The air insideThe air inside the body is quite important, especially for the low range
on the instrument. It can vibrate a little like the air in a bottle when
you blow across the top. In fact if you sing a note somewhere between
F#2 and A2 (it depends on the guitar) while holding your ear close to the
sound hole, you will hear the air in
the body resonating. This is called the Helmholtz resonance and is introduced below. Another way to hear the effect of this
resonance is to play the open A string and, while it is sounding, move
a piece of cardboard or paper back and forth across the soundhole. This
stops the resonance (or shifts it to a lower frequency) and you will notice
the loss of bass response when you close up the hole. The air inside is
also coupled effectively to the lowest resonance of the top plate. Together
they give a strong resonance at about an octave above the main air resonance. The air also couples the motion of the top and back plates to some extent.
The Helmholtz resonance of a guitar is due to the air at the
soundhole oscillating, driven by the springiness of the air inside the body. I expect that everyone has blown across the top of a bottle and enjoyed the
surprisingly low pitched note that results. This lowest guitar resonance is
similar. Air is springy: when you compress it, its pressure increases. Consider
a 'lump' of air at the soundhole. If this moves into the body a small distance, it compresses the internal air. That pressure now drives the 'lump' of air out but,
when it gets to its original position, its momentum takes it on outside the
body a small distance. This rarifies the air inside the body, which then
sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring.
In practice, it is not just the compression of the air in the body, but also
the distension of the body itself which generates the higher pressure. This is analysed quantitatively in Helmholtz Resonance.
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