Voicedness and voicelessness or the state of the glottis
Activity: Place your fingers on your larynx (that is the part of your neck referred to as the Adamís apple with boys) and slowly say the words serve and surf while concentrating on the last sound. Do you feel the difference?
When saying serve you can feel vibrations beneath your fingers whereas you do not feel them when you say the word surf. Why is that so?
All English sounds, as they are egressive-pulmonic, have to pass through the glottis (the gap between the vocal folds in the voice box) on their way to the oral cavity.
If the vocal folds are apart, there is a gap between them and the glottis is open. Air coming from the lungs can pass relatively freely through. These sounds are called voiceless.
If the vocal folds are close together, the glottis is narrow and the upcoming air has to force its way through causing the vocal folds to vibrate. This is what you can sense with your fingers if you place them on your larynx when you say the last sound in the word serve. These sounds are called voiced.
If the vocal folds are pressed together, the airstream is cut off and glottis is completely closed. This is called a glottal stop. It is not a distinctive feature in the English language. It does, however, occur in a British accent. In the so-called Cockney Accent the glottal stop is used as an allophone for the sound /t/ and, though less frequently, for /k/ and /p/ causing the words light and like to sound identical.
Intensity of Articulation: Breath Force
The state of the glottis is, however, not the only difference between voiced and voiceless sounds. They also differ in the force with which the airstream is expelled from the lungs.
Activity: Put your hand in front of your mouth and pronounce the words cap and cab, heart and hard, laugh and love while concentrating on the respective last sound. What do you notice?
You should be able to sense a stronger airstream at the end of the words cap, laugh and heart than at the end of cab, love and heart.
Now whisper the words. In whispering all sounds become voiceless. But even then you can still distinguish between them. Why?
The reason is that voiced and voiceless sounds are usually accompanied by a difference in the force with which the air is pushed up from the lungs, their intensity of articulation. Voiced sounds are usually accompanied by weak or lenis breath force (as in cab, hard and love) whereas voiceless sounds are created with rather strong or fortis breath force (as in cap, heart and laugh).
Fortis sounds are always voiced as they require higher muscular tension. Lenis sounds are usually voiceless. They can, however, be devoiced (which means that they can be articulated voicelessly), for example when they occur at the end of a word (especially in connected speech).
Thus, the last sound in hard is always pronounced with weaker force than the last sound in heart and that is why we can still distinguish between them although the main criterion voice does not apply anymore when the sounds are whispered.