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Novel Open Baffle Construction Techniques
Novel Open Baffle Construction Techniques
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Old 13th March 2014, 01:00 PM   #21
gainphile is offline gainphile  Australia
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Originally Posted by Z transform View Post

Attachment 404714
MF-HF Directivity

I will let the data speak for itself, but everyone who has heard them has been impressed with the rock solid imaging and tonal balance. I was slightly disappointed with a "shoulder" in the directivity plot around 1.3kHz, but overall it's very good. Response is +/-2dB from 35-20kHz on axis except for a 3.5dB dip around 1.3kHz that's on purpose. The true standout though, is the bass; the directivity is an almost perfect dipole as the outdoor measurements below show (un-gated, 2m ground bounce). A double bass, cello or a kick drum sounds like it's in the room, really. The speakers don't get at all muddy if you want to listen to Slipknot (which I don't). Toms hit you in the chest. You can feel no bass vibration in the baffle, the woofer force-canceling really works. Mid frequency vibration in the baffle is very low, the damping layer works.
Hi Z, this is a really well executed project. I like the way you cut the side holes of the mid/tweeter to enhance directivity. Been thinking about this for quite sometimes for my own speakers but I can't bear thinking of ruining my mahogany baffle in case I make mistakes, which I do often

Like you I have been thinking to just CNC my next speakers. Enough MDF dust !!!.... 'Luckily' I haven't build anything new since.....
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Old 13th March 2014, 07:19 PM   #22
john k... is offline john k...  United States
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Z, nice design. I like it a lot. And thanks for the further conformation to my work that the slot loaded woofer system is indeed a dipole at low frequency.
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Old 14th March 2014, 04:37 AM   #23
gainphile is offline gainphile  Australia
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What was the the thought before about slot loaded open baffle? They clearly look dipole to me?
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Old 14th March 2014, 10:41 PM   #24
Z transform is offline Z transform  United States
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What was the the thought before about slot loaded open baffle? They clearly look dipole to me?
Nelson Pass claimed in a report he published that a slot loaded woofer had up to 9bB of gain because of the high velocity of the air in the slot. Here's that report: The Slot Loaded Open Baffle Project Article By Nelson Pass

This claim appeared to bother JohnK, because it was only a near-field effect. He protested it was misleading and "there was too much fuss about it" on a PE tech talk forum post: From Nelson Pass: Slot Loaded Open Baffle Project

I also found a very well done technical manuscript by John on the topic where he showed that the only "real" gain was from slot resonance, but I can't find that anymore. Perhaps John can share it.

In my case, the slot configuration was used because my previous dipole with 12 of the Aura drivers had a lot of "bulk" movement of the speaker from the bass. The slot configuration promised to cancel the forces and eliminate this- It worked great for that in this design. My slot ratio was also quite small; I think around 2x driver diaphragm area to slot area, so the front velocity is not increased as much. That's probably why at even only 2m it measures so much like a dipole.
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Old 15th March 2014, 12:12 AM   #25
BuildMeSomething is offline BuildMeSomething  United Kingdom
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Hi Z, glad you got time to do a write up about your slot loaded open baffle build. Loads of useful info to digest and new areas to explore.

On the subject of constrained layer dampening. You wrote 'Ice Shield', would this be Grace Ice & Water Shield? Not a brand easily available here in the UK, though chanced upon this from IKO.

Paul
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Old 15th March 2014, 12:30 AM   #26
john k... is offline john k...  United States
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Here is the link to my discussion of the slot loaded woofer.

Nelson Pass Slot loaded OB

Also I think a lot of people don't understand the behavior of a dipole source as you move closer to it.

http://musicanddesign.com/Dipole-axis.html

which is why near field measurements make it look like there is some additional gain, slot loaded or otherwise.
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Old 15th March 2014, 07:57 PM   #27
Z transform is offline Z transform  United States
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Default Constrained Layer Materials

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Hi Z,
...On the subject of constrained layer dampening. You wrote 'Ice Shield', would this be Grace Ice & Water Shield? Not a brand easily available here in the UK, though chanced upon this from IKO.

Paul
Paul,

Here is a photo of all of the consumables I used for the project including the ice shield which is a GAF product: "StormGuard" GAF | StormGuard Water & Ice Shield. (In the MSDS datasheet you'll find the composition.) The IKO product is about twice as thick, I think a little too thick.

Materials.jpg
Consumable Materials

Wish they made this stuff with the PSA on both sides, then things would be real easy. The "top" gray side has a very thin layer of what I assume is polyethylene or some similar low surface energy material. Despite it being micro textured (so the roofers don't slip on it) I still assumed it would be hard to bond. So, I used the super aggressive 3M #90 spray adhesive, coating the StormGuard and the MDF before bonding and then pressed it on good with a rolling pin (don't tell my wife).

A word of caution. Don't press a panel together and then try to cut or route it. It works, but it makes a gooey black tar-like mess of the tools and the edge. Here's an example of a 3-layer (Baltic-birch/MDF/Baltic-birch) panel from another project I routed after making the sandwich, ugly.

3-layer CLD panel.jpg
Don't Route after Assembly!

It has nothing to do with the CLD layer, but the photo above shows everything else that was used to build the cabinets- just wood glue and clear shellac. The shellac was applied by an HVLP gun that was bought at Harbor Freight. I normally hate everything in Harbor Freight, usually just plain crap IMHO. But several posts on woodworking forums said the $20 gun was excellent and they were right. Even though I made the newbie mistake of not cutting the shellac, the finish came out great. It dries so fast you can put another coat on one speaker while the other one dries... Ad infinitum. The only other tools used for assembly were a knife, a screwdriver and wood clamps (and shhhhh, the rolling pin). I really did not use sandpaper- The power of CNC!

Spray Gun.jpg
Economical and Excellent Spray Gun

I was intending that the shellac was just going to be to sealing coat, but it really looks great. Next project I am going to try dieing the shellac- red/black to give the MDF a dark crimson color. It would be less forgiving to apply than clear, but it would look cool.

Glad everyone is so appreciative of the info. I have to go in to work to get them, but tomorrow I hope to post the SolidWorks files.

Paul, If you want to have a sample of the ice shield so you can compare to local UK products, PM me your physical address and I'll mail you a scrap.

Last edited by Z transform; 15th March 2014 at 08:02 PM.
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Old 15th March 2014, 08:53 PM   #28
BuildMeSomething is offline BuildMeSomething  United Kingdom
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Thanks Z, great info once again.

Should be ok with just the datasheet for the GAF Ice & Water Shield... looked at a few different makes of roofing underlayment here, most didn't look heavy enough. Think the IKO one linked won't be to far off.

Quite familiar with Titebond glue and 3M spray adhesives, I use 'Photo Mount' fairly regular... I have done airbrush work for a few years. Unfortunately, I don't think my compressor & tank is of a high enough flow rate or capacity to make using a larger spray gun an easy task. Will have to take another look as spraying is lots more fun than brushing the stuff on.

For dies, have a look at the ColorTone range. The liquid stains can be cut with water or alcohol, so might be usable with cut shellac.

Seen a thread here about a modular OB design where the Eminence Alpha 15A and FR drivers are mounted on separate square cut baffles, then mounted on a metal sub frame. I liked the way the squares helped break up the vertical lines and wondered about doing something similar, but mounting the front baffle onto an MDF panel with the membrane between...

CNC is the way forward, it's just getting the work done for the right price. I'll stick with the hand tools while my builds are still fairly simple
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Old 16th March 2014, 12:13 AM   #29
Z transform is offline Z transform  United States
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Thanks Z, great info once again.

For dies, have a look at the ColorTone range. The liquid stains can be cut with water or alcohol, so might be usable with cut shellac.
Paul,

Thanks for the tip on the dies, I'll see if I can get them on this side of the pond.
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Old 16th March 2014, 12:36 AM   #30
BuildMeSomething is offline BuildMeSomething  United Kingdom
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Those ColorTones are an American make so you should be ok

Had a look at HVLP spray guns, there's some nice one's available through Amazon at attractive prices. Plus found a new 24L 1.5HP 6.35cfm air compressor for 65 inc. P&P

Need to make my mind up now, try out a straight forward OB build first, or jump straight into the deep end and try building a slot loaded OB... choices
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