Vowel equalization is a technique that can be used by singers to achieve a more balanced vocal resonance, or chiaroscuro, by balancing corresponding front and back vowels, which share approximate tongue heights, and also high and low vowels by means of a more neutral or centralized lingual posture. Vowel equalization helps singers increase their voice projection while at the same time reducing strain on their vocal cords.
To do an exercise in vowel equalization, a singer quickly alternates between near and distant vowel sounds, which forces the tongue to find the most efficient movement between vowels. For example, a singer may alternate quickly from the "ee" sound in "feet" to the "you" sound in Utah. The exercise may sound a funny, but should produce an EE-YOU-EE-YOU-EE-YOU sound. After doing this for a few seconds, the vibrations in the mouth, which have vacillated from front to back, will stabilize somewhere in the middle. When that happens, these vowel sounds have been successfully equalized.
Another exercise involves moving through the sounds EE-AY-AH-AW-OO. The first three should be sung without moving the lips at all, in a smile position. All five vowel sounds should be sung in the same, level tone, moving smoothly from one to the next.
Through exercising, the vowels are sensed in a different place in the mouth and the vowels can be heard in a different place in the mouth. The movement of the tongue becomes more subtle, allowed the different sounds to become more connected.