Humming is one of the most effective exercises for practicing head resonance.
Humming is a kind of singing in which the voice is redirected into the nasal cavity. The voice does not exit through the mouth but through the nose. The purpose of this exercise is to eliminate the palatopharyngeal closure: the path through the mouth is blocked and so we automatically open the path to the nasal cavity P31. Leading the voice this way achieves the setting of the head resonance much more easily than when singing vowels.
A quality tone of a hummed or sung vowel requires more than the mere elimination of the palatopharyngeal closure; it also requires the soft palate to move upward - what we call the arching of the soft palate. This position, along with a relaxed jaw and chin slightly pushed back, greatly increases the space within the oral cavity. Maintaining the connection with the nasal cavity also gives us two spacious and cooperating resonators.

We distinguish between open and closed humming.
As the name suggests, closed humming is performed with a closed mouth while in open humming, the path through the mouth is blocked by the tongue, in the rear palatal position for N.

The most frequent mistakes while humming:
when the jaws are clamped together in closed humming D86, or
when the chin is shifted forward in both types of humming D87, or
when the soft palate is not lifted enough, or
when the tongue is placed incorrectly.

The tongue is placed incorrectly when its tip is not anchored behind the lower incisors and the tongue mass is concentrated at its base.
The difference is best understood by looking at the following two figures, showing a correct and an incorrect singing of the vowel ĀH (when singing the vowel ĀH the tongue position is similar to closed humming). The two fundamental differences between the correct and incorrect example of singing are the palatopharyngeal closure and the tongue position. During the correct tone formation, the tongue is concentrated at its tip, which is anchored behind the lower incisors P34. During an incorrect tone formation, the tongue is concentrated at its base and the tip is not anchored P35.
Another mistake may lie in the fact that the tongue is not relaxed. The tongue stiffness transfers to the entire singing apparatus.

Correctly performed humming follows the principle:
"The nose must be in the tone, but the tone must not be in the nose!"
This epigram means that thanks to the resonance form (arching of the palate, connection of both cavities), we vibrate air in the resonance cavities and thus achieve the resonating of all the facial bones, including the nasal bones - "the nose is in the tone" . If, however, we try to "push" the tone into resonance using the root of the tongue with the soft palate dropped, we narrow the space for the non-constricted passage of the tone and the voice takes on an unpleasant, nasal timbre - the tone is in the nose .
This problem also concerns singing vowels , .
It is therefore important to remember the "smelling a flower"trick, which guarantees a correct setting of the resonance form. Close attention must be paid to tongue position, with the tip anchored behind the lower incisors P13.