Illuminator Monitor
Copyright 2012-21 Troels Gravesen


The article here was meant to be a quick review of the ScanSpeak 18WU/8741T00 driver paired with either 7100 or 6600 tweeters. A two-way from said driver and the 6600 tweeter was quickly in place and I started on the implementation of the 7100 tweeter. Another two-way in a few days. My simulation software is a great tool - and toy, and the more I played around, the more options became available, thus numerous crossovers were made for evaluation, each time taking performance a little further. Every time the "quick'n dirty" turned out not to reveal the full potential of the drivers. I used to think 3-ways were more trouble than two-ways, but now it seems to be the other way around. Due to the limitations of a two-way, care must be taken in overall balancing of the system. It takes time adjusting to the limited sound stage from a small speaker, being used to DTQWT and Jenzen speakers. The traditional 6+1 really can't play very loud and the weight of the bottom end is missing. Anyway, below the best - to my ears - is presented. No 6600 version will be presented.

Illumination can mean enlightenment, insight, etc., and this is probably what the ScanSpeak guys were thinking of when designing these speaker drivers. A driver that would enable us to dig further into our recordings and discover new details previously hidden by flexing cones that may provide an overall smooth frequency response, but never really deliver the final micro details.
Former top of the range drivers from ScanSpeak featured sliced paper cones, the Revelator series. Revelator drivers are a dream when it comes to crossovers. Due to an inherent smooth frequency response they're easy when it comes to crossovers, the heart of the speaker. It goes without saying that slicing a paper cone and gluing it together again with flexible glue comes at a price. We may have solved the problem of damping resonance - mechanically - but we may have lost the ability to reproduce micro detail.
Getting more detail we need more rigid cones like ceramics or metal cones. The price to pay is usually serious resonance at higher frequencies with subsequent need for notch filters or high-sloped filters, e.g. magnesium cones.

A compromise between soft cones and hard cones are sandwich cones used by e.g. Eton and AudioTechnology. Eton favors the hexacone technology, while AudioTecnology has adopted a sandwich of paper-foam-paper, but only for bass drivers. Both technologies work well and deliver pistonic movement within their operating range. Another driver (cone) that impressed me was the Chario Sonnet midbass, which is actually a Rohacell cone. Rohacell is a foam material made from polymethacrylimide and the trick is to make a sandwich of two layers of plastic polymer with the Rohacell foam in between; rigid, light-weight and with high internal damping. Trouble part is designing cone geometry so that we also get a smooth frequency response.

ScanSpeak decided to stick to paper, but here we have two identical ribbed paper cones glued together by something. What this "something" is remains a secret, possible only glue. The result is a rather rigid cone helped by the curvilinear cone geometry. And the dust cap - the inevitable - and troublesome - dust cap is not just a standard hard pressed paper dome, but made from a pulp similar to the cone itself.  Whether the dust cap is a sandwich I can't tell. I guess it is.
Next they designed a low-reflecting magnet system that would at least divert rear energy away from the driver and not bounce back into the cone, thus a fancy neodym magnet system with a sloped upper pole piece allowing a deep magnetic gap of 20 mm! The voice coil height is only 8 mm, thus a linear excursion of +/- 9 mm according to SS specs. 9 mm will take the voice coil out of the magnet gap it seems. What we have here is an under-hung voice coil contrary to most speakers fitted with a short magnet gap and a long voice coil, the over-hung system. The benefit from this is arguable, but we don't have a coil outside the magnet gap picking up eddy current from the magnet gap. At least that's what we learn about underhung voice coils. There have been made many great speakers over the years without symmetric drive and with overhung voice coils and to my knowledge we have never seen distortion measurements from two otherwise identical drivers having these features.
What I leaned from Per Skaaning of AudioTechnology is that an under-hung system usually provides a slightly concave frequency response contrary to an over-hung system delivering a convex frequency response. It looks like this:

Looking at the frequency response of the 18WU drivers suggest there is something to this generalisation, a slightly recessed midband compared to the response at e.g. 100 Hz and 1 kHz.
I've been hesitant to dig into the Illuminator series due to three reasons: Higher price (compared to Revelator), lower efficiency and a ragged frequency response. These are good reasons, aren't they? 
If we look at 18W/8545-00, 18W/8531G00 and 18WU/8741T00 we have a claimed sensitivity (2.8V) of 88 dB, 87 dB and 85.4 dB respectively, the low efficiency of 18WU partly due to underhung voice coil. To my thinking this is the wrong way. What we get is lower bass from smaller enclosures at the expense of efficiency and there may be good marketing rationales for this and I fully acknowledge the need for black bottom line figures rather than producing items a few people love, but nobody buys.
So, the Illuminator series has stayed put until someone offered me a pair of 8 ohms 18WU for loan. I had just acquired a pair of 6600 and 7100 domes, so the obvious thing to do was to set up a simple two-way from these tweeters and the 18WU/8741T00 drivers in my 21 liter test cabs, actually ready-made cabs from PartsExpress, type 302-730, 0.75 cubic feet - for you over there.

Useful links (Please read before writing!):

FAQ (Please read before writing!):
You cannot change cabinet front panel dimensions and drivers' placement without needing a new crossover - and I cannot help.
You cannot use any other drivers with the crossover shown here.
Please read these files before e-maling:

The Drivers

D2904/710003 tweeters. Illuminator 18WU/8741-T00
Click to view data sheet. Click images to view large.


Click image to view large.

I suggest 20 mm Baltic birch for cabinet. Place bracing across cabinet height as seen on sketch and make curved cutout for midbass driver. Make front panel from two layers of 19-22 mm MDF. Make sure you chamfer midbass driver hole as seen on sketch.
Place tweeter crossover at bottom and make sure it can get through the bass driver hole - or make a hatch at the rear side. With super caps and wax coils you need to make sure you have sufficient space. The bass crossover is placed on rear panel between the two braces.
I strongly suggest attaching the rear panel with screws for easy access to the crossover. Small cabs are always a challenge when it comes to space for the crossover.

Cabinet damping: Please check: Cabinet damping and this one: QUATTRO.htm for ideas. Damping materials included in kit.

Felt for port: Cut 13 mm felt to 100 x ~195 mm, slightly trapezoid like seen on photo:

I've recently seen this trick launched as the eight wonder of the world by Sonus Faber; it's called "Stealth Reflex", there you go! 
This was used by Spendor four decades ago.

From SF website:
The "Stealth Reflex Port" is Sonus Faber's patented solution for the realization of a para-aperiodic enclosure ventilation system. The properties of this system allows for deep, extended bass with the same or less cabinet volume. In addition, it has all the advantages of conventional ports but eliminates the inherent "chuffing" sound completely.

Test cab front panels.



As mentioned above, you can make a floorstander from this construction with a 22 cm wide front panel. Use 22 mm MDF throughout. Maintain top chamfering of front panel, reduce side chamfering to 45 mm width at top. Cab height is 100 cm + 30 mm base plate. Use feet or spikes to make top panel some 105 cm above floor level.
30 liter cab volume is close to ideal for the 18WU/8741T00 driver making F3 = 37 Hz. Port tuning = 35 Hz, Use 68 x 220 mm port (Jantzen #900018). And no felt in this port! 
Cabinet dimensions from 22 mm MDF: 220 mm width x 1000 mm height x 344 mm depth, all external dimensions.
The rationale for the deep box is to reduce early reflections. Now we have a low-reflection speaker driver there's no need to spoil the good work of ScanSpeak but adding a rear panel up against its back.

Click images to view large.

Check out here for damping. Use 13 mm grey felt on all internal panels, except front panel. Place two layers of 30 mm acoustilux on top and rear panel behind drivers, not behind port. Place three layers of 30 mm acoustilux below port.

The Sound

What struck me the most listening for the first time was the bass response. Not since the unfortunate 2.5 clone had I heard such low bass from a 6" driver, and this coming from a meager 21 liter. Quite impressive although you should not expect a full-bodied sound like a Jenzen speaker, which were in place when these were taken to the living room.
Lots of low-level detail too thanks to the rigid cone and high mechanical Q. These rigid sandwich cones indeed do reveal a few details former Revelators may be short of at higher SPLs. But don't overestimate a "6+1" system, there are limits to how much power it takes. As-is they deliver a smooth midrange with good upper-mid integration and should these speaker sound harsh, look at your gear - or you're playing too loud. Maybe a bold statement, but I'll stick my neck out.
And BTW: 6" or 7". There seems to be some inflation in driver size as SS really market these as 7" drivers. They are big 6" drivers with a membrane area of 154 cm2. Usually a 6" makes 130-140 cm2. Most 8" drivers make 220 cm2. So, I guess a 7" should manage something in the range of 170-180 cm2, like the Audio Technology 6I52 having 174 cm2. The Accutons are marketed as 7" driver and really only makes 132 cm2 and are nowhere near a 7" driver, rather a small 6" driver. Anyway...
This 21 liter stand-mount will challenge any 30 liter floor-stander and should your spouse find 21 liter on a stand less intrusive on interior decor compared to a 30 liter floor-stander, this may may be an option for getting some good hifi. And while we're at it, you can extend the cabinet to some 28-32 liter and get even lower bass. Keep panel width and driver placement with regard to top of cabinet.
As always, tweeter level needs some experimentation, thus a range of series resistors added to the crossover kit. It MUST be tried to suit room-placement, room damping, listening distance, front-end gear and not least personal preference. Taste cannot be argued and if you want a ruler flat response at 2-3 meter listening distance, be my guest. Due to the low point of crossover, dispersion is significant in lower-treble range and from a flat tuning it can be too much - and to my ears, just doesn't sound natural. We badly miss the tone controls of former line-stages to balance the sound in our rooms and the result may be endless tests of gear and cables to find a proper balance. Quite often a 1 USD resistor is all it takes to tame a too aggressive treble level. 


The crossover follows an LR2 topology. For the bass an RC circuit across series coil provides damping of peak at 4-6 kHz. The tweeter high-pass filter is a bit unusual as it provides compensation for the delayed response from the woofer and allows drivers to be connected with the same polarity. Bottom line is that we can manage a close-to-perfect LR2 topology. We could have made a stepped front panel with the tweeter retracted 35 mm, throw out the all-pass filter, connect the tweeter with inverted polarity and we would get the same response as shown below. 35 mm retraction is likely to cause some disturbance for the the tweeter response with subsequent corrections. In short, it wasn't tried. And yes, the tweeter PLUS goes to what appears to be minus but as can be seen from step response below both drivers now have positive response due to the all-pass section.

Numerous crossovers went before the one shown above but they never really matched the easy flow of upper mid/lower treble. The quick'n dirty goes like this:

Works well on all parameters - except sound. Not the level of transparency the drivers are capable of and some harshness to vocals and strings. The quick'n dirty is basically a 2nd order filter providing a 3rd order acoustic roll-off (Butterworth). It provides slightly better power handling but at the expense of other parameters as pointed out.

Tweeter attenuation: I recently had a mail for a builder complaining about lack of treble from his construction and fortunately he had measuring equipment allowing comparison. It turned out he really like his current speakers better due to 6-7 dB higher treble level. Now, that is a lot! If you take a look at measurements show below and elevate treble level some 5-6 dB we're there. With the 7100 tweeter it could actually be done, but it would sound terrible. The graphs below is from R1011 = 2.2 ohm providing an almost flat response. Lots of detail and see-through. I suggest trying 3.3 and 4.7 ohm for R1011 to render a more true presentation of the treble range. Additional resistors provided with the kit. 


Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this ports may contribute to bass extension. Most of us diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.  
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell anything unless seriously bad and most modern drivers display low distortion within their specified operating range. 
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are way more to good sound than what can be extracted from a few graphs. I think I speak for all of us doing speaker measurements when I say that we learn new things every time we do one. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.

Left: Response of system from 400-10000 Hz with input normalised for 2.8V/1m. System sensitivity = ~85 dB/2.8 volts. Right: 200-30000 Hz displayed.

Left: Point of crossover around 1.8 kHz, 12 dB/octave. Right: Impact of RC circuit across L2011.

Left: The tweeter roll-off could be further smoothed by an LCR circuit across tweeter terminals, e.g. 1 mH/68uF/4R7. Listening tests at quite loud levels didn't suggest this tweak imperative. Right: System impedance, minimum = 6 ohms. No particular difficult load on the amplifier.

Left: Vent tuning from port as-is (black) and with felt inserted (red). Right: Horizontal dispersion at 0o, 10o, 20o and 30o

Left: CSD at 25 dB scaling. Right: Step response.

Left: 2nd and 3rd harmonic dist. shown @ 2.8V input. Right: THD shown @ 2.8V input. Equivalent to around 85 dB/1 meter.

Left: 2nd and 3rd harmonic dist. shown @ 5.6V input (disregard red graph). Right: THD shown @ 5.6V input. Equivalent to around 91 dB/1 meter.


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Crossover Layout


Click images to view large. The terms "upper" coil and cap refers to the orientation of components shown in schematics.
0.33 uF does not come as Superior-Z, thus Silver-Z.
If you use standard PP caps and wire coils all of this will take up significant less space - but sound will deteriorate.