Well, I'm NOT a 'speaker guy', so I'm going on what may be flawed understanding, too. Still, FWIW, I'll put it up: anybody who knows better please correct me.
The idea behind a 'bass reflex enclosure' is twofold: to capture some of the energy off the back of the speaker, and to give a more even response. It turns out that the best way to get the even response is to have two resonant systems, both with high damping, that are tuned to the same pitch, or very nearly the same.
You would expect that two coupled resonators tuned to the asme pitch would give a tall, narrow output peak, but this doesn't happen. Instead you get one of them changing phase relative to the other as you approach the resonant frequency, so that the overall output of the system actually drops. You can see this if you look up a 'Butterworth filter' which is essentially the same thing implimented in 'tank circuits'; capacitors (stiffness elements), inductors (masses), and resistors (loss elements). The output becomes a two-hump spectrum, with the dip occuring at what would be the resonant peak frequencies of the seperate tanks.
How 'flat' the spectrum is will depend on the overall resistance of the circuits: if that's high the height difference between the peaks and dips can be less than 3dB, which is more or less inaudible. The overall circuit also maintains relatively high output as you go up from the double-hump in the low range.
In a bass reflex speaker cab it is assumed that the cone itself has fairly high damping: it's light weight, and has to move something like it's own mass in air as it vibrates, so the amount of energy stored in the system is small relative to the output. The cab itself is usually lined with acoustic foam or fiberglass: something to broaden out the Helmholtz resonant peak. Using a sleeve in the port does two things: it adds damping, and it also allows you to establish the 'upper cuttoff frequency' of the radiation from the port independantly of the Helmholtz frequency.
As you go upward in frequency, the port becomes more and more efficient. As the wave length of the sound it's trying to put out approaches the diameter of the port it works better and better. At some point the port becomes essentially 100% efficient. The smaller the port, the higher the upper cuttoff frequency. On a speaker cab that is meant to be part of a system you'd want to be able to predict the relationship between the port's upper cuttoff and the crossover frequency of the tweeter driver, so you need to have the port be a certain diameter. You tune the Helmholtz pitch by altering the length of the sleeve.
Of course, with high damping in both elements, you don't need to get the frequency match too exact. This allows you to mass produce acceptible bass reflex cabs assuming decent quality control in the speaker construction. I have read, though, of audiophiles measuring the exact resonant pitches of speakers so that they could be matched up with the appropriate cabs, or tuning the cabs by changing the port lengths.
Closing the port or covering the soundhole on the guitar eliminates the Helmholtz resonance, and thus the low peak of the 'bass reflex' output. There's not as much cyclic pressure change in the box for the internal mic to pick up because there's no flow through the hole.
|
