SL3-eXtreme Center channel speaker – a design and build story

[Robin]

Beautiful Trifield Center...

And here is an animation of the front view. First is the bare system, then the black acoustically transparent masking which is required to cut the light spill through the screen. Otherwise, even with 18% grey walls, the light bounce from behind would affect the on-screen image. Also, that big metalized ESL panel is mighty reflective as you can see in some of the shots, so we need to tame its light reflections.
Finally, the Screen itself is up. It fills the entire space between the monoliths (80” wide plus frame).

Jon,

I love you design and craftsmanship. You are a very talented and gifted speaker designer and cabinet maker as well. You could work for Martin Logan as a designer, you'd be great!

I was just wondering about your 80" wide front projector screen being in front of the top part (ESL and mid-range speakers), of your new center channel speaker... wouldn't the sound be blocked during film presentation by the viewing screen? Even with a audio perforated screen? It seems like your new center channel would voice better, laying on it's side, just under the screen... with nothing in front of the center speaker. I could be wrong I don't know, what do you think?

 

[JonFo]

Hi Robin,

Thanks for the kudos on design, but the craftsmanship with wood has to be shared with a good friend and my wife. My friend is a master woodworker with six figures worth of tools in a shop also worth well north of six figures. My wife loves tools (gave her a router for Xmas two years ago ) and is very adept at measuring physical stuff. Me, I’m more about systems, electronics and acoustics.

Like I tell my staff at work, it’s the team that delivers the results.

As for the whether a center should go behind a screen or above/below it. The answer is that for maximum imaging and sound field cohesion, it needs to be behind the screen.
Now, the screen must be acoustically transparent. And while my screen is a Stewart Studiotek 130 with micro perforations, which is supposed to be ‘acoustically transparent’, it actually impacts the frequency response a bit. But FR can be equalized back in (reason #42 for a DriveRack).

Since I bought my screen, there is a new material called ‘ScreenPix2’ which is trully acoustically transparent.
John Kotches just reviewed this in WSR this month and found it to be perfectly ‘transparent’ to sound save for a 1db drop in level.
See article here:
http://www.widescreenreview.com/wsr_attractions.php

If you are subscriber, you can download the PDF.

Here is an excerpt:

Audio Performance

I have held several home theatre meets
at my home since the ClearPix2 screen has
been installed into my system. One of my
favorite demonstrations is to raise the
screen while playing a track from James
Taylor’s Hourglass, which features a hardpanned
center channel lead vocal. This
shows the almost non-existent signature of
the ClearPix2 on something that we are
quite familiar with, a singing voice. I vacillate
between screen-up and screen-down
when listening to music as I find that the
screen’s influence is so minimal that it
makes no real difference to my conclusions
in audio reviews. Even test tones are indistinguishable
across the front soundstage on
my identical Fontaine IIs, with the screen up
or down, and only a 1.0 dB bump accounts
for the screen’s presence when it is retracted.
I should note that only the center
Fontaine II resides behind the screen. I
could write a few paragraphs, pulling out a
few audiophile clichés for you to read, but
in the end it would be merely typing practice
for me and page filler for you to sort
through. The fact of the matter is that the
ClearPix2 fabric in front of a loudspeaker is
well nigh undetectable save for the slight
drop in level.

Bottom line, the tradeoff towards ideal placement is greater than the minor (if any) losses incurred with the screen there.

I listened to the system for two weekends with the screen down, and now it’s been many straight days of solid listening (and movie watching) with the screen up. Frankly, can’t really tell the difference anymore. At first I though the highs we’re a bit more ‘congested’, but that was really a combo of time-smearing with the other speakers and the overall FR, which I had yet to EQ to ‘perfection’.

The only thing on my agenda is to upgrade to a ClearPix2 material when I get a digital projector (and do the switch to a 16:9 aspect ratio at that time as well).

Cheers,

Jonathan

 

[JonFo]

Screen materials

And since we’re on the topic of screen materials, here’s a good article reviewing some of the choices out there, including the Stewart I have.

It clearly knocks it for inducing comb filtering. Taming some of those reflections is my next goal. I’ll add felt to the front of the line array box and also to the SL3 rails. The rear wall will also get some form of high-frequency absorption.

 

[JonFo]

Jon,

I love you design and craftsmanship. You are a very talented and gifted speaker designer and cabinet maker as well. You could work for Martin Logan as a designer, you'd be great!

...

LOL!

Thanks for the vote of confidence, but I think I’d rather volunteer to do some serious beta testing for them.
I do have two other listening rooms that do not have ML’s in them yet

 

[DTB300]

.....but I think I’d rather volunteer to do some serious beta testing for them.

ML needs to learn from your Center Channel Design and apply it to their thoughts and designs - they need to stray away from the real world and explore avenues like you have.

I really applaud your design and finished product!!! Nothing like having a team together as you noted to get to the final product. The only thing with your "product" is the verticle design. Some of us do not have the verticle space like you do.

If you were space limited, any thoughts on how you would accomplish a horizontal version? The only issue I would see is the panel would disperse up and down instead of side to side and limit the sweet spot. Which is why ML designs their Center with the panels either convex or concave.

Dan

 

[JonFo]

SL3XC vs Logos behind the screen

OK, Robin, you asked for this

So, what really goes on behind a screen?

Reflections, for one. That’s the real issue with putting speakers back there, which is why the more transparent acoustically the screen is, the better, as it will let more sound through and have fewer reflections. The ScreenPix material uses biased weaves and un-equal sized holes to achieve a smoother acoustical profile.
Unfortunately for me, my Stewart is equal-sized holes on a very tight material that does indeed reflect at high frequencies.

As we can see in this graph, the old Logos (blue) had a serious hit about 1 ms in (which correlates to the fact that it was about 14” inches from the back of the screen.
The new SL3XC also has a screen bounce at about 0.4ms (as it’s only 6” from the screen), but notice how attenuated it is. We’ll discuss that in a minute.

The SL3XC is a full dipole, unlike the Logos, whose rear wave is canceled by the woofer box. Even though the stators sit out about 3 inches from the woofer box, the reflections had nowhere to go (but back through the stator, smearing the sound as can be seen in the impulse trace roughness. Therefore the SL3XC (red) has a full-frequency (well, >450hz) signal that will hit the rear wall. Guess what? Major reflections back there. (see next post with naked wall measurements), that cause a spike. With some diffusion, that reflection is minimized and spread out over time a bit.

 

[JonFo]

Back wall bounce

Here is a solo shot of the SL3XC (screen down) with no treatment on the wall behind it.

Big spike from that rear wave at 4ms. Some of the sound gets reflected right back at the ESL, smearing its response.

The solution is to diffuse the sound in the horizontal plane, so most of the energy is not going right back at the ESL.

 

[JonFo]

With diffusion

And here it is with the horizontal diffusion in place (red trace). I combine the two traces, so you see the overall impact of the diffusion.

Not only does it dramatically reduce the amplitude of the rear wave reflection, it also diffuses it in time.

I believe I need to also add some 500Hz to 3Khz absorption back there. Like maybe take it down 3db or so in the FR.

{2008 edit} Yet another clear example absorption is needed, and now that it is covered with MiniTraps HF, that noticeable bounce is completely gone {edit}

 

[JonFo]

Screen down vs screen up impulse at 5m

And here is a combo of the impulses of before and after the screen being put up at 5m.

Not too major an impact. Other than the slight screen bounce at 0.4ms (and its echoes, which is what causes the negative going element at 1.8ms).
In general, this is a pretty minor impact from all that material hanging in front of the speaker system.

 

[JonFo]

Frequency response - Screen impact

And to cap off the measurements of the screen impact, here is a shot of the 3rd-octave frequency responses before and after the screen.
The biggest impact is the sharp roll-off in the extreme high frequencies. Pretty much after 9K, the screen is muting pretty hard. I’ve adjusted the EQ to compensate a bit and could add some more; I’m just not a fan of big boosts (>4db) with EQ’s.
From purely a sound quality perspective, it sounds just fine right now with the screen there.

 

[JonFo]

Discussion of point source vs line arrays and reduction of reverberant artifacts

All of the above reveals an interesting fact, and that’s the relatively smooth frequency response of the SL3XC behind the screen. The lack of significant reflection artifacts is also of interest.

One reason for this is that unlike point source speakers, the line array of mid-basses and the 4’ of ESL radiation work to smooth out the frequency response overall. As the ear (or the mic) is receiving impulses from a large surface area, with different arrival times.
That’s one reason the impulses look a little ragged, as we are picking up slightly varying elements of the same tone from the varying distances to the emitting surfaces (again 4’ linear feet for 80hz on up)
However, at the listening position, these slightly varying sources combine into a smoother FR overall (as noted in the 5 meter measurements earlier in the thread). .

This is one of the reasons I believe that the ultimate center for a big Martin Logan rig has to be of this configuration. It’s what most closely matches the dispersion and FR of the left and right speakers.

Point source systems like the Logos have a serious disadvantage in that the response of the mid-bass and tweeter obeys the inverse square law of diminishing levels, whereas the Line Array and ESL broadcast a deep ‘near field’ that extends into my seating areas.

The one down-side (besides requiring a front projection system with acoustically transparent screens in a dark room ) is that the limited horizontal dispersion requires a large minimum distance from the speaker.
But then, for some strange reason, that’s just how my room is designed. Nice

 

[JonFo]

ML needs to learn from your Center Channel Design and apply it to their thoughts and designs - they need to stray away from the real world and explore avenues like you have.

I really applaud your design and finished product!!! Nothing like having a team together as you noted to get to the final product. The only thing with your "product" is the verticle design. Some of us do not have the verticle space like you do.

If you were space limited, any thoughts on how you would accomplish a horizontal version? The only issue I would see is the panel would disperse up and down instead of side to side and limit the sweet spot. Which is why ML designs their Center with the panels either convex or concave.

Dan

Dan,

Thanks, I agree with ML doing extreme designs. One of reasons I applaud their Statement products is that they push the limits. Then the rest of the line can benefit.

Unfortunately, there is nothing practical about my design. It really requires the characteristics of my room or of similarly configured dedicated theaters to really work,
As a commercial design, it would be a flop. I figure to do this commercially; ML would have to charge around $8K (or more) just to break even.
And the install and tuning costs would be another major issue. As you’ve seen in this thread, that’s just as critical.

To do a horizontal design, well, ML has been cracking at that for 15 years now, and I believe that they are doing a great job at it. Hard to compete.

But if I were to try something, it would not be placing a stock ESL on its side, as you noted the vertical dispersion is all wrong, and the horizontal window would be limited to the width of the ESL.

But now that I think about it, I guess one could take two 4’x13” ESL panels, put them sideways, end-to-end so we have an 8 foot long ESL, which should be wide enough for most rooms. Benefit is no horizontal room boundary reflections. But everyone better stay within the width of the unit.
Secondly, I’d do something radical, like mount ESL’s so they are Concave (i.e. using the rear stator to face out) then to control vertical directivity, I’d surround them with ‘wings’ that would keep the sound from hitting the ceiling or floor too early.

The rear wave would be mostly absorbed, with the lower frequencies from the panel being redirected to top mounted ports that would reinforce the lowest frequencies from the front of the panel.

This would be mounted to a mid-bass mount hosting five 6.5” drivers in a Bessel array (using DSP to generate the Bessel transfer functions). Sort of radical to do Bessel arrays, but they are the only kind of horizontal array that has decent vertical directivity.
Taking a regular line array (like mine) and laying it on its side would not yield good vertical dispersion.
Anyway, something to play with.

Here’s a quick rendering of what the side profile would look like, showing the mid-bass box, with a pivot mount for the ESL elements (which are mounted concave) and the directivity control ‘wings’, which would be both articulated and some form of a compound curve (vs straight as in this drawing). Some serious waves modeling would help determine that.

This is sort of semi-horn hybrid thing, with a full ESL energizer.
The limited horizontal dispersion (pretty much nil outside the width of the unit) would make for better room acoustics, as long as the ceiling bounce was well controlled. Most floors are carpeted, so less worries about that.

Dan, now see what you made me do. Some other crazy center channel idea is now out there

 

[Robin]

Interesting Audio Technical information...

Jon,

Thank you for explaining how your new SL3CX pretty much maintains quality sound with your Stewart Studiotek 130 viewing screen down. Your information is very well thought out and clearly researched as well. The graphs you made are very good at illustrating the sound.

I hope ML sees what you have done, as they check out our web-site often, according to Tom. Maybe it will give them some ideas of what ML enthusiast would like in ESL speakers.

Again thank you for sharing some of your information and research with me (us).

 

[akm3]

Wow!

I just saw this for the first time.

I love the "Statement" design direction in your Mini-Statement.

It answers a lot of questions I have about a practical matched ML Center speaker and the tradeoffs of doing it both this way and the ML way.

I still don't know what I want to do for MY way.

Question: For your system, couldn't you have achieved your goals similarly by just using a stock SL3 behind your screen as the Center channel? You'd get dispersion and panel characterstics more similar to your monoliths than the Cinema, with an already decent woofer/panel melding.

Or, making three of your unique speaker to use for your front field, for uniformity.

-Allen

 

[JonFo]

...

Question: For your system, couldn't you have achieved your goals similarly by just using a stock SL3 behind your screen as the Center channel? You'd get dispersion and panel characterstics more similar to your monoliths than the Cinema, with an already decent woofer/panel melding.

Or, making three of your unique speaker to use for your front field, for uniformity.

-Allen

Allen, thanks. Using a stock SL3, I would not have reached my primary goal:
Consistent power curve from 80Hz to 20Khz up to (and beyond) 105dB SPL.

FYI- A power curve is the measure of Frequency response at various power levels (as measured in dB SPL). The idea is to see a uniform response from 80dB all the way past 105dB. Which is especially hard from 200 Hz on down for an ESL (even a hybrid like the SL3).

A stock SL3 can't do it (I'll use my 'Spare' SL3 and compare next time I whip out the measurement rig and post here). Heck, even my big Monoliths with their new panels, and updated and customized woofer aren’t in the same league as this Line Array hybrid.
The other critical difference would be the active crossover. As noted elsewhere in the thread, active beats passive in every way imaginable.

I agree that three identical speakers across the front is very tempting, and don’t think for a moment that I haven’t actually measured and planned out how to re-use my rear Sequels in such a scheme

I just wonder how extravagant (and cool) it would be to have Monoliths as my rears

 

[Pcar928fan]

I just wonder how extravagant (and cool) it would be to have Monoliths as my rears

That would certainly make a statement! WOW!

 

[akm3]

Allen, thanks. Using a stock SL3, I would not have reached my primary goal:
Consistent power curve from 80Hz to 20Khz up to (and beyond) 105dB SPL.

FYI- A power curve is the measure of Frequency response at various power levels (as measured in dB SPL). The idea is to see a uniform response form 80dB all the way past 105dB. Which is especially hard from 200 Hz on down for an ESL (even a hybrid like the SL3).

A stock SL3 can't do it (I'll use my 'Spare' SL3 and compare next time I whip out the measurement rig and post here). Heck, even my big Monoliths with their new panels, and updated and customized woofer aren’t in the same league as this Line Array hybrid.
The other critical difference would be the active crossover. As noted elsewhere in the thread, active beats passive in every way imaginable.

I agree that three identical speakers across the front is very tempting, and don’t think for moment that I haven’t actually measured and planned out how to re-use my rear Sequels in such a scheme

I just wonder how extravagant (and cool) it would be to have Monoliths as my rears

I guess you are right, I was thinking of MY primary goal not yours. MY primary goal would be to have a center that performs/radiates similarly to my mains with no dead spots. You already had that part and just needed equal power outputs.

-Allen

 

[JohnA]

Very interesting thread.:rocker:

Builds on the kick-ass implementation of IB subwoofer - hardcore stuff (until you realise that with the SL3panel on top you'd like to move the whole contraption 2 feet forwards, ooops )

Picking on the 'concave' idea, I've been wondering what the rear dispersion pattern looks like on the curved ML panels.
Yes, we know it spreads 30 degrees at the front ---- but how does it work at the back? Does it spread outwards, or does it 'focus' to a point that defines the limit of the back wall?

 

[MotorToad]

Wow, what a build! Thanks for the write-up.

 

[JasonR]

To achieve a reasonable load on the amp with six 7 to 8 Ohm speakers, you need to wire them up in such a way that they produce a load somewhere in the range of 4 to 16 Ohms.
Therefore, we will combine series and parallel wiring techniques to achieve our goal.

Use two series wired sets of three speakers each. Each set is paralleled into the amp for a total load of around 11 Ohms.

Jon,

This is easily one of the coolest DIY projects I've seen. Just a thought... not that you'd have any interest in changing the wiring of an already assembled and finely tuned speaker, but had you considered a different wiring setup to result in a single 4 2/3 ohm load with these six 7 ohm speakers? If you put three of the drivers in parallel, you'll have a 2 1/3 ohm load. Then put each of those sets of three in series with one another, for a single 4.67 ohm setup.

It's nice to drive the array with two amplifier channels as I believe you indicated you've done, but the somewhat high impedance load would probably not bring out the best of the amp's capabilities.

Anyway... just a thought. Thanks for fully documenting such a fantastic project!

- Jason