All-new ultimate 3-way proj. with Accutons and Eton

[CeramicMan]

It begins.

I'm about to start making a pair of loudspeakers that I want to be of Avalon-beating, close-to-unbeatable fidelity. They will use:
Accuton C12-6 25mm concave ceramic dome tweeter, with f.fluid and chamber,
Accuton C44-8 50mm concave ceramic dome midrange, with f.fluid,
Eton 7-360 seven inch kevlar midwoofer.
I'll probably also chuck in a sub to do the low frequencies and keep the neighbours unhappy , but that's the easy bit and I'll leave it 'til later. Currently the drivers are enroute to NZ, so I've got a bit of time to think.

It gets technical!

There will be 3 separate enclosures. The factory-built chamber for the tweeter will have some kind of soft wave-shaping exterior put around it. It'll probably end up looking like a big ball of felt hanging off the back of the tweeter's face-plate, with 2 wires sticking out, dunno about the feet yet. (The tweeter and midrange are definitely not going in combined enclosures, but I'm hoping they'll still be spaced close together).

The midrange is the hard bit I think: anything other than an infinite baffle is out of the question. HP @ about 500Hz, LP @ about 8kHz. The enclosure will probably be around 10L+ and may be larger than the midwoofer's box. Here the difficulty is trying to avoid standing waves between 500Hz and 3kHz or so. At some frequency there's inevitably a changeover from pressure-loading to standing waves with modes at various frequencies, and since this produces a boxy characteristic I want to minimise it as much as possible. Lots of damping material like fibreglass or something + internal panels for diffusion + the smallness of the cone all helps, but the enclosure material needs special consideration too. I was thinking of using something plastic (like putty, not necessarily made of polymers) to absorb high-frequency vibrations. It could also be nicely shaped to reduce diffraction effects. Fimo modelling clay seems good it but costs quite a bit, and it might be too hard when it cures after oven-heating. It would preferably be something that stays a little bit pliable and doesn't gradually harden with age. How about doping it with sand for extra mass? Or what about some recipes for "home-brewed" modelling clay? I haven't abandoned the idea of double-glazing the enclosure walls too, and leaving an air-gap in between.

The woofer will most likely go in a sealed cube that measures around (20cm)^3 internally. Since the longest standing wave in box like that would be around 860Hz and the crossover is at 500Hz, the box will therefore be pressure-loaded throughout the woofer's range, and even have some margin above the crossover frequency. This means that as long as the outer walls are hard enough then the box won't need damping material or even bracing or internal panels. I think 25mm plywood will do a good job. If the resonant frequencies of the plywood (knuckle-tap test) are too low I'll add bracing. Just to be perfect I'll probably damp the walls of the enclosure anyway to stop external sounds or frequencies slightly above the crossover frequency from exciting the various resonances.

Crossovers!

I haven't quite decided what to do for the crossovers. Apart from the usual 3rd/4th/5th-order active filters at around 500Hz and 8kHz, there need to be DC-blocking capacitors for the midrange and tweeter. These caps are unavoidable and I will have to tune the active filters to take them into account.

Accuton/Thiel & Partner go to an unusual amount of effort with modifying the domes of their midranges and tweeters to damp ultrasonic resonances, and I suspect that without it there could be some IM distortion due to huge resonant peaks somewhere above 20kHz. For this reason I'm considering a low-pass filter for the tweeter. If it's passive, it will reduce the electrical damping factor at high freq. and mechanical cone resonances from distortion etc may be worse, but electrical signals won't directly generate these frequencies. If the filter goes before the amp however, any spurious ultrasonic noise generated by the power amp will not be filtered. I think that with a good power-amp the second option is best though. At some stage I will upgrade to a digital crossover system to fix other problems like time delays, nasty room modes, and box resonances. In the meantime, I'll measure the responses with a microphone in the room, and base the active filters on those measurements. What do ya reckon?

CM

[Gregm]

Just a thought -- Why not run the mid & the Eton open baffle -- esp. since you're considering a pair of subwoofs to complete the scenario... It might make it even more "airy" & fast in the mid/lowmid region.

Ok, that's complicating things -- BUT why make it simple when you can do it complicated, right

BTW, isn't the Thiel-Eton combo identical to the Kharma Ceramiques?
Cheers

[CeramicMan]

I've never really been a big fan of open-baffle speakers - they seem to require special positioning and I can't listen to them comfortably at off-axis angles. I find it unnatural if I move off to the side and sound cancellation makes the tone of the music change drastically and the midbass disappears - it stuffs up the room interactions and makes it all sound a bit unreal. But that's just my preference. My existing speakers are 2-way bookshelf ones with the Accuton C23-6 and C94-8 which I can listen to anywhere in the room without having to worry about sound-cancellation and things like that, so I want something like that just bigger and better.

Originally posted by Gregm
...isn't the Thiel-Eton combo identical to the Kharma Ceramiques?
Cheers

The Accuton C12 is quite common, but no-one seems to use the C44-8, they all use the bigger C79 midrange instead. I think that's an important point of difference, and since I chose the C44-8 midrange for its super-smooth water-fall plot I have to work around its non-existent bass response. This is where the generic midwoofer comes in, I chose the 7" Eton, but I also considered 7" and 8" Seas and Accuton drivers. Because the requirements for the midwoofer in this project aren't that demanding (up to 500Hz, low bass not required), I couldn't really justify the extra cost of Accuton or Seas woofers.

Other points of difference between Avalons/Kharmas/etc and my ones are that they won't be generic MDF monoliths. They won't have midbass ports to bloat the bass, and they won't have a 1/4 tonne of ineffective dead wood weighing them down. I also don't want problems with size and directivity caused by a huge difference in size between the woofer and midrange. Regardless of how good the crossover is, it can't make a 10" woofer more omni-directional at 500Hz and a 2" midrange less omni-directional at 500Hz.

What are other people's experiences with these sorts of things? The midrange enclosure design is starting to worry me, I don't want to stuff it up if the modelling clay doesn't turn out to be any good. Thick sheets of polypropylene for the walls of the midrange box was another option, but now I think it'll be too light and will require weighing down and damping with something else anyway.

CM

[5th element]

Hmm I wouldnt knock the up to 500hz region, remember almost all the fundamentals of the intruments will be produced here, and although its tha harmonic content that sets them appart you still need good reproduction in this frequency range.

I had my W15cy's crossed at 300hz to a peerless 850146, then I lowered it to 150hz, which sounded better?? The 150hz because the W15 is better quality then the peerless. Although this was not a big big difference, it was worth having. After all if this is close to unbeatable fidelity then you would be wise to choose the seas or accuton, BUT the etons are still very very highly regarded and I bet would give the seas a run for its money anyway.

[Andreas]

Regarding the mid cabinet.
make it from laminated MDF and lead sheets glued together with viscoelastic glue. Something like 6mm mdf, 1mm glue, 2mm lead, 1mm glue, 6mm MDF.
ad some internal bracing an stuffing. The lead laminate is really good in converting the vibrations into heat (which is what damping here is all about).

cheers
andreas

[Gregm]

Hi Ceramic-

I've never really been a big fan of open-baffle speakers - they seem to require special positioning

Well, this is where "de gustibus... etc" applies. Never found placement difficult though -- as long as you allow some distance from the back walls.

In another vein,

HP @ about 500Hz

As per 5th, I too find the 500Hz cut a bit tricky. I mean, that lands you smack-bang in the voices region.

I'm no experienced designer by any means -- but I have tried ~400 & 800Hz with subjectively better results higher rather than lower (note: with ATC drive units -- not yours).
Looking at the c44's plots it DOES look happier higher rather than lower...

OTOH, the Eton looks happy enough even at ~1,5kHz... which of course, begs the question of your liking (or not) the Eton reproducing of most of the vocals...

[BAM]

Bass drivers for subwoofers: Go to PartsExpress and look at the JBL Sub 1500. It has a very low inductance, which means that it will have some great transient response. It also has shorting rings and a T-shaped pole piece for very low distortion. Kevlar-impregnated paper cone. (This is Kevlar done right, not just with stiffening epoxy.) 4" voice coil. Butyl rubber surround. Originally made for Revel, then used in JBL TiK series, and now phased out. PE has the last remaining stock in the world.

Addendum:
Oh yeah, 25Hz Fs.

[leadbelly]

Originally posted by CeramicMan
It begins.

I'm about to start making a pair of loudspeakers that I want to be of Avalon-beating, close-to-unbeatable fidelity. They will use:
Accuton C12-6 25mm concave ceramic dome tweeter, with f.fluid and chamber,
Accuton C44-8 50mm concave ceramic dome midrange, with f.fluid,
Eton 7-360 seven inch kevlar midwoofer.

Sorry, cannot be of much help myself as design work is over my head, but I did not see you mention Tony Gee, and since he is working on an Avalon clone (and is one sharp cookie), it cannot hurt to touch base with him:

http://home.hetnet.nl/~geenius/news.html

[CeramicMan]

Thanks for all the replies so far people, keep 'em coming!

With these sorts of projects, it just seems that the more you know about DIY audio, the harder it gets. But it's all good! In the pursuit of perfection, I've found that more and more obstacles seem to pop up out of nowhere and you get to the point that you realize that you practically can't make "perfect" loudspeakers, but how close you can get depends on.... blah bla-blah, you get the idea.

No such thing as a perfect midwoofer box
I don't want to overestimate the performance of the woofer because even though its lowest break-up frequency looks like ~3kHz, box resonances will start much lower. If I go ahead with an 8L to 10L box (which is already pretty small) measuring 20cm - 22cm in a cube shape, then the shortest wavelength that definitely won't cause any resonances will be longer than about 80cm to 88cm - which is already under 400Hz. The lowest resonance will occur when a 1/4 wavelength of air will fit inside the box between the cone and any hard surface. At that frequency the cone will move with a reduced mechanical load, which could be problematic as it might cause ringing and/or a change in sensitivity.

Other box resonances (where the cone is an anti-node and the walls are nodes) will occur when 3/4, 5/4, 7/4, 9/4.... wavelengths fit inside the box. Because these waves are complex 3-D entities (or 4-D I should say ) they don't have to be harmonically related and there might be 100s of them below 3kHz. The higher the frequency the more easily these resonances are absorbed with damping material, but there's also more and more of them.

This is all assuming that the walls of the box are perfect nodes, ie: perfectly still, and that only the cone is affected because of its less-than-perfect stiffness and electrical damping. In reality the walls of the box will also resonate above a certain frequency, which could be even worse because this can't be fixed with digital speaker-calibration systems. So, to avoid problems with the woofer operating at high frequencies, then a lower crossover frequency is better.

When considering the midrange though, its +-0.8mm linear excursion has to be taken into account, and that it has a slightly ringing response at about 400Hz. It can be seen on the speaker's step-response graph and I think it's caused by the low electrical damping of the speaker's mechanical resonance.graphs To achieve reasonable power-handling without too much pk-pk cone movement the crossover frequency should be relatively high.

CM

[Blitz]

I would highly suggest to you to have a look at Doede's site:

www.dddac.de

He made good experience with the chassis you are planning to use. I heard his system last week and it shows already in the test-phase right now very high potential.

Doede uses the Diamond tweeter (ouch, 4000 $ a pair) and very important in Doede's and as well in my point of view a Acouton 8" driver from 40Hz on. Forget the Eton at this stage, it will not work. The 8" woofer will play significant into the mids and therefore it is import to use the same material as your midranger has. As big as 8" gives you a very nice warm but fast fundament as the small mid-rnager will sound otherwise to analytic and has a lack of colour and anyhow not enough dynamic.

To have the ultimate system I would take the three chassis mentioned above, put them in an enclosure and have a corner-horn for the last octave. Nothing, absoutely nothing beats a corner-horn in the way it makes music. Real music and not a bum-bum at the lower end. Sorry to say that even Doede's system would sound better with that (and he agrees on that with me).

When going this route, I can promise you that you will have better imaging than an electrotat, warmth, color, huge dynamics with fairly small satelites and a bass system which you might not even see (in the corner), but which can beam you into a church.

Best Regards