Hi Speakerfreak,

RE "The rectangular cut in the right is the bass Port for the two 2205 drivers. There is some kind of fabric gleud from the cabinet inside."

This type of vent with fabric covering it is typically called a resistive vent. They were popular in the 60's (even prior) and sometime during the 70's before vented box workings were well known following Thiele and Small's monumental speaker design works. I can't recall a "modern era" JBL model that used resistive vents. Simply because generally they're not a good idea, except for very few situations like "correcting" or salvaging a LF alignment.

A free vent tuning has the advantage of reducing cone travel and LF driver distortion on a notable range of frequencies. A resistive vent restricts that bonus markedly. The more opaque the fabric is the more vent restriction. Making the cabinet/driver combo behave as if it was partly vented and partly sealed box at the same time, not getting the best of both worlds as some may think, rather a fraction of each.

Moreover, the 2205 isn't really a "good candidate" for a resistive vent, assuming such driver exists. In fact, reducing cone travel on the older 2205 driver is a very good idea since it doesn't have a lot of cone travel capability (Xmax) for higher level bass output. I know I own a pair of these, but driver has other qualities...

I think you should seriously consider removing the cloth covering the vent in order to get maximum benefit from vent action, more so in view that Will Audio's cabinet is a sound reinforcement box, more likely to be fed with lots of power.

Naturally, this implies first knowing box size (Vb) and vent dimensions to determine what would be the box tuning frequency (Fb), before removing the cloth, and establish the predicted non-cloth LF alignment, and/or modifications to it, as well as to the vent, if any. I might be able to help you with that.

In any case, take a look at the following:

"According to Thiele's analysis, resistive vents (series resistance in the form of fibrous stuffing or a tight cloth placed over the vent), ... result in the following changes in normal vent-box operation: 1. increase in F3, 2. decrease in output (efficiency), 3. increased cone excursion near cutoff, 4. lowered Qts.

Thiele's point is that if a regular alignment with no resistance has a lower cutoff and higher efficiency than the same size box with resistive loading, why bother?" (Vance Dickason, The Loudspeaker Design Cookbook, 5th ed., P.62)

Based on Thiele's items # 2 and 3, I would add a fifth item to his list: increased distortion, since efficiency and distortion are usually inversely related in well designed systems, plus cone excursion and distortion are usually directly related. As for item # 4, well the 2205 already has a pretty low Qts, no real need to shift it even lower. Food for thought... Regards,

Richard