Odense Elektro Days Show

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I attended the annual Odense domestic AV electronics show with high hopes for hearing some great audio and plenty of visual toys. The vast majority of the show seemed to revolve around two large halls of large, flat TVs.  3-D headlines were being splashed everywhere but I counted only four pairs of 3D glasses to go around several thousand potential wanna-sees.

Somebody really should be publicly thrashed for the long delays when there was nothing [at all] to see on many of the wall mounted screens. If they can't get rid if the endless cues, long black blanks and pretentiously silly credits, for every short film sequence, then they really should seriously consider the attention span of the average customer at such a show! Especially me!

Many of the screens showed jerky sideways movement on moving images which wasn't making me at all hungry for a better screen than my modest JVC 100Hz 37" LCD. My LCD doesn't judder on fast moving sideways action! Not ever! And, it's only "HD ready" with two HDMI sockets! So there!

There was one huge, wall-sized LCD screen but the image quality, while acceptable, wasn't exactly crisp. Probably ideal for despot's giving long, boring speeches to keep the unpleasant peasants at a safe distance. It certainly kept me at a distance. Most of the other screens varied considerably in PQ depending largely on source material. Which seemed too brutally honest even for a cynic like me.   

The "real audio" was tucked well away from the main halls in much smaller rooms at the far end of a long corridor. A stack of Naim electronics, which in total would probably cost as much as a house, was all racked up on their incredibly expensive, Fraim support system.

The kit was playing through white, fire hoses to Audiovector S3 "Avantguard Arretté" speakers at over £5k a pair. Smart floor-standers with two 6-7" bass-drivers and a fancy looking tweeter above, consisting of a series of sub-elements on a recessed plate.

A picture of the very pricey Naim kit. Probably no change from £2k on any of the boxes on display. With several running from £12k for the CDP to well over £15k each for the pre and power amps.

I failed to capture the computer monitor perched right up on top. This automatically displayed track details and the album artwork after dropping a CD into the player. Not forgetting the inevitable puck. Naim kit is always hard to photograph but the room wasn't well lit anyway. I should have moved further back because the wide angle lens has distorted the geometry of the kit. The room was a bit too crowded for relaxed photography. 

It all sounded very Hifi with plenty of drive, wallop and the ability to go quite loud. Bass was easily strong enough but it completely lacked any real extension, realism or physical presence. There were a few tracks with acoustic bass but it didn't sounds as if it was in the room.  Unless it was a cello being plucked by a dwarf?

All thoroughly enjoyable if you don't mind the very obvious distortion on vocal pushes and the poor imitation of real bass. It would easily satisfy a great many listeners who know no better. People like investment bankers, estate agents and footballers in particular. Basically anybody who can drive an Audi really badly. While suffering delusions of grandeur as to their true value to our world. 

As usual, my attention was immediately drawn to the slightly brash, Naim 'signature' sound.  I have owned a modest preamp/PS/power amp Naim combo for nearly fifteen years. So I'm not just picking on the Naim kit for the sake of it. The forwardness is soon forgotten but can make some speakers sound "too loud" and tiring to listen to. Though not in this case. It was very well balanced without actually being at all wonderful.

There was the usual grainy SQ from CD and the tizzy edge to vocals. There seemed to be no escape from the forward presence regardless of the wide range of music played. Somebody had brought along a  favourite artist's CD and this sounded very HiFi too. It was well recorded but certainly didn't make me want to go out and buy it like some systems do. Nor could I tap my feet reliably to anything which was played. This is completely unforgivable in any Hifi system! Particularly one from Naim! PRAT! (Pace, Rhythm and Timing, to the uninitiated)

Basically it sounded just how Hifi is expected to sound. Exciting and loud in places. Unfortunately it didn't sound at all real. Not even remotely. The imagery told us there was a soloist surrounded by a group of musicians up front somewhere or other. That didn't mean it sounded as if there was somebody actually performing right in front of our seats. There was no sense of being able to put one's hand out and touch anybody living and breathing. I moved about a bit as seats became available but could not improve on the basic offering.

Just to add to the fun somebody was demonstrating "HD Audio" via a computer right outside the open door! Another tragic case of mismanagement by the show organisers! One expects to be able to wallow in background peace and luxury at this price level.

It seems I missed some demo rooms due to closed doors being completely unmarked. Why the hell was Bose allowed into this area to take the largest room available? It speaks volumes of how the organising electrical chain has moved down-market into white goods. Probably in a desperate attempt to stay afloat in a recession. I think one may safely assume Bose offers a good mark-up and a ready willingness to pay for exhibition space.

In the high-end "chatter room" I found a massive, thread-driven platter, a 3/4" bearing shaft and a weird 12" SME 3012 clone. With (horror of horrors!) a detachable headshell!?!  I have a couple of old SME3009s knocking about somewhere. One with a detachable headshell. The other the 3009 'Improved.' (i.e.Non-detachable headshell) Including the SME silicone damping trough on one of them. I think this was an attempt to kill the knife-edge rattles from the new breed of low compliance moving coil cartridges. These were more usefully attached to an [upstart] Linn Sondek/Ittok turntable. The new legend on the "Subjective" block which crushed SME's stranglehold on almost affordable "High End Hi-FI" for those who could actually afford this level of kit.  Or was it all about damping record-warp-related, cone movement? It all seems a very long time ago now but I remember it all being horribly expensive. 

The room had several Avid turntables, lots of flashy, "traditionally" chromed, valve (tube) electronics and a variety of speakers. I recognised no names on most of the other tin boxes except for Tim de Paravicini. (From a mention in a magazine somewhere in my hazy past.)  A pair of promising, blond, Spendor, floor-standers were pushed expectantly towards the front. Though a modestly-sized pair of OBs were playing when I went in. At least they looked like badly undersized OBs. They certainly weren't grabbing anybody's attention, at all.  In fact the room was stuffed with Hifi boors all talking very loudly indeed! I could hardly hear the bløødy music. 'Nuff said? Though so.

Then we [the unwashed] all had to stand up while "they" rotated all the chairs by 90 degrees so we could all be subjected to a new lot of kit. This time with a large pair of forward leaning, piano black speakers and another, rather more stealth-like, black, Avid turntable. This system immediately sounded so absolutely dire that I had to leave the room to stop myself from literally bursting out laughing! Or complaining out loud in my "pidgin" Danish! You know when you put a bit of greaseproof paper around a comb to make a simple kazoo when you're a kid? Well, now you've got the picture of how it sounded! I kid you not.

When I glanced back into the Naim/Audiovector room it looked as if the kit had packed up. Or some fault had developed in the, now lonely, demonstrator's skills.  Probably just tired batteries in the terrifyingly expensive, Naim remote. Sadly for him I was much too well brought up to actually mention it aloud.

So, all in all, the free pass to get into the show was well worth the asking price. Perhaps I should have put in a claim for my travelling expenses?  I bet they do when setting up customer's systems at home. Probably charge for time spent drinking the customer's coffee or booze too. Well, they really aren't much better than glorified electricians in posh salesmen's hats and the former charge for everything several times over. Which, come to think of it, so do the latter. ;-)

The best part of any show is always coming home to a modest system which completely and utterly trashes anything you heard, at any price, on the day. Bung on "Gaucho" to make sure you weren't terribly mistaken about the high-end kit's hideous graininess, total lack of warmth, realism, bass or depth. Now wallow in that silky sweetness. "Hi-Honey, I'm home!" :-)

Me negative? Only in the sense that I didn't need to spend any more money to remain happy with my music. If I get a free ticket next year I'm definitely taking my favourite Franck organ CD. That'll show 'em!  :-)

For the humour-challenged, obsessive-compulsive, hi-fi phreak, who dresses formally in a hair shirt for "listening sessions at the hifi altar," none of the above should be taken too seriously. It's just that some of us have moved on from treating overpriced boxes, cables and pucks as sacred objects. We no longer erect sacred shrines in our listening rooms. Now we just listen to the music and pretentious hi-fi just gets in the way. As a magazine equipment reviewer I'd last about two minutes. If that. I have no respect whatsoever for anything which doesn't play music regardless of price. I certainly heard nothing today which could remotely manage it. Until I came home, of course.

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As you were.

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Well, I have now replaced the older drivers back in the bottom four holes of the big manifold. Since I had spent some time adjusting filters on the BFD for the experimental "sealed" boxes I had to start from scratch. It would have been more sensible to have started a new filter series but one thing led to another. A small change just to see what happens and then another...

A quick check with a 9V battery on the speaker cables, at the EP2500 amp end, confirmed that the drivers were all moving in unison. I had wired them in series-parallel again to obtain a combined load of 4 Ohms.

I started with an REW sweep  of the newer drivers with no BFD. (or rather with the BFD bypassed and no LED lit on the top, right button.)  No obvious problems with 1/3rd octave smoothing. I'm not going to bother trying to improve this with the BFD.

 The older drivers proved not to be quite so accommodating. I added two filters. A boost at 20Hz and a -5dB cut at 40Hz to flatten the hump. That was all that was required for the result below:

Interestingly(?) there is a phase anomaly between the two sets of drivers. The new version of REW can now display phase though I omitted to record it. The effect of this anomaly is that the older drivers suck out the bottom end of the newer drivers unless their phase is reversed. I chose not to reverse the phase because of the unwanted effects higher up the frequency range.

Here is the combined effect with the drivers level matched and in parallel:

Not a bad result from just two BFD filters on the older set of drivers and leaving the new ones quite untouched. No cheating here. This is exactly as measured at the listening position at ear height.

My next objective is to better match the IB to the speakers. The images above are with a much higher crossover point setting on the CX2310 to match the different needs of the "sealed" boxes. I shall have to experiment with a lower crossover point to ensure there is no bass source localisation. Though none was noticed upon listening to a few samples of music. The crossover is probably somewhere up around 200Hz, at the moment, but there was no more time to play after dinner.

Below is shown the awful "raw" result from having no BFD but with the speakers playing as well as both sets of IB drivers:

The deep trough, centred on 180Hz, has returned with a vengeance. I have now decreased the IB level relative to the speakers but need much more time to play. It still sounds amazing compared with the previous "sealed" boxes. These were intended as mini IBs but I was never allowed to make new holes in the walls for heat loss reasons.

I have had another idea though. The ceiling below the AV stage needs to be redone. I have already boarded the other half of the ceiling in the sitting room with pine. I could use the stage area as an OB (open baffle) with the manifolds opening into the room below. The distance to the edge of the baffle would only be about five feet to the open stairwell at my feet. Still enough to  be well worthwhile if deep infrasonics are not required.

The older 32Hz drivers don't do very deep bass anyway without massive boost. The best thing about this idea is no connection to the outside world. So no heat loss. The area below the stage is a transition area to the main sitting room area so a couple of ceiling grills will pass completely unnoticed.

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ib + sealed boxes

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Having built the two new, 2 x15" manifolds some domestic resistance was encountered to cutting holes in the walls. So they were simply planted open side down on the carpet at the back of the stage for the moment. The true IB remained as the top half of the tall manifold with the four newer drivers. A shelf  halfway down isolates the drivers from the rest of  the box. Heavy cardboard has been temporarily stapled over the open driver cut-outs.

I did some REW sweeps to see how the new boxes were performing alone and together with the IB. As usual these older drivers (Fs32Hz) rolled off very early without serious bass boost. So I added their usual +16dB @ 20Hz 120BW steroid injection with the DSP1124 BFD. This  lifted the bottom end nicely leaving a very broad hump peaking around 90Hz. I doubt they will suffer from over-excursion in such small boxes if they aren't harmed in an IB. Besides, they have the four 15" drivers in the IB to support them.

 This is the IB + the two boxes + the main speakers up to 500Hz with 1/3 octave smoothing. 120Hz indicated setting on the CX2310, 24dB/0ctave, active crossover. This probably means they are crossing over a bit higher going on past measurements. The idea of the two boxes was to allow a higher crossover point. I shall experiment further with this. The new manifolds/boxes contribute nicely to fill in the former deep troughs between the IB and the speakers.

I have had a quick listen to Loreena McKennitt and some Franck organ music but no films yet. I  normally have the subwoofers rather more "warm" than this. Early days yet as I can easily add more bass to taste. The Behringer EP2500 is throttled right back on the control knobs. So has plenty in reserve.

As an afterthought I have switched input channels on the EP2500 to bring the boxes onto the Right channel and the IB on the Left. This should ensure what remains of any stereo information in the bass is maintained. The IB is on one channel and the two boxes are on the other. Bass sounds "drier" and lighter than before but still with good depth. Detail and weight is certainly not lacking.

The REW Waterfall graph generated from the response curve above.

I shall bide my time until resistance crumbles and I can open up the plywood wall to let the manifolds breathe as true IBs. The reason for not opening the manifolds to the under-roof space is thermal loss in winter. I'm considering a plywood slide to close off the openings when the subs are not in use. The drivers are fitted magnets outwards to the boxes so that the metalwork will remain in the room to avoid condensation.  I could also hinge the boxes to the wall. Then slip a piece of thin plywood in to seal the wall when the subs are not in active use. I'll think of something to satisfy the "Management Committee Chairperson".

I moved both of the new manifolds right to the back of the stage today and then watched The Matrix Revolutions rather loudly. The manifolds are still operating as grossly undersized sealed boxes with opposed drivers. The combined subs managed to "melt the floor" under our chairs a few times. The polythene DPM out in the enclosure was rippling in time to the visible driver excursion of the IB. I probably saw +/- 1/4" cone excursion (+/-6mm) during the big fight scene in the rain. No doubt it would have been even more exciting with a "hotter" setting on the subs. I suppose I should have used the SPL meter but I was too busy enjoying the film. It's ages since I last saw it.  What a truly remarkable example of special effects. One which actually works. Even the actors are well up to the task for an SF action film.

An update: I have fiddled with the phase, crossover point and levels of the two sealed manifolds and the IB relative to the speakers. The idea was to fill the trough around 180Hz. The new curve looks slightly worse than the last one. What is much worse is the sound quality. Lifting the levels on the sealed boxes has muddied everything.

Switching between the subs using the push buttons on the CX2310 it is easy to hear the difference in SQ on organ music. On films and rock music there is more drive and attack. We watched S.W.A.T last night which has lots of rock music tracks. Drum and bass both sounded better. On classical organ there is no detail or subtlety. Just loud "subwoofer" bass.

I'm still getting resistance to the idea of cutting more holes in the house.  So I may just return the older drivers to the big manifold. I am thinking of switching the newer drivers to the bottom of the box. The older drivers can go up in the top section. This may not achieve anything useful but it's something I haven't tried yet.

I have a couple of 10" SEAS 25F-EWRX drivers which I built into a stereo pair of 6th order series bandpass boxes. They have been resting unused for years but could be fitted into a couple of sealed boxes just to fill the 180Hz trough. The problem always comes down to the necessary 3-way crossover and amp to drive them. They might add the missing "wallop" which the present 180Hz trough is denying me. I have tried every imaginable crossover point from around 40hz up to several hundred Hz but nothing helps to fill the trough. It is obviously room related but with a twist. The interaction between the main speakers and offset IB is still a problem.

Logic suggests that I need a balancing IB on the right of the stage. There just isn't anywhere to fit one sensibly. There is a box dormer to the right of the stage where I sit at my computer. Beyond that is solid cupboard space on both sides of the central chimney. The ceiling of the dormer is flat with the open roof space above the insulation accessible but thermal losses come into play. It's a shame because it would be otherwise ideal. Probably with room for a compact 4 x 15" manifold up there. I have been over this so many times in my mind but nothing suggests itself. At least nothing that I can easily get away with and still have a clear conscience. :-)

Update: 25th Aug. I have returned the four older drivers to the big manifold.  




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Labyrinthine musings:

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As usual I am just thinking aloud again:

Having failed with an OB sub I am still feeling an urge to do something constructive with the four, older, AEIB15 drivers. (Vinyl cones, big magnets, Fs 32Hz, Xmax~15mm, remember them?)

While the present IB performance is fine I still want to reduce the IB's Total Vas Ratio to a more normal figure. I would like still more impact on drums and bass guitar than I have at the moment. A more "Hifi" sound if you want to be really picky. My Mission 753F, main speakers have only four 4" diameter bass/mid drivers with only the bottom two reflex ported.

Which is where the four older 15" drivers could excel if given the opportunity. Ideally they should be moved from the big manifold to a central position between the main speakers. Where a higher crossover point could be used if necessary or found desirable.

I may have achieved a reasonable balance between the two sets of drivers with the BFD. But it is obvious that they are still very different in electro-mechanical character. The newer drivers are being handicapped in a much reduced enclosure volume to driver ratio. Which is rolling them off early. The older drivers can't do low bass without massive boost. So the older drivers have to come out of the tall manifold but still be used for something useful. I hated to have them just sitting in boxes while I listened to the new drivers. It was such a waste of (huge) potential. Which is why I built the big manifold in the first place.

Behind and above the TV and main speakers is a 45 degree, sloping wall. Above this wall is the roof itself. There is at least 4" wide continuous eaves ventilation from end to end of the roof.  With 12-16" of rockwool resting above the flat ceiling and sloping surfaces of my attic AV room.

I am now thinking of fitting the older sub drivers so that they exhaust backwards through the knee wall. Either from baffles or manifolds. This will depend whether I decide to have two drivers in front and two behind me in an identical arrangement. (this might need a separate amp and EQ for each pair) Or have all four drivers in front which is easiest.

The rear wave would find itself opening upwards into a very wide, and quite deep, wedge-shaped area above the insulation but still under the roof surface. Some sound will inevitably escape through the continuous roof vents but I doubt this will matter given our rural situation. There would be no air resistance (at all) other than that required to negotiate the upward bend just beyond the knee wall. So in some senses it is an IB but with an extended path to freedom. Hence the term "labyrinth" mentioned in the title.

I am thinking about thermal losses while still having my audio "cake and eating it". A very low position in the room will avoid the higher temperatures up near the ceiling due to stratification in our two story, open plan home. Open plan probably sounds a bit too posh for an open stairwell in the middle of my AV room.  This stairwell is what has been putting me off fitting an IB in the ceiling. I could not possibly tolerate the heat loss through the cones in winter. The drivers already sweat in the unheated IB enclosure if the magnets are placed outside the manifold.

Any shutter on the warm side of ceiling-mounted drivers would cause the drivers to cool rapidly to outside temperatures. With subsequent and massive condensation upon reopening the shutter! Seven feet lower down at first floor level should reduce this problem considerably. Moreover, the drivers will not be directly subjected to outside temperatures. Nor will they be radiating room temperature to the open roof above the insulation. A manifold could  provide an exit to the cold outdoors as small as 17" square per pair of drivers. Far better than having the driver cones fully exposed to the open roof space itself.

The baskets would be placed on the warm side of the wall so that they enjoy the heat input from the room's ambient temperature. Driver orientation makes no great difference to sound quality with subwoofers. Placing the baskets in the room allows a simple, flat, fine wire mesh screen to protect the driver cones from any birds or other wildlife which might get past the careful screening at the eaves. (I had a tiny wren exploring the IB enclosure one day!)

The list of advantages goes on: The drivers would find themselves placed in a dark, invisible, storage area behind the TV stand. No special finish would be required as an existing curtain drops in front of this area to conceal the few storage boxes hidden back there. The very low position at the throat of the large horn, formed by the floor and sloping ceiling, should enjoy considerable boundary gain. The central position should help to reduce the present troughs between the speakers and the present IB.

While no offset bass effects are noticed in the soundfield, with the present IB location,  placing drivers centrally can't do any harm. To avoid stereo channel confusion I think I will use the 300 watt Bash amp from my SVS cylinder with all its extra controls. Though it is limited to a 120Hz maximum crossover point with a 12dB/octave slope this should be enough to let the IB15s provide a bit of extra impact.  

Shrinking the big IB manifold down to four opposed drivers would let much more light in through the glazed doors. It would allow a stiffer manifold, closer to the floor, for yet more boundary gain and better SQ. The big amp could go back on top of the manifold out of the way. I  am also looking at ideas for a sand-filled manifold rather like St.Louis Bob's superb OBs. Or even cast concrete panels. Or paving slabs for shelves or manifold reinforcement, cladding or resistive mass. A four driver manifold allows much more space between the drivers for such fun and games without the manifold box increasing too much in size.

You might be asking why not move all eight drivers to this "very desirable situation" in the sloping wall? Well, the present IB is a proven SQ winner with just four drivers. The enclosure volume is ideal for four drivers but not eight. The peculiar quasi-IB/ labyrinthine/ bandpass arrangement, which I am discussing here, may not offer the same quantity or quality of bass.  It remains an unknown quantity until it is built and tested half to death. Eight drivers would also take up much more room and require four manifolds.

I already have enough ready-cut baffles to throw two compact, two-driver manifolds together to check if this idea actually works.

27June 2010: Update: I built a quick and dirty opposed 2 driver manifold out of 3/4" plywood but have yet to cut new holes in the construction of the "happy home". Instead I turned the box opening downwards to achieve a sort of seriously undersized, sealed box. This sounded louder and deeper than having it open to the room. I tried reversing the polarity relative to the remaining two in the big manifold but could hear no difference subjectively. Have yet to try REW on the new box. The two sets of older drivers sound much quieter than the four newer drivers in the big manifold when I mute each box in turn on the active crossover.

Having removed a small area of the house roof covering I was able to peer down into the insulation filled void from above. Just to remind myself what I did when I was building the new roof. Clearance between the inner side of the 4" external, block house wall and the present plywood knee wall is only about 7".  I can most easily add a new knee wall/ baffle further into the room to allow more clearance if necessary. I could use weld mesh to fix the insulation on either side to achieve a very wide and deep, insulation-lined slot to the underside of the roof covering. (and thence up over the top of the 12"-16" of rockwool roof insulation) Any leakage to the great outdoors (at the well ventilated eaves) would be on the opposite side of the house to the nearest neighbours. I deliberately opened a dormer window while playing a loud and bassy CD today and found sound leakage well down. Not the least bit problematic.

It is quite shocking how much room two 15" drivers with magnets outwards on a modest 12" wide box take up indoors. For greater compactness, I am sorely tempted to build a horizontal line array despite my previous negative experience with such long baffles. I am thinking that the driver assembly will be working against the bottom of the rafters, floor joists and the entire roof mass above. (rather than sitting in a flexible wall between flimsy, original studs in the former vertical array)  The bulky magnets and baskets can best be lost in the space under the 45 degree sloping ceiling behind the baffle. Though this does raise thermal issues with cold drivers in winter. I can easily add more mass in the form of paving slabs and local bracing if necessary. As well as fixing the baffle/ knee wall firmly to the surrounding timbers.

Avoiding the present, +16dB bass boost at 20Hz will reduce reaction forces dramatically. I confirmed this today when the temporary sealing boards started rattling over the empty driver cut-outs in the manifold. Removing the boost instantly silenced the vicious buzzing. I was then able to raise the boost to +6dB without further problems. An hour of listening to Bass Outlaw's CD "Illegal Bass" was both interesting and entertaining. I found it impossible to localise the bass to the new box even when I had my head right beside it. All of the bass seemed to be coming only from the nearest speaker two feet away on the other side of my head. A very odd sensation since I had already raised the crossover point to 120hz.

I built the second manifold and placed that too open side down on the carpet behind the other main speaker. It was time to fire up the new [2010] version of REW.

N.B. No effort was made to EQ the results. Only to roughly balance the various units and ensure they were all connected in the best polarity relative to each other. The manifold boxes were still much quieter than the remaining 4 x 15" IB but I adjusted the gain until they matched well enough for listening tests. The new boxes peaked at 90hz rolling away slowly to below 20hz. The various combinations appear in the image below:

The red trace is all subs together using a 80Hz 24dB/octave active crossover but no speakers.

Yellow is with the speakers connected but deliberately set out of phase for comparison.

Blue is the best match between speakers, 4 x15" IB and the two box subs. (2 x 2 x15")

The 180Hz and 280Hz troughs are reduced relative to the 8 driver IB alone though not absent.

There is more energy in the upper bass now. Which is exactly what was hoped for with music.

The question now is whether it is worth hacking holes in the sloping wall behind the speakers to allow the two new boxes to breath as quasi-IBs? Or should I build a new horizontal line array (IB) for all four older drivers? The two new manifolds owe me nothing in time or materials as they were thrown together from scrap 3/4" plywood and simply screwed together.

As the rear wall slopes at 45 degrees I have various options regarding the height of the final design. The higher I place them the greater the likely heat loss through the cones. Though this does bring them nearer to the plane of the main speaker baffles. If  I raise a line array just high enough it won't upset the present storage arrangements at all.

A simple (long) box with bottom and front at right angles would bring the driver baffle to the vertical. Triangular ends would seal the box to the sloping wall.  Internal triangular braces would help to stiffen the whole arrangement.

Placing the array box three feet off the stage floor would offer the greatest freedom for air movement between the sloping indoor wall and outer roof surface. I have a weird roof construction which I built myself to make room for lots of rockwool insulation. By overlaying a new set of rafters at a lower angle I gained plenty of space without needing hugely deep rafters. The original roof was 45 degrees and very probably thatched when built. The new roof surface is 40 degrees. The small difference in angle provides plenty of space for well ventilated insulation. The large overhang was designed to allow heavy external insulation but I haven't got round to doing this yet. This rough drawing is not remotely to scale: Green is 12-16" of rockwool.

 As can be seen; exhausting the box to the space above the insulation would constitute a true IB as near as matters. Not only is the free volume above the insulation very large in area but the eaves are also well ventilated. The only thing which stopped me going ahead years ago was worrying about heat loss though the driver cones. The 12" insulating blanket would have to be pierced to let the drivers breathe. The area where the manifold/array would sit would lie behind existing, concealing curtains. I could always have a hinged, super-insulated, drop-down flap to cover the driver cones when not in use, I suppose. Though I would greatly prefer the insulation on the cold side of the drivers.

Getting such an idea past the "Head Gardener" is the greatest hurdle to fruition. Her patience with my IB building is wearing rather thin. Nothing I have tried so far reduces her love for  the old SVS cylinder. At least this tension offers me some leverage.  I can always claim I am only building new and better IBs in her interest. Nah, I wouldn't buy that story either. :-)
 

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Filling the trough.

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It is ages since I added anything to this blog. I floated the idea of open baffles on the IB Cult