terryforsythe
Major Contributor
1/2 wavelength corresponding to the hole diameter for a 6.5" woofer.
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1/2 wavelength corresponding to the hole diameter for a 6.5" woofer.
terryforsythe
Major Contributor
I would configure it like RickS suggested.
- Thread Starter
- #103
Thanks to @terryforsythe , @Paweł L, and @RickS for the guidance on this.
One bit of background data: The prototype was chamfered at 22.5 deg around 2/3 of its full 33 mm thickness, as shown below. It showed no evidence of any issues in the 1.0-1.2 KHz area. The full thickness of the baffle is 38 mm. The inside edge of the chamfer comes very close to the enclosure wall.
For the new baffle, I've already cut the 19 mm deep rabbet on the inside. I think I have two options:
1. Chamfer at 22.5 degrees as before, except where the mounting screws come through. More material has been removed than in the prototype, due to the straight portion where the rabbet has been cut, but of course the shape is different and there are the lands where the screws go.. The side wall is now shown for reference. This looks similar to the example that @RickS provided.
2. Chamfer at 45 degrees, except where the mounting screws come through.
Suggestions?
One bit of background data: The prototype was chamfered at 22.5 deg around 2/3 of its full 33 mm thickness, as shown below. It showed no evidence of any issues in the 1.0-1.2 KHz area. The full thickness of the baffle is 38 mm. The inside edge of the chamfer comes very close to the enclosure wall.
For the new baffle, I've already cut the 19 mm deep rabbet on the inside. I think I have two options:
1. Chamfer at 22.5 degrees as before, except where the mounting screws come through. More material has been removed than in the prototype, due to the straight portion where the rabbet has been cut, but of course the shape is different and there are the lands where the screws go.. The side wall is now shown for reference. This looks similar to the example that @RickS provided.
2. Chamfer at 45 degrees, except where the mounting screws come through.
Suggestions?
terryforsythe
Major Contributor
I probably would go 45 degrees, but I don't have any experience with 22.5 degrees to opine on that.
thunderchicken
Active Member
I did 45, but mainly because that’s the router bit I had available. It was available because the kit specified 45 and I bought it to follow directions, not because of any actual data.
- Thread Starter
- #107
Since it's been three weeks, one pic from the process of fitting the baffles to the enclosures. I was not happy with the enclosure finish so I've taken a lot of time to get it right. We want a relatively flat finish. Flat water-based polyurethanes are notorious, I've learned, for showing brush strokes. I don't have a sprayer, so I've reverted to something I've done before: Apply glossy finish then buff down with abrasive pads to 0000 steel wool. I'm close to being finished with repairing remaining flaws.
Meanwhile, the baffle mounting blocks with elongated holes, along with a thin spacer that sits between the enclosure and baffle are in place. The spacer, glued to the enclosure with silicone adhesive, fulfills two purposes: It provides a visual break between the enclosure and baffle, and also a properly dimensioned O-ring groove for the seal.
And yes - I do need to place holes for the terminals!
Meanwhile, the baffle mounting blocks with elongated holes, along with a thin spacer that sits between the enclosure and baffle are in place. The spacer, glued to the enclosure with silicone adhesive, fulfills two purposes: It provides a visual break between the enclosure and baffle, and also a properly dimensioned O-ring groove for the seal.
And yes - I do need to place holes for the terminals!
- Thread Starter
- #110
The prototype (5-ply big box store plywood) used a single layer of 1" thick shredded denim, folded to cover the sides, top, and bottom. I'm expecting to stay with that.
- Thread Starter
- #111
Thank you! Though I'm not yet entirely happy with the black. I'm still struggling with those brush lines that you can see and feel...
- Thread Starter
- #112
First fit of drivers, PR, and terminals. I mounted the DXT using polymer washer-sealed through-holes as with the Purifi. Clearance of regular mounting screws in the mounting holes probably provided for enough movement in most climates but at the risk of the screws being obviously off-center in the recesses. I'm still fussing with the finish on the second enclosure but it's just about finished.
- Thread Starter
- #114
The folks at Purifi may or may not be amused by that!
Seriously, though - I know that the design is credited with reducing surround resonances, and for contributing to linearity with a very large Xmax. Having done work in the past with devices with rolling diaphragms, I'm curious about how the details were determined. For simple large-displacement rolling diaphragms, like you'd have in a sealed piston or a hot water expansion tank, there are design rules for longevity. But I suspect there's more involved here - radial stiffness, circumferential stiffness, near-zero spring constant for movement along the axis, and longevity. The "wrinkly" shape is probably pretty important.
Seriously, though - I know that the design is credited with reducing surround resonances, and for contributing to linearity with a very large Xmax. Having done work in the past with devices with rolling diaphragms, I'm curious about how the details were determined. For simple large-displacement rolling diaphragms, like you'd have in a sealed piston or a hot water expansion tank, there are design rules for longevity. But I suspect there's more involved here - radial stiffness, circumferential stiffness, near-zero spring constant for movement along the axis, and longevity. The "wrinkly" shape is probably pretty important.
Purifi is happy to explain how their surround works. It’s about keeping all of their T/S parameters the same regardless of excursion.
It’s under Sd modulation.
purifi-audio.com
It’s under Sd modulation.
Some Speaker Problems That Needed Solving
purifi-audio.com
- Thread Starter
- #116
Thanks for that, @D!sco. Constant Sd is a bit different from what I was thinking.Purifi is happy to explain how their surround works. It’s about keeping all of their T/S parameters the same regardless of excursion.
It’s under Sd modulation.
Some Speaker Problems That Needed Solving
"We chose to construct an “Sd-neutral” surround, where alternating normal and inverse roll segments cancel out each other’s Sd variations. The tricky bit is transitioning between them without producing torque or torsion on the cone. The W-folds and dimples are co-optimised to minimise the problem." [Emphasis added]
They don't explain here how the dimples and W folds work together. I suspect (though I surmised incorrectly before) that the dimples add stiffness to those portions of the folds. From my experience with rolling diaphragms, this has to be done really carefully to get good durability and it appears that they've succeeded.
There’s a great video with their analysis of the different surrounds moving in a simulation which shows how it remains constant even while under rapid movement and extreme distances. I just don’t have the time to link it right now. Besides the patentable details, Purifi puts everything about their products on the table. How else can they demand such a high price if they have no evidence? Their transparency as a company and the transparency of their transducers set them apart from a lot of other esoteric and expensive driver manufacturers. Maybe someday they’ll be more affordable as well.
Hi Alan J apologies if this has been asked before but do you intend to use grills on the front of the speakers either separate grills for the tweeter and woofer or an all in one design, if so would you also consider using coloured grills as a way of emphasising your overall design (Naim Mu-so for instance). Secondly given you’re not too enamoured with the paint finish on the plywood have you considered vinyl wrap as an alternative to paint. Cheers
- Thread Starter
- #119
We have no plans to add grills. It's a bit of a risk since these will be sharing a space with small children 
. If I had to add one, my first thought is a flat black metal screen on standoffs that use the existing driver mounting holes - along with reading up on the acoustic effects.
I'm now happy enough with the finish, and the future owners are thrilled with it. The problem, apparently common with water-based urethanes, was brush lines, especially with the flat finish I was using. I did better with glossy finish that's applied in very thin coats (contrary to manufacturer's suggestion of thick coats that are supposed to self-level), then buffed down with synthetic hand pads (equivalent to steel wool). The surface was maple to begin with and any grain that would show through has been thoroughly filled. So the end result wis a bit like a piano type finish with very little sheen. It's pretty pleasing, and exactly what the future owners wanted. Of course, I can still see the flaws!
If I were doing this again, I might (a) practice more in the specific (seasonal) conditions, (b) consider solvent-based urethane that's easier to work with and/or available in a spray can, (c) get a sprayer for the water-based material, or (d) consider spray paint followed by buffing.
. If I had to add one, my first thought is a flat black metal screen on standoffs that use the existing driver mounting holes - along with reading up on the acoustic effects.I'm now happy enough with the finish, and the future owners are thrilled with it. The problem, apparently common with water-based urethanes, was brush lines, especially with the flat finish I was using. I did better with glossy finish that's applied in very thin coats (contrary to manufacturer's suggestion of thick coats that are supposed to self-level), then buffed down with synthetic hand pads (equivalent to steel wool). The surface was maple to begin with and any grain that would show through has been thoroughly filled. So the end result wis a bit like a piano type finish with very little sheen. It's pretty pleasing, and exactly what the future owners wanted. Of course, I can still see the flaws!
If I were doing this again, I might (a) practice more in the specific (seasonal) conditions, (b) consider solvent-based urethane that's easier to work with and/or available in a spray can, (c) get a sprayer for the water-based material, or (d) consider spray paint followed by buffing.
- Thread Starter
- #120
One unit is completely assembled. I have 16 gauge wire on the drivers and a short segment of 18 gauge that, with effort, can pass through the binding post holes. There's a 1" thick layer of shredded denim batting around the perimeter (top, bottom and sides). I have 40 grams on the PR which is about where I expect to end up. As a check on overall integrity, here's an impedance sweep, with the prototype in blue and the new box in purple:
The peaks are blunted and lower Q, due, I think, to the absence of a complete seal where the wires pass through the holes (really low Ql). The ~40 g on the PR is also a bit more than the ~37 g I had on the prototype, and the volume is about 15% larger, consistent with the direction of the shift in the lower peak.
The blip at around 410 Hz is still there:
It was suggested before that this is attributable to a known resonance in the Purifi basket in that general area. With a contact stethoscope, 410 Hz doesn't seem to be any louder than 350 or 450.
I'm not seeing need for added bracing. Any informed opinions to the contrary?
The plan from here:
The peaks are blunted and lower Q, due, I think, to the absence of a complete seal where the wires pass through the holes (really low Ql). The ~40 g on the PR is also a bit more than the ~37 g I had on the prototype, and the volume is about 15% larger, consistent with the direction of the shift in the lower peak.
The blip at around 410 Hz is still there:
It was suggested before that this is attributable to a known resonance in the Purifi basket in that general area. With a contact stethoscope, 410 Hz doesn't seem to be any louder than 350 or 450.
I'm not seeing need for added bracing. Any informed opinions to the contrary?
The plan from here:
- Take new driver data
- Crossover design
- Crossover build and test
- With crossover in place, revisit bass alignment and finalize PR mass.
- Collect data on the finished speaker
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