The + 3dB shelf?

The changes to the IB had wrought doubt regarding the overall frequency response. Testing my speakers was not an area with which I had exercised myself too much. Subwoofers are far more fun and don't leave jagged spikes all over the REW graph box. Smoothing is beneficial to improve both the response curve and one's sense of well being. Audio paranoia is never far from the surface.

With 1/3 octave smoothing the results were far from pretty! Several hours passed while every possible variation in microphone and speaker position was run through REW. In phase, out of phase and changing the roll-off point were all tried in combination with the above. Even with the shortest sweep fatigue quickly sets in when testing speakers. One can test subwoofers all day long with much longer sweeps without growing tired of the resulting, deep, soft whoop. Speakers are more audible and whistle into the bargain. Each new graph had 1 octave smoothing applied to follow the trend as I raised the speakers on 4" wooden blocks one moment and then removed the concrete slabs on which they sit the next. Nothing helped. Nor did moving the test microphone back and forth and up and down. (I used my Galaxy 140 SPL meter) Eventually the speakers ended up 5' apart instead of the former 7'.

Then Jim on the IB Cult suggested I try fitting a shelf. I found the shelves I had made and even painted but had never used. One slotted into the middle and I snugged up the nuts on the studding to pinch it tight. The result was absolutely remarkable on both the REW graph and sound quality: Here one octave smoothing is applied.

The magenta curve is with the shelf in place. The IB's bass plateau is more extended from 50Hz upwards to well beyond 100Hz. The trough just above is filled in a little more and then the response is generally smoother beyond that point until the upper roll off point. Far, far beyond my own ears' ability to hear anything. The black curve is without the shelf and is shown for comparison.

Organ bass suddenly sounded much harder edged. With much more character and timbre. I was surprised how much louder it sounded too. There seemed to be bass growling along where I had never noticed bass before. We are not talking nuances here!

The real proof of the pudding? My wife commented immediately we started watching "Hot Fuzz" (for the third time) how the sound quality had improved dramatically. The bass is now absolutely stunning! Effortless, brutal and real! Oh, and loud! None of your shapeless, deep rumbles. This is a whole new experience! Whoopee! Is that enough exclamation marks for one paragraph? Exaggeration? Not a bit of it! The sound quality from top to bottom just seemed to be improved. BTW:

"Hot Fuzz" is one of the funniest, most wicked films ever made. Such a shame that you have to be British to understand all the inside jokes. No stone goes unturned in the search for English humour. Never mind the language. Enjoy the carnage! Bring the noise!