I went down to Olympia this weekend to visit my old Evergreen State College audio engineering professor Terry Setter; he is also a composer, microphone builder and flamenco guitarist. We talked about music and tried out guitars. I told him about a recent instrument that I made with a sound port and we got on to the subject of Helmholtz frequency and Q. His brief explanation is one of the best and easiest to understand that I have heard, so I thought I might share it:
"As for Q, here's the quick version (and possibly all you'll ever really need to know about it). Any form of resonance has a primary frequency at which it occurs - like "F" in many guitar cavities. However, you can measure how much activity takes place at this primary frequency (often called the "center frequency") and how much occurs on either side of it by comparison - that's why they call the primary frequency the "center frequency", because there's always an effect on both sides of it.
Lets say that a Helmholtz resonator is tuned to 100 Hz. That means that it will amplify a 100HZ tone, but it will also amplify 90HZ and 110HZ. The rate at which its ability to amplify falls off, above and below the center frequency, is called the "Q" of the resonance. In audio, we measure that in dB/octave. So, if the amplification at 100HZ is exactly 12dB higher than it is at 50HZ or 200HZ (the octave below and above the center frequency), then the response of that resonator has a 12dB/octave "shape" when graphed. "Q", scientifically, is the amount of amplification divided by the width of the effected frequencies. So, if a resonator is built to 100HZ, but only effects the frequencies near to either side of 100HZ, it has a high Q factor (a narrow spike when graphed), and if it effects a wide range of frequencies on either side of the center frequency (100HZ in this example) it has a low Q.
Therefore, if putting a port in a guitar lowers the Q of the resonance, that could mean that although the primary resonant frequency is not amplified as much as without the port, a WIDER range of notes IS being amplified, thus smoothing the low frequency response of the instrument across more of its lowest tones. It would be easy to measure, and maybe some builders have written up the results of tests they did with ported bodies."
My general impression of the sound port is that it increases the volume to the player. It didn't seem to increase the volume in front of the guitar at all. If it does, it is by such a small amount that it doesn't really matter. It definitely raised the Helmholtz frequency of the body (by about a semi-tone), but it seemed not to amplify that frequency as much which would make sense bases on Terry's explanation.
Personally, I'm not really a fan of sound ports. I find them to be unnecessary and somewhat confusing. That being said, I understand how some people would find them helpful and I try not to be too much of a curmudgeon about it.