The recent drama on the Lionsgate intro has been bothering me ever since. It sounds as if the bass has been turned up from normal. Certainly as if something had changed. I just couldn't put my finger on it. All the films we had watched since were very bassy. I stared at the bars on the mixer and BFD but there was no sign of obvious excess.
Inevitably a window had to be found to run REW to see what was actually happening. The response was rather worse than I remembered but only a couple of dB hot compared with the speakers. The old drivers are still getting maximum boost at 20Hz. (+16dB 120B/W) The newer drivers have no filters running at all.
I found the same problem as before with the mute buttons on the Behringer CX2310 active crossover. It took a few attempts to get them to switch on by rapidly tapping the button. Until then I could not rely on them.
This is the waterfall plot of all 8 drivers and speakers combined. Somehow I failed to save the simple SPL curve.
A bit of a mess with a deep valley between IB and speakers. No wonder it sounds bassy even though the sub isn't running hot. I checked up to 500Hz and the speakers extended on upwards at much the same level.
Here are the overlaid curves for all 8 IB drivers, single sets of four drivers, with and without speakers. The blue line is with the speakers set out of phase with the IB. The heavy violet line the in-phase response. It's a toss up between having a wide valley or an abyssal trench! Quite a mess with either option. It is no wonder there isn't much impact on drums or plucked bass! Though there is a free 10Hz hump to provide some deep infrasonics.
Time for a complete rethink. Only the upper (newer) set of 4 IB drivers causes the valley. I really ought to put these drivers in the bottom half of the IB manifold. Then use the older drivers in some other alignment between the speakers. There is a fair bit of room under the 45 degree sloping ceiling for boxes. Though I'd prefer a large baffle or IB.
Beneath the stage is a wide open space leading to the rest of the L-shaped, living room. The ceiling below the stage is covered with plasterboard tiles. These need replacing with T&G pine boards to match the rest of the living room ceiling.
One would have to be blind to miss the obvious chance to use the floor of the stage as a deep baffle for the older drivers. True, the baffle would only be about 6' deep from front to back. Before it reaches edge of the stair well. But, the older drivers need not dig very deep and won't anyway with their 32Hz Fs. So I can forget the massive +16dB boost and let the drivers behave as big woofers. Let them roll off naturally.
My idea is to mount the four old IB15 drivers vertically on a thick baffle as far back on the stage as possible. It would be sealed all round or even made into an open bottomed, triangular box sealed to the floor. The rear of the baffle "box" would exhaust straight down through a wide slot in the ceiling below. Probably covered with a "camouflage" cloth grill. A sort of understated IB, if you like. Certainly much bigger than any normal, standing, open baffle!
So I should be finally able to reinforce the missing bass in the deep valley between the real IB and the main speakers. The reduction by four drivers will allow more room to breathe for the four remaining drivers in the IB enclosure. I can chop the manifold in half to return to my former layout.
The problem is getting permission from my wife to launch into further bouts of DIY after years of putting up with it. Taking down the old ceiling tiles would make a bit of a mess. Though I planned to make a heavy poly tent to work inside.This would keep the dust inside. Instead of spreading right through the house. The tiles could be bagged for removal outside.
Click on any image for an enlargement.
