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Old 24th August 2012, 02:10 AM   #1
Bon is offline Bon  Australia
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Default 3-way active, time aligned, constrained layer construction

This is a report on my latest speaker project which has occupied most of my free time over the last six months. Since I am getting on in years, I expect this will be my last serious speakers, but I always say that. Typically I get about 10 years before the itch becomes intolerable and I have to scratch it again. Since my speakers are getting more massive as I get more feeble, this may well be it for me.
I went for top grade drivers, Scanspeak Tweeter D2904-710001, Scanspeak 6.5” Mid-Woofer Revelator 18W8531G00, twin Peerless 10” Woofers P830452. I want to crossover the mid quite low, around 150-180 Hz, so the mid has to have good power handling which is why I choose the 6.5” Revelator rather than the 5.5". This means the mid-treble crossover will be closer to 2 kHz than 3 kHz. I have plenty of choice with power amps since I have run active 3-ways for over 10 years now. I plan to run with 400 wpc into 4 ohms bass, 200 wpc into 8 ohm mid, 100 wpc into 8 ohm treble, so headroom is not a problem. I am a convert to active crossovers for equalization, time and phase alignment. The best way I can describe the benefits is that it is like when a camera image snaps into focus as the the focusing ring is adjusted. Once you have become used to a time and phase aligned speaker system, most passive crossover speakers sound vague and unnatural by comparison, especially on transients such as percussion, cymbals etc. I have found a well aligned speaker does a great job with reverberation and ambient sounds, which makes the stereo image very convincing, especially width and depth. Controlling edge diffraction is essential to obtain the benefits of good time and phase alignment. Drivers must be properly flush rebated and sharp enclosure edges removed (as shown by the classic Harry Olson paper (JAES November 1951). Here are the issues I have tried to address.

Enclosure features:

1. Constrained layer damped wall construction.
2. Rigid multiple 25 mm plywood braced enclosure walls.
3. Separate bass enclosure to minimize interference and intermodulation.
4. Non-rectangular shape to control internal air resonances.
5. Angled edge to tweeter-mid section to optimize diffraction performance.
6. Massive cast resin angled tweeter-mid baffle.
7. Inner box separated from outer box by damping layer.
8. Bass drivers rigidly attached to inner box with no direct contact to outer layers.
9. Massive four layer mdf bass baffle.
10. Bass drivers attached to internal machined steel 10 mm threaded plates by machine bolts.
11. Marine ply outer stiffening layer.

Acoustical features:

12. Time aligned drivers.
13. Phase aligned drivers.
14. Active crossover.
15. Linear phase (Bessel) crossover alignment.
16. Critically damped sealed bass enclosure loading
17. Tri-amped configuration.
18. Equalised driver frequency bands.
19. Flat overall on-axis room frequency response.
20. Low distortion drivers.
21. High power handling drivers.
22. Wideband drivers.
23. Flush mounted drivers.
24. Felt damped tweeter-mid baffle.

I learned a lot during this construction. I am getting close to installing the drivers and performing the final testing/alignment/tuning process which I will perform with my measuring microphone, RTA and various test signal sources (including actual recorded music).
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Last edited by Bon; 24th August 2012 at 04:15 AM. Reason: Typo. Of course it is Harry Olson, not Henry
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Old 24th August 2012, 02:43 AM   #2
GDJ is offline GDJ  Canada
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Wow! Nice construction techniques!
Looking forward to seeing these completed.
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Old 24th August 2012, 02:45 AM   #3
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Looking good so far Bon. The 452s are nice in that they work in smaller boxes, but you might need an LT on them to get the extension you're after. They do give nice performance up higher then you'd usually get out of a sub driver, but crossing them around 150Hz is a good idea imo as it also lets you get the most out of the revelator mid. You might of course find that the system sounds even better crossed over even lower, but that's all a part of the fun once you get the drivers installed.
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Old 24th August 2012, 03:45 AM   #4
Bon is offline Bon  Australia
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Quote:
Originally Posted by 5th element View Post
you might need an LT on them to get the extension you're after.
I used the 12" 830500 in my current speakers and they are equalised flat down to 25 Hz in a sealed enclosure. There are only a few suitable bass drivers with the required power handling and low distortion high excursion. It is a challenge though to find a bass amp with the balls to deliver the current. I have had plenty of reputable amps clipping or going into protection on high level bass transients. One good thing about going active is that bass clipping is does not make the whole sound stage collapse since the mid and treble are still clean. I try and avoid it though. It's not good for the amps. I have never detected the Peerless 830500 speaker in distress even when my bass clipping indicators are flicking on at 400 watt peaks.
Quote:
Originally Posted by 5th element View Post
but crossing them around 150Hz is a good idea imo as it also lets you get the most out of the revelator mid. You might of course find that the system sounds even better crossed over even lower,
That is what I am thinking too
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Old 24th August 2012, 06:01 AM   #5
jerryo is offline jerryo  Isle of Man
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Massive respect for your woodworking skills. Awesome!
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Old 24th August 2012, 07:49 AM   #6
Bon is offline Bon  Australia
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Originally Posted by jerryo View Post
Massive respect for your woodworking skills. Awesome!
After many years of making do with the garage, I finally took the plunge and built myself a proper separate workshop. Then I seriously tooled up for woodworking. I have a very nice table mounted router, 12" compound sliding saw, deWalt vernier bench saw, small band saw, compressor, HVLP guns and lots more (especially clamps, you can never have too many). These speakers are my first big project with the new facilities. So I suppose I should add the cost of the shed and tools to the speaker costs
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Old 24th August 2012, 08:04 AM   #7
jerryo is offline jerryo  Isle of Man
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Hi Bon,
having the right sort of space to work in, is essentially, something I can only dream about where I live. I am envious. I am a very creative person who has to make do with a minute amount of space to do what I wish too.
I hope this changes before I am to old and knackered.
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Old 24th August 2012, 08:21 AM   #8
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Awesome work Bon!
Can you go into a bit more detail on how you made that cast resin baffle?
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Old 24th August 2012, 09:43 AM   #9
Bon is offline Bon  Australia
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Originally Posted by David Gatti View Post
Awesome work Bon!
Can you go into a bit more detail on how you made that cast resin baffle?
Hi David. The baffle of the tweeter-mid enclosure is a fibreglass (isopthalic) resin core inside a 25 mm mdf skin. I discovered the damping properties of resin a while ago, when investigating material for turntable plinths. For plinths the resin requires fibreglass ply re-inforcement for strength but for the baffle, the 25 mm mdf skins provide sufficient strength. The Audio Qualia site plinths - Audio qualia contains good information about the audio damping properties of various materials. I fabricated the baffle outer skin from mdf with compound mitre and table saw cuts. The outer skins were glued up with Selleys polyurethane glue. I chose this glue over PVA because of its space filling properties. Be careful with the vapours from polyurethane glue because it is isocyanate based, which has some bad potential health effects. (2-pac sprays are similarly dangerous to inhale or even absorb through the skin). I filled the underside of the glued up baffle over a couple of days (no more than 10 mm at a time, since the curing generates significant heat and this heat can affect the strength, cause cracks or even spontaneously combust). The final core is 90 mm thick at its deepest. The finished baffle is attached by compression of a layer of glazing silicone.
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Old 24th August 2012, 09:55 AM   #10
Bon is offline Bon  Australia
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Quote:
Originally Posted by David Gatti View Post
Awesome work Bon!
Can you go into a bit more detail on how you made that cast resin baffle?
The baffle gets glued to the enclosure under compression. The adhesive is Bostik V60 glazing silicone. The complete baffle is approximately 10 kg.
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