Howard, I found that this project was very rewarding and enjoyable to do. If is alot of fun to play with single-driver systems, because alot of the complexity of making speakers(crossover design and building/tweaking) is eliminated. Basically if you can do a good job of putting the cabinet together, you'll have good results. This is already a proven cabinet, and has been around for about 70 years now, originally designed by P. Voigt who was the original designer of the Lowther driver. Also, as can be seen by the Cain & Cain Abbey, and some other Voigt Pipe designs commercially available, this cabinet has the characteristics of working as intended, even when a variety of drivers are used. Fostex 8" Sigma drivers work fine, all the 8" Lowther drivers, AER 8" drivers, REPS-1 drivers, even little FE-103(4") have been used successfully in these Voigt Pipe cabinets. That's one of the things that makes Voigt Pipes so popular with DIY people.
One of the key issues with Voigts is to make sure the cabinet stuffing is done right, or the tonal balance will be off because of the lack of "blending" the port output with the direct radiator output. This has to be done properly. The cabinet stuffing determines how strong the midrange output will be, and has an big effect on the flatness of the frequency response. If you have too much midrange coming from the port, then there is a hump in the midrange response(and also an imaging shift) and below, and if you don't have enough, then there is a dip in the midrange response and below. Adding more stuffing makes less midrange output of the port, and taking away stuffing makes more midrange output at the port. You strive for an even balance, so that the port does not call attention to itself.
Many articles on Voigt Pipes make a big issue out of the possible "comb filtering" effects of the interactions of the direct radiator and the port mouth. They say it results is some harmonic cancellation of certain frequencies that are multiples of the cabinet resonance tuning. In practice, I have observed none of this, and either I got very lucky, or they are making a mountain out of a mole-hill. I wouldn't worry too much about comb-filtering on these.
Lowthers and most single-drivers take a very long time to properly break in. The mids and highs come in quite early, but the bass response is weak until at least 100 hours of play time. It continues to improve up to around 500 hours of play.
Even after break in, there can be a desire for a little more bass impact, and the "swinging door" mods that I designed and implemented on my speakers address this issue. It is related to the boundary effect, and wave-launch off the front baffle, and it is called "baffle-step loss". The narrow cabinet fails to reinforce the lower frequencies and results in a 5db loss in response below about 128Hz. The wider baffle face that is provided by the "swinging door" mod handles this by acoustically providing the added surface that is needed.
