This set of pages shows the results of various modifications made to a Vifa D27TG-45 (purchased in 1995). The rear chamber was removed and tests were made with no chamber and with a custom chamber (a 2" pipe end cap, actually!) which was tested with and without felt stuffing.

It seems that manufacturers actually know what they're doing! There is a significant improvement with a rear chamber (duh). If there is no felt, then the chamber is of little help, and would actually be detrimental when compared to the other alternate form commonly found, that is, the same tweeter with a non-vented backplate.

An additional note, I tried using the end cap on a small extension which made the total chamber size about 4" in length. It was basically identical to the end cap only.

There is a droop in the SPL meaurment with the larger stuffed chamber, about 1db centered around a secondary resonance, as yet unidentified as to origin. Maybe my stuffing job, maybe introduced by the larger chamber, I don't know. But this can be accounted for in the design and is of very low Q, so in the stop-band it's unlikely to be a major concern. But it must be handled.

My conclusion after this series of modifications is that there is a benefit to increasing the rear chamber size, but care must be taken to stuff it properly and design the crossover for this change.

One final note. This lowered resonance will make it easier to cross the driver lower. This will not change the excursion capability of the driver, so distortion will occur at a lower SPL if a lower crossover frequency is used. It will stress the driver more.