Consequently, the quality of the vowels being sung is crucial. This has to do with chiaroscuro. Chiaro = clear and Scuro = dark. We can certainly understand someone's vowels even if they are overly bright and nasal. That doesn't mean we want to hear the vowels sung that way. For example, think of the character Olive Oil from Popeye. We can clearly understand her worlds and vowels, but it is not a nice sound for singing. Conversely we might also be able to understand words if someone is singing overly dark and woofy, lacking clarity, but we also do not necessarily want to hear those sounds being sung either. The ideal sounds are vowels which are dark, having full resonance, but also that are clear having what the Italians called squillante. Squillante is the overtones produced that give a "ringing" quality to a singer's voice. Those overtones are shaped by the vocal tract - mainly the position of the tongue - in order for the vowels to be clearly distinguished. You can test this out yourself by simply looking in the mirror at your tongue while you say or sing "AH", "EH", "EE", "OH" in succession. You will see that the tongue must move to form the vowel. Some of the things that can happen which distort the singing voice are that the vowel is not properly balanced, it is not centered directly in the pitch, the vowel sound fluctuates while holding the notes, and/or several vowel sounds are added which are unnecessary; e.g., diphthongs. A diphthong is when two vowel sounds are added together to form a word. For example, the word "SAY" is made up of the vowels EH and EE. S..EH...EE. In singing we have to be careful with diphthongs as they can distort the voice. Imagine singing the word SAY and instead of holding out the EH part of the word you go right to the EE part. That would be very odd sounding to an English speaker. Also, many times a singer might hold a note on a vowel and while they are holding the note the vowel does not maintain a consistent sound. Take the vowel "AH" in the word father. Imagine holding the AH and instead of it staying clearly AH we hear AH....UH....UHL....AH. Inconsistent in other words. That will make the singer sound quite unskilled and it will cause the listener to feel unsettled. As singers we should have in our minds and in our aural image that the proper vowel sound (chiaroscuro) is what sounds the pitch. Just as as piano player strikes the middle C key with their finger, the singer should be sounding the same pitch with their voice by the vowel. Listeners can easily start distinguishing great singing from mediocre singing just by this one fundamental element alone. The following clips are great examples of the vowel and pitch quality in the signing voice:
Now with the men singing you will notice as they go into their higher range there is a bit of a change in the sound. That is called covering. It is a muscular switch that happens which allows the sound to remain dark while keeping full voice all the way up to the highest notes in their range. The vowels should, however, remain clearly understood. Just as we can understand vowels spoken in a myriad of ways; i.e., nasal, bright, dark, light etc. we should still understand the vowels in the upper range even when covered. You will see that with Del Monaco you can still understand his words even in the top of his voice. One thing that is taught today is the idea of "vowel modification". This is coming at it the wrong way where the vowels are not understood in the top. Instead the "covering" switch should happen and the vowels should still be clear. This means that one must be taught how to cover properly and that is not by distorting the vowel. In classical women's voices you will notice that in the very highest notes - from about F or g 5 and above - you can no longer understand any vowels except AH or UH. That is due to the fact that the pitch is higher than the overtones needed to form a clear vowel sound. Still, women should be clearly understood in everywhere else in their range. Women who sing non classically should be understood the same way men are understood since they use a similar setup.