In my view this box gives the best compromise between f3 and linearity of response.  I consider this the optimal box.

Don (Moron) posted on the Audioholics forum about adding a sub woofer to his speakers.

His speaker system was a reflex loaded MTM system, using dual Focal 7W4411 woofers and Focal axiom TLR tweeter in MTM configuration.  Don had built the speakers which had been professionally designed and the crossovers supplied by the designer.

Don put this post on the Audioholics Subwoofer forum on 12/19/2008.  Don’s main complaint about his speakers was severe lack of bass.  However in further correspondence via Email, it became there were other issues.  Don complained, that the sound was generally poor and that he did not listen to music much anymore.  Speech clarity was so poor, and he had had to resort to using an old cheap RCA speaker as center channel to make speech intelligible.

Obviously with drivers of that quality something had to be seriously amiss.  Attention was paid to the bass tuning.  On information supplied by Don it seemed that the bass alignment was optimal.  The F3 was an excellent 41 Hz, so the speakers should not be bass shy.  Don did however volunteer that all internal surfaces were covered with 1″ Black Hole sound absorbing material.  I advised him to reduce coverage to 50% of the internal surface, but to make sure the rear wall remained covered.

From Don’s plans it seemed the enclosure was well designed and braced.  Don reassured me about the constructional integrity of the speakers.

So suspicion naturally turned to the crossovers.  These were Don�s crossovers.  They are fourth order Linkwitz/Riley crossing over at 2.4 kHz  There is a frequency response chasm from 200 Hz to 4 kHz, centered on 1.5 kHz.  It is easy to see why Don was unhappy with his speakers.  The major part of the dip is across the speech discrimination band from 400 Hz to 2 kHz.  So it is easy to understand how speech intelligibility was compromised.  The tweeter level is two high, so the speaker is poorly balanced, leading to the perception of lack of bass.

So new crossovers were designed.  A second order impedance compensated crossover was designed crossing over at 3 kHz.  The response in the crossover region is +/- 2db.  when combined with the driver roll offs, this constructs a composite fourth order crossover.  The tweeter is 90 degrees ahead of the woofers at crossover, so tweeter polarity is not reversed.  The crossover is impedance compensated, with impedance of 4 to 5 ohms except for a rise to 16 ohm at crossover.

The crossovers were constructed, and Don had a friend help with installation.

Don has been very pleased with the result.  We have confirmed Fb  is very close to 35 Hz. With Dons Radio Shack spl meter, the speakers would apear to be 4 Hz down at 40 Hz.  There is useful audible output to 30 Hz.

Since this modification Don has used his speaker more, and the old RCA center channel speaker is gone.  Speech clarity is reported as excellent, with a good stable center image.  Don now finds the bass adequate and no longer feels a need for a sub.

This is Don’s impression of his “new speakers,” in his own words.

“I can barely believe the sound of my “new” stereo system. The crossovers you designed and made, and your suggestion of removing about half of the insulation, has turned a rinky-dink system into a concert of beautiful music. I can now easily understand voices – which I couldn’t before – and the music is beautiful, AND, you saved me hundreds of dollars, because the bass is so much deeper and clearer that I don’t feel the need of a subwoofer. I use the stereo soley for music and TV.

I paid a considerable amount of money to have a “professional” chief engineer of an audio company design the cabinets and crossovers for my componts, and the sound was pretty lousy: Couldn’t understand voices and pretty much a jumble of music. Seemed like the tweeters and woofers were in conflict.

As I admitted, I’m a moron regarding the topic of stereos, so I have no idea what the terms you used mean, or why your crossovers have made such a difference, but the sound that my speakers now reproduce is stunning. I had no idea a home stereo system could produce such beautiful music.”

Don’s post is interesting and shows that things are not always what they seem.  If Don were so inclined his system would now likely have improved performance with a sub, were as previously a sub would have provided little benefit.

It is gratifying when a long range call for help can have such as satisfying outcome.