I have been listening to my favourite classical organ CDs all morning to assess the new drivers.
These include: Amongst many others of:
"Bombarde!" French organ classics: Liverpool Cathedral: Ian Tracey. Chandos CHAN 9716
"For Weddings" Chichester Cathedral : Kevin Bowyer. Nimbus NI712
"Tsar of Instruments" Organ music from Russia: Winchester Cathedral: Iain Quinn. Chandos: CHAN 10043
Cesar Franck's Complete Organ Works , Jean Guillou, organ at St.Eastache, Paris. Brilliant Classics 92282. On 2 Cds this 1989 recording is simply stunning for its clarity and capture of the building's reverberation at all frequencies. The bass quality and weight is phenomenal with the timbre of pedal stops and pipes laid completely bare by a good subwoofer. The inner detail of the big pipes and their complex interactions is entrancing.
The deep tones from the great pipes sound suitably heavy and effortless with fine rendering of timbre. The way the pipes stop and start is vital to enjoyment in my experience. Some pipes have a slow way of breathing themselves gently into audibility. Others chuff or start and stop abruptly. Some change pitch as they play their tone. All have different tonal qualities and this should be heard without effort. It is never enough to just reproduce low frequencies. The reproduction must be done with low distortion and great subtlety of tone, timbre and dynamics. It is no use just filling the room with loud sinewaves. (or square waves for that matter) That is not what organ music is all about.
The tones one hears must be nimble to match the footwork of the organist on the pedals. Most cathedral organs have various ranks of great pipes which can be played alone or in unison. The 16 foot pipes sometimes dance on a foundation of 32 foot stops. These huge pipes can produce inaudible tones which modulate the sounds of the shorter pipes. Sometimes the great pipes beat together producing a "tremolo" or "warble" effect with varying frequency. Sometimes the beating has clear gaps between the beats. Which themselves are constantly changing in speed and strength. I have heard beating as slow as a half second cycle. Putting one's head in the manifold brings out even more detail suggesting that there is much more to be had if only it could be brought out into the room. The danger is in overemphasis of the bass by playing the IB too hot. Very impressive but a little unrealistic.
All this detail must be reproduced for the listener's ears without artefact or distortion and without making him or her work hard to hear it all. The only way to do this well seems to involve many large drivers working in unison in an IB.
A single 12" driver can reproduce low frequencies in large quantities particularly if placed in a large, ported enclosure. Though it does so with too many false harmonics. Which often mask the fundamental with unwanted distortion. All detail is lost to a muddy rendition of loud sound waves which have little to do with the original musical performance. It could be described as a black and white cartoon rendition of the music. Bold, but completely lacking in the fine detail which we take completely for granted in the cathedral, hall or church when listening to live organ music.
The IB subwoofer fills in the details like a colour cine film. Everything is constantly moving, changing in tone, colour and contrast. One can hear the great pipes start, sustain and decay. The IB adds huge weight to the sound no matter how briefly a pipe may speak. The incredibly deep tones dance nimbly on the jumbled infrasonic ice field hidden just beneath the more audible surface.
Here's an image of the new drivers installed in the same IB manifold. The drivers have been brought magnet to magnet inside the manifold to avoid the metalwork "sweating" in cold Danish winters. My IB enclosure is unheated, though frost free and subject to rapid temperature changes in afternoon sunshine. A perfect solar oven were it not for the white curtains drawn across the glass gable end. Endlessly delayed plans to fit exterior louvres to block summer sunshine have never matured into reality.
Warning! : Large 300KB image if you click on the above: Not for slow modems.It has been pointed out that my T-nuts are not properly bedded into the plywood to lock them firmly into place. This is because I originally planned to line the box with OSB or double the thickness of the plywood. As I may have to replace the original manifold I haven't bothered to secure the T-nuts properly yet.
The next plan is to build another manifold to stack on top of the existing one. This new box will support the four original drivers. Bringing the tally up to eight fifteen inch drivers. The Total Vas ratio to enclosure volume will fall to around 4.5:1. It will be interesting to hear how this arrangement performs when driven by the same amplifier. I may need to examine other installation ideas if it fails to please. These plans have been put on hold while I examine further alternatives for the four old drivers. I might build an open backed baffle over on the right hand side of the main speakers. The idea is to boost upper bass/midrange to allow higher levels and dynamic range. With the present performance level of the IB I don't need to do anything in reality but would like to use these older drivers for something useful.
I am examining ways to avoid the magnets coming so close together. I may stagger the drivers relative to each other in a new box manifold. To make the magnets alternate from one side to the other. This will make cross bracing between the drivers difficult or impossible though. Not that I've ever noticed much flexure in the manifold sides so far despite the use of only single layer of 3/4" (18mm) plywood.
It would not be too difficult to arrange something to go between the magnets to stiffen the structure further. Any inter-magnet bracing would need to allow the pole vents to function normally. It will have to wait for warmer weather before I am really tempted to start making more sawdust. In the meantime I shall be considering different ways of doing things. I am an inveterate modifier and a martyr to perfectionism.
There is little doubt that, should they ever suffer the misfortune of my arrival in any sort of heaven, I shall be offering ideas for improvement from day one. True perfection would be intolerable for me to enjoy. Others sometimes think I am being negative or criticising when the complete opposite is true. I automatically seek alternative and possibly better ways of doing things. This is the habit of a lifetime. Ideas are simply thrown up in the air. To be shot down or caught and given a bit more spin before being lofted again.
If I should ever earn a proper gravestone I shall have a very simple message carved there:
"IF IT AIN'T BROKE: DON'T MEND IT!"
And carved just below: "No tools to be left near this grave!"
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