So that is the science of vocal resonation. Now I'd like to talk about it in terms of practical application.
What does vocal resonation do for a voice?
In a nutshell, it gives the voice access to tone color options. We use different vocal tones to deliver different kinds of spoken or sung messages. While there is communication that calls for thin, harsh, whispery or dark tones, most confident sounds are richly resonant, and so most speakers and singers will be needing full access to all vocal resonators (larynx, mouth, pharynx, nose, and some say chest).
What can a singer or speaker do to gain access to vocal resonators?
1. Open the pathway from the larynx to those resonators. I call this pathway the open throat. Tall, flexible posture is everything... head forward and crunched into shoulders will tighten this pathway. This is why in my Power, Path and Performance vocal training method I give fully 1/3 of my teaching to the voice path through the open throat, which should open up, down and back.
2. Use enough breath correctly and efficiently to sound those resonators. This calls for a balance between breath support (air you are sending to your vocal cords) and breath control (air you are witholding from your vocal cords).
3. Stay flexible in your spine and jaw and don't tense your shoulders. Your resonation will and should move around as you hit different pitches.
4. In performance... set yourself up well and stop thinking! After you have opened and powered your voice, focus your intention to communicate a specific feeling. Trust this clear intention to choose from the tones your resonation zones make available!
How to feel your resonation zones:
Put your hand on your chest... you'll be starting there. Making a sliding siren sound (ooooooo) from your low chest voice up into your head voice. Make sure you keep a loose jaw and pull this sound instead of push it or your voice may break. Notice where you feel the vibration occuring... it should change places from your sternum, through your facial mask and up to the middle of your upper head. Notice that if you freeze you will make it hard for the vibration to smoothly travel.
What happens if your vocal cord vibrations don't reach your resonators effectively?
Pretty simple: Your voice will sound like a cheap instrument instead of an expensive one! More correctly, it's like you have great speakers but you haven't plugged them in. Consider playing an electric guitar not plugged in vs playing that same instrument plugged in to an amplifier. You get the idea.
OK, any specific questions about resonation I didn't cover here (or wasn't in the Wikipedia article)?