The nasal passages are resonating chambers which help colour the sound of your voice both in speech and in singing and permit you to articulate nasal consonants such as m, n and ng which is written as [ŋ] in IPA (International Phonetic Alphabet). [ŋ] is similar to the french [ɲ]. French also has nasalized vowels which we do not have in English namely [ɑ̃, ɔ̃, ɛ̃, œ̃].
The sinuses are spaces in the bone of the skull that are mainly full of air. There are four pairs of sinuses making eight all together and all are attached to the nasal cavity by small openings. The sinuses are also chambers that resonate to a lesser degree than the nose. I know of professional singers who are missing a sinus and it does not diminish the beauty of their voice in any way.
The lining of the nose and sinuses produces a clear fluid called mucus which constantly cleans them of unwanted material. This fluid passes through the nose where it is expelled either by sneezing, blowing your nose or draining to the throat where it can enter the mouth and be coughed out or swallowed. This drainage to the throat happens continually, although we are usually unaware of it. When excess fluid is produced it is often known as phlegm, or catarrh. It can produce a chronic irritation in the throat known as post-nasal drip which can irritate your vocal cords, sometimes causing mild swelling. The two main causes of inflammation are from allergies and infection.
The nasal cavities & sinuses are empty spaces in the head which are resonating chambers for the voice, located within bone and cartilage. Since they are not soft tissue, they cannot be manipulated in any way. They are simply present and balance the voice in both speech and singing.
Some teachers try to further access nasal resonance by various manipulations of the soft palate. These teachers erroneously think that there is air passing through both the nose and mouth when you sing and that putting more air through the nose will add nasal resonance or focus to your overall sound. This is simply not true and a waste of your time. The soft palate closes off the nasal cavity for most vowels and consonants and should not be played with. Any attempts to change your sound along these lines will add tension to your voice and should be avoided (see Soft Palate).
The only thing you can do is keep the nasal cavities & sinuses from becoming blocked and by trying to avoid excessive postnasal drip. Allergies and infections are not always avoidable. Medications to dry out the excess phlegm and/or catarrh also dries the epithelial lining of the vocal cords which is not good for singing and should be avoided where possible. There are various natural remedies to unblocking the nasal cavity which are preferable to allergy medications and nasal sprays (see Nose & Throat Care).
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