I’ve thought about the examples MDEW provided, and would like to propose these following experiments. Most of these are “open” (not enclosed) acoustic explanations, and are incorrect for the vocal tract. Enclosed system explanation must be explained, e.g. a guitar.
If one has two metal bars separated and suspended in air using strings, and one vibrates, the other only will vibrate only very, very slightly, and only after some time. This is caused by air frequency pulsations.
If the two metal bars are connected at base using metal– a tuning fork– and one side vibrates, the other will vibrate strongly. The sound heard is the two metals’ frequencies pulsating the air between the two metal stems. The air pulsations generate sound and reinforces the metals’ frequencies, but the power remains with the metal frequencies. Resonance is the metals’ and air-in-between common frequencies reinforcing each other. This explains MDEW’s statement that a tuning fork is placed on a piano and not in the air contained with the piano–because the bulk of the frequency power is carried not by the air, but by hard surfaces.
In a bass drum, the large diaphragms provide not only a different natural frequency, but also more air power. Is the bass drum’s vibrating resonating frequency’s power driven by the drum’s open space air or by the sides of the drums? If we take a paper cup close to our ear, and hit the closed end, we would hear loudly the air power. If we again take the paper cup and cup it to our ear and hit the closed end, we would not only hear a louder hit, but also feel the vibrations of the hit. Thereby, I believe in the bass drum, most of the frequency power is still carried by the sides; the diaphragms’ do carry more air power and the air power do create the sound and reinforce the internal resonance, but the bulk of the power remains with the physical drums’ sides’ transmssions.
Another example shows this. If we take two huge gongs, line these up separated, and hit first with force, will the second gong vibrate? Yes, but very little. Will the two resonate? Yes, but very little, as air transmission transferring significant power is very slow. Air power cause sound, can reinforce the frequencies when resonanting, but air power doesn’t carry sufficient power.
With the head, “sound” is created in the air filled empty cavities–the throat, mouth, nasal cavity, chest, and sinus. But the bulk of the power of carrier of frequency is through the bones. Soft tissues, such as the tongue, soft palate dampen the “air” sound and if obstructing, can muffle the sound. Hard surfaces– the hard palate, all bony parts, carry the bulk of the frequencies’ power. Hence, the analogy of the hard palate acting as a sound board is correct.
The power of the frequencies is carried by the bones, obstructed and dampened by soft tissues, propagated as sound by air space, and resonated by the bones and air as resonance. The head voice is resonanted by both the sinuses and nasal cavity (because I don’t believe the nasal cavity is especially filled with soft tissues, like the tongue).
Singing and posture--see Categories, About, Launch 4-2014 
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