From the larynx the airstream enters the speech tract (Du: aanzetstuk), a tube extending all the way from the vocal cords to the lips and/or nostrils. It consists of the pharynx (Du: keel), the mouth, and – assuming that the soft palate is down – the nasal cavity. The soft palate (Du: zachte gehemelte) is a valve which closes off the entrance to the nasal cavity when it is pressed up, but opens the cavity when it is allowed to hang down, as in ordinary breathing. (When we have a cold, the entrance to the nasal cavity may be blocked by mucus, which forces us to breathe through the mouth.) It is in this speech tract that the airstream coming from the larynx is (further) modified so as to produce all the different shades of sound – in vowels as well as in consonants – that we can produce.
In essence, there are two factors that play a part in bringing about these differences: resonance and friction.