The voicing parameter specifies whether the vocal folds are vibrating. The vocal folds or vocal cords have a pair of muscular bands controlling the flow of air to and from the lungs. The two muscular bands of tissue are stretched from front to back in the larynx, behind the Adam’s apple. In normal breathing, the vocal cords are relaxed and wide apart allowing air to flow freely from the lungs. When the vocal cords are apart, the space in between is known as the glottis. In speech, the right amount of air and tension of the two bands of muscles cause the vocal cords to vibrate. Many speech sounds are produced with the vocal folds vibrating/phonating.
In English, the difference between voiced and voiceless consonants tends to coincide with gentle and strong aspiration; also referred to as lenis and fortis (Underhill, 2005). This means that voiced consonants are expressed with weaker aspiration (force) of air or lenis while voiceless consonants are expressed with stronger aspiration of air or fortis.
This distinction is particularly useful when differentiating English consonant sounds that are essentially uttered in similar manner except one with voicing (vibration) and the other, without. For e.g. the sounds /f/ and /v/ are both labiodental fricatives but /f/ is voiceless and fortis while /v/ is voiced and lenis.