There are many consonants that are produced with the glottis held open, as in ordinary breathing. Such sounds are called voiceless (Du: stemloos), and we hear them because other speech organs, usually the tongue or the lips, are involved in their production. Examples of voiceless sounds are [f] and [ʃ] in GA fish and [t] in AN eten.
When the vocal cords are allowed to vibrate, they produce voice. The closed glottis is subjected to gentle air pressure, sufficient to blow the vocal cords apart, but not strong enough to prevent them from falling together again; when they have fallen together, they are immediately blown apart again, resulting in vibration. Opening and closing cycles typically repeat themselves more than a hundred times per second for the larger and laxer vocal cords of men, and over two hundred times per second for the smaller vocal cords of women and children. You produce voice when saying [mː], a voiced sound; now say [sː], a voiceless sound, and feel the difference. Sonorants and vowels (except when they are whispered) are voiced (Du: stemhebbend).
The AN obstruents /b, d, (g); (v), z, (ʒ), (ɣ)/ are also voiced. The lenis obstruents of GA have voiced allophones. GA fortis obstruents are always voiceless.
Types of voice.
Instead of what is called modal voice, which is produced when no particular adjustments are made, it is possible to produce breathy voice. It is produced when part of the glottis is held open and part is allowed to vibrate. (The same effect is apparently produced when the closing phase of the vibration is not complete, so that air can be allowed to flow through with friction during phonation.) Breathy voice (Du: licht hese stem) is not uncommon in Dutch and may be more typical of female than of male speech. In addition, note that AN /h/ is pronounced as a vowel with breathy voice. In English, breathy voice is sometimes jocularly used to create the effect of a sexy voice.
Another special phonation type is creaky voice. It is produced with tensed vocal cords, and sounds as if you can hear the opening actions of the vocal cords separately. (The effect may remind you of the sound produced when you run your fingernail along the teeth of a comb.) Creaky voice is also known as vocal fry, and has been popular for some time now among GA-speaking (young) women who break into creak at the ends of their utterances, when the pitch is very low.