Found this small extract
"Vowel sounds originate from the vibration of your vocal folds. Consonants are created with an exhaled air stream and are formed by your mouth. If stressed, consonants will push out too much air and tense the muscles in your throat and mouth. This condition makes it difficult for your voice to work well. In response, your may find yourself additionally tightening throat and tongue muscles in a fight to make the note. This will produce strain, choke your sound, sing off pitch, miss the note entirely, run into register break, and result in vocal fatigue. The problem usually increases as you sing higher and louder. Vowels, worked with correctly, will relax the acoustic chamber of your throat and mouth and increase your volume through resonance. Consonants should not be stressed as you sing. Let the vowels take the spotlight."
Little easy science that should help!