johnnyjazz:Sorry guys, think i,m a bit thick on this, my understanding on tuning is that when you tune an instrument you alter the pitch of the note, therefore if a hole has not got a note to start with then how can you alter it,s tuning ie pitch. By the way a guitar is standard tuned to 440Hz, is 40Hz not quite low ? I,m so confused
What you are doing is tuning a resonant cavity, or a mass of air. A quick Google on Helmholtz Resonator will give you more detailed info.
The port tube in a speaker is tuned to resonate at a certain frequency - say 65Hz. This moves the energy around under the frequency response curve. It doesn't extend the bass in absolute terms. A closed box will generally have a more extended bass response. It just allows the designer to move the energy around - within limits.
Two ports tuned to the same frequency (i.e. having the same mass of air) but with different diameters and lengths will have different effects.
The same effect is used to absorb energy in a bass-trap. At a certain frequency the bass-trap resonates rather than the room.
The same principle applies with hollow-bodied guitars. The cavity has a certain resonant frequency. This contributes to the individual sound of an instrument. It also provides the resonance that creates feedback.