds lie open in a V shape, and as they close over the larynx, the rising air causes them to vibrate together, producing a sonorous sound. If the pleats don’t move or close completely, they don’t vibrate.
All sound action is at the vocal cords. Once air enters the mouth, the articulators can add friction or temporarily cut off the airflow, but the sound remains muted. This quality may be the only audible difference between words in many cases, such as “ton” and “done,” where “t” is voiceless and “d” is voiced.
The movement of the vocal cords can be affected by surrounding sounds, adding and subtracting voice if one sound is surrounded by others that have the opposite quality. For example, if voiceless consonants surround a voiceless vowel, the vowel may lose its sonority and become deaf. This doesn’t happen in all languages, but it does happen in some, like Japanese. There’s nothing wrong with the vocal folds: they just don’t close as much as they normally would because the two surrounding sounds require them to stay open.
A dull sound can also occur when the listener perceives a loud sound. When the articulators block the air from moving, the sound this produces is called a stop or plosive, such as a “t.” In languages like English, sometimes a non-voiced stop has an extra puff of air following its pronunciation, which is a phenomenon called aspiration. Not all languages have aspirated sounds, however, and for those accustomed to hearing only aspirated unvoiced sounds, unaspirated unvoiced sounds can actually sound voiced even if the vocal cords are separated. Linguists transcribing languages must be very careful to distinguish between unaspirated and voiced stops.
The lack of pitch and vibration in a dull sound is evident when the sound is detailed on a graph called a spectrogram. This is a type of recording in which the airflow produced in speech is visually displayed. Dark bands indicate voiced sounds, but unvoiced sounds appear as lightly shaded or unshaded areas, depending on how much any nearby voice affects the vocal folds during the production of the unvoiced sound.
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