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konacat:
Interesting: one of my students did much the same thing [without the dB meter] on a 12-string reso that he built. He kept making the port bigger until it started sounding worse, and then put in a finish ring. The original hole was too small, and it 'choked', so he needed to do a 'tracheotomy'.
I did a rather long experinent on 'ports' several years ago. I built a guitar with twenty holes in the right side that could be plugged with corks and took it to the H'burg show for players to test out. Then I spent a month measuring the response in various ways with various holes open.
All guitars have a number of resonant modes of the air inside the box. Some of these communicate through the 'normal' soundhole, and some don't. The ones that don't can still effect the tone by changing the way the top works.
The most important of these air resonances is the 'main air' mode, which works the same way as the sound you get blowing across the mouth of a soda bottle. It's a 'Helmholtz' type mode, with the pressure in the body rising and falling as the air moves in and out of the hole. This 'main air' mode works with the 'main top' resonance, which acts like a loudspeaker cone, to form a 'bass reflex couple'. This bass reflex action actually accounts for most of the sound you hear coming from the guitar, all the way up to the fundamental of the highest notes you can play.
When you put a port in the side of the guitar it 'vents' that 'main air' resonance a bit. The pitch of the resonance goes up a bit, but the amplitude drops a little. The air is less effective at pushing on the top, and on the back, too. We don't normally think of the back as producing a lot of power, and it doesn't make much, but it can have an important effect on tone color, and, to the extent that 'venting' the main air mode drops the activity of the back, the color can suffer. More importantly, though, the 'bass reflex' gets a little weaker. Overall, then, putting in a port actually causes a small _drop_ in the output of the instrument when you average over most of the spectrum.
However, that port can also 'hear' internal air resonances that are not normally effective at producing sound directly. These are generally fairly 'narrow band'; effecive only over a small range of frequencies of a semitone or less. Still, the output for those frequencies can be very much greater with the port than without: as much as 4dB, which is more than double the power. Any note with a partial in that range will sound louder.
Thus, when you average over the whole spectrum of the guitar there is a _small_ increase in output. I measured overall gains of a bit less that 3%, and Chaigne, with better equipment, got an increase of about 1% in a separate experiment.
Three things to note about this:
1) Your senses are set up to detect changes, since things that change are more likely to be either edible or dangerous. When you open a port you _will_ pick up the change, and it will often seem huge. This was noticed by players and listeners at H'burg, who often said something like: "Wow! That's much louder" when they opened a hole. However, with a little more playing they would generally say: "Now, wait a minute; that's not really all that much different".
2) You are generally much more sensitive to high frequencies than low. Much of the power loss when a port is opened is in the low range, while the gains are all at higher frequencies. Even with the same amount of power, that redistribution can sound louder.
3) The player almost always loses a lot of the high-end sound, which is 'beamed' away from him by the top and the soundhole: you only hear the reflections. The low-end sound spreads out much more evenly in all directions. A port that faces the player will send some of that high-end sound in his direction, and make the guitar sound louder.
Generally speaking, the closer the port is to the 'main' soundhole, the less change there will be in the timbre of the instrument. Similarly, the larger the hole, the more change in the timbre. However, as far as I could tell, making the hole bigger did _not_ increase the output overall: the narrow-band gain is almost exactly balanced by the wide-band loss, so the overall sum stays much the same, with only a change in balance.
Adding a port _can_ improve a guitar, depending on what it lacked to begin with. It can be kind of tricky to figure out just where to punch the hole for good results.
A port of about 1" diameter somewhere near the wide part of the upper bout makes a nice 'monitor' if you're playing for your own pleasure or in a somewhat noisy and 'dead' room, as for a restaurant gig.
So that's my 'take'.
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