Polishing the gilding.

Having had a listen to the beast on organ music I tried REW to see what was happening to the response. With the two completely different sets of drivers at work one might expect all sorts of problems. Would the old and new drivers compliment each other?

My first mistake was trying to match the drivers at the usual peak of 12-13Hz. With an Fs of 32Hz the old drivers were never going to match the bottom end power of the new ones. So using 13Hz as a hinge point forced the old drivers to do all the work. I ran the REW Calibration subwoofer pink noise signal and then touched the driver surrounds on each side of the box with my finger tips. It was obvious that the older drivers were quite busy but the new ones felt almost inactive. First things first: I bypassed the BFD to ensure neither driver was getting a free ride:

It seemed my attempts to lift the old drivers by 8dB on the BFD was removing the newer drivers from the equation. They might as well not be there. Which was not the idea at all.

So I returned the gain settings to normal on the CX2310 and tried again:

That's better! Now the drivers are working equally hard and the meeting point has been lifted to about 23Hz or almost a whole octave above 12-13hz. The combined response is showing a saddle between the VLF peak and a peak at around 27Hz. I'd like to flatten this result a little with the BFD.

Here is the improvement after 15 minutes of playing with different BFD filters and the resulting curve given 1/3 octave smoothing. I have since reduced the output at the EP2500 to 26 on the dials as the bass was beautifully expressive [and impressive] but just too heavy. (loud!) This resulted in rather too much distracting LF ambient noise from the cathedral. Now the bass is not so much in my face but amazingly detailed and muscular. The pedal organ has taken on a superb new realism with a more descriptive rendering of the timbre and interplay of the great pipes. The character of each pipe and rank now seems more obvious than before.

This subwoofer plus speakers trace above shows there is a serious trough between them despite an attempt to reverse their relative polarity. (and then returning it to normal) Without any means to alter relative phase or delay I'm rather stuck with this trough unless I start moving the speakers around. The other active crossovers in the Behringer stable offer more facilities than the basic CX2310. I might have a look at the potential of a better crossover.

The exterior of the manifold seen from inside the enclosure. The picture is distorted due to the closeness of the camera to the box. It is difficult to get far enough away to take a decent picture. On its supporting timber block the manifold stands over 6'4" high. The large, galvanised, roofing washers and nuts from the screwed rod braces are visible.

Out of curiosity I checked the sound levels out in the enclosure with my Galaxy 140 SPL meter and found the SPLs were about 2dB lower than in the room. The room levels drop rapidly with distance from the manifold. Out in the enclosure the levels were fairly stable regardless of distance from the box.

The doubled manifold thickness certainly feels and sounds more dead to a knuckle rap than my old manifold which had only one thickness of 3/4" plywood. Yet pressing my ear against the box proves it to be quite transparent to higher frequencies! Perhaps I should have used MDF and flake board? (OSB)

I've tried a few film scenes. The difference is not so much of quantity but of kind. Brutal realism without warmth or hangover. The battering ram on LOTR Two Towers had a real wood splintering malevolence despite hitting only a 108dB(C) peak. The walking trees shook the room but there was no overlying warmth. The collapsing underground bridge scene was also nicely realistic.

Xmen: Last Stand bridge drop scene hit 111dB(C). There seems to be less rumble for effect and much more violent content. Cone movement hasn't exceeded a few millimetres so far. Now I'm enjoying Blue Man Group's Audio at 85-90dB(C). Very tight with clear and clean vibrations from the VLF instruments. No visible cone movement. Same with Bass Outlaws' Extreme Woofer Test.

Mike Oldfield's
Millennium Bell is another favourite bass test. There are great quantities of deep drums and beats. Try track four: Sunlight Shining Through Clouds at 85dB(C) average for an excellent example of what I'm talking about. The beat is almost nauseatingly hard hitting with a deep underlying pulse. Track 7: Mastermind also has plenty of deep bass. The last track played at a steady 95dB(C) is good fun too.

Screwing the controls
on EP2500 back up to full results in fairground levels of bass but it is too exhausting to experience for very long. After several hours of listening to music and films at robust levels the IB enclosure is now at 84F. With a reading of 92F on a digital thermometer with its sensor resting on the amp top plate.

A remarkable recording of Messiaen's Les Corps Glorieux is played by Vibeke Astner on the Maribo Cathedral organ on the Helikon label. Track 4 "The battle between life and death" has the most subtle, deep breathy bass at times. At the absolute limit of audibility this is probably a quiet theme on one of the 32' stops of which there are two: The Bordun and the Tuba.

It is shame that Messiaen is not very accessible to the average listener. The quiet, wistful upper theme is hauntingly beautiful but full of pain and anguish. The real magic to this piece lies in the subtlety of the, sometimes almost infrasonic, underpinning. The great pipes restlessly rise and fall in pitch with changing character and strength. Like an oily swell lapping menacingly on the seaweed-encrusted steps of an ancient harbour amidst blackened rocks. A 22 minute masterpiece which will probably pass without much notice to most owners of ordinary subwoofers. They may hear an occasional, diesel-like rumble but nothing more. The real truth about the subterranean depths beyond the visible surface is clearly depicted by the IB. The counter to the beauty of light and life lies in the suggestion of unspoken fears below in the inky darkness. Always just out of reach. Hidden from view, but a constant presence, behind the barrier of in/audibility. Death is always just beyond our visible gaze. The unknown and the unknowable beyond yet another barrier to our ordinary senses.

All chapters in my IB blog are regularly corrected and sometimes sections are rewritten. Try reloading the page to ensure you are reading the latest edition.