Head voice is not just for tenors, IMO. I believe that all singing voices are entitled to this healthy aspect of vocalism, and that it can be learned by almost all who study and practice to learn it, and by many just by playing around. 'Finding' a head voice is most problematical for those who have sung by forcing... by trying to sing high with the same registration as they use for the lower part of the voice. Next after that in difficulty (IMO) is the singer whose singing habits are very light, wispy tones. Over time, these singers have become used to expect that their voices should feel a certain way when they sing, and those expectations (and their associated mental images and physical sensations) are habit.
To learn to sing head voice, which is a co-ordinated use of the musculature of the voice, the student must progressively replace their habits (and expectations) with new, practised ones. The expectations (and images) seem to play leapfrog with the muscle coordinations during this time, in something like 6 week cycles, in my experience. The teacher guides the process by (in my approach) moving the student's mental image by asking the student to make sung sounds that are a little different, maybe even experimental.
The earlier discussion about lip trills, 'wonky' falsetto-like whoops and hollers are reasonable examples, based on the student's starting point. For example, for the 16-yr-old bass with nothing above middle C, and altogether too much 'muscle' feeling and concept in the tone, the teacher might start with messa di voce (cresc-decresc) patterns, and coax the singer to include softer dynamics into the student capabilities. The very act of trying this, over time, will induce a gentleness of thought which will allow the voice to rebalance a little, dropping some of the weight at the softer dynamics.
From there, the student must learn how to move the voice from pitch to pitch without 'selling up' the higher pitches. Its remarkable that, after all the 'performance' weight has been removed, that a given voice will move nicely up, readjusting the balance of registration in a nice gradual fashion.
At a certain point in this development, I think the teacher gets the sense, partly by watching, partly by listening, that the singer is ready for a breakthrough to a head voice co-ordination. Its commonly felt that many fine singers have a noticeable sensation transition as their voice moves into head voice on the way up, or when coming out of it. These sensation transitions can be substantial, but as a general rule singers work to 'allow' such a transition to occur without adding unnecessary extra work and tension in the throat, jaw and tongue regions. Under the guidance of a good teacher, the student will learn how these sensations change note to note as the teacher assists them in finding their right 'sound' in that area, and begin to accept the fact that these new sensations, and their accompanying sounds are 'right' for their voice.
From an exercise point of view, I think that low tension, well supported 'leaps' across the change region allow the voice to 'go wonky' a little, and spontaneously respond to the pitch change in the student mental concept. With a little, non-judgemental vocal play, the student can 'find' a nicely co-ordinated head voice. I think its best to expect it to occur accidentally, spontaneously, as any 'control' mental images that the student is using, or concerted efforts to 'make' it happen, seem to induce just enough restricting tension to prevent the spontaneous reco-ordination.
by Steven Fraser