Building a new OB

JohninCR · « **Reply #160 on:** 12 Mar 2007, 02:07 pm »

Dan,

Unless you listen to a narrow type of music or only at very modest levels, all OBs require some type of XO (electrical or mechanical). I figured out how to address the vibration noise from my styrofoam roundovers. I'll cut holes in the backside and get most of the styrofoam out with a hot wire cutter, and then fill in the void using concrete with small bit of styrofoam mixed in to keep the weight down. Note that I'll screw a number of screws into the wood leaving the head exposed for imbedding in the concrete, and give everything a coat of epoxy to ensure a good bond with the concrete. I'll do each in 2 pours, one laying on their face and one on their back. Here's a pic of where they stand and a drawing of what I plan to do:

DanTheMan · « **Reply #161 on:** 12 Mar 2007, 07:12 pm »

mmmmm. Those look tasty. Hopefully that cures the styrofoam sound. I would bet it does.

JohninCR · « **Reply #162 on:** 12 Mar 2007, 09:56 pm »

Oh well, those roundovers are for the scrap pile. The epoxy layer was too thin. That's what I get for not using wood. At least I learned a few things, including that this avenue is worth pursuing. I've started on proper wooden roundovers using some nice mountain laurel to go on my almost identical pair, which is slightly larger, has a changeable driver mounting plate and can accomodate up to an 8" driver. Hopefully they'll be ready tomorrow and I can do a shootout of the B200, FE108EZ, & phase plugged FE167. I have the 167's going today with just the PVC magnet mount, and they sound better than they have any right to with no baffle at all due to the accompanying big dipole peaks and nulls above 700hz. They're much smoother than my FE206's. I can't wait to hear them in the waveguides.

I also decided to go stealthy with my PVC magnet mount. When I'm done, it will look like the support pipe comes up directly from the floor. Styrofoam doesn't work, but it would be nice to come up with a minimal and lightweight baffle to complete the look, shaped to smooth the dipole ripples and doesn't become a soundboard.

Bob in St. Louis · « **Reply #163 on:** 13 Mar 2007, 02:17 am »

John, I'm glad I wasn't around when you realized the epoxy was too thin!

Did you maintain your cool, or go ballistic and start cussing and throwing handfull of wet concrete?

Stiff wire mesh and expanding foam John.
I'm sure you'd have the mesh down there, but I think we've talked about the foam and it's hard to find down there, right?
(The mesh I used is a base layer for a stucco or brick facade.)

Bob

JohninCR · « **Reply #164 on:** 13 Mar 2007, 02:56 am »

No Bob,

No wet concrete, I took the easy route and tried thinner. It ate the styrofoam, but the remaining epoxy was soft. It probably would have been different with some fiberglass cloth as part of the epoxy layer, but I wasn't planning on needing strength, just something to protect the styrofoam. No more composite stuff for me. That's just added work, unless I do a mold for some curved fiberglass waveguides.

On a positive note, I tested a new baffle shape today that worked really well. I've seen a couple of people use it and thought it was hideous, and it is, but I think I figured a way around that too. I'll have a couple of interesting things up and running by the weekend.

Bob in St. Louis · « **Reply #165 on:** 13 Mar 2007, 01:31 pm »

Can't recall seeing too many hideous baffle designs.
You've got my curiousity up now.

Bob

JohninCR · « **Reply #166 on:** 15 Mar 2007, 11:27 pm »

Sorry, forgot to link a pic in my post the other day. The white one is the nautilus shape I've never liked as a baffle. The 3 test baffles pictured all work very similar. A prize goes to whoever figures out the function of the black one first. After this week's tests baffles, I'm not sold on the premise that very thin flat baffles eliminate edge diffraction. The front and rear wave diffraction may net to zero, but as Rudolph pointed out a while back, there seems to be a lot of directionality in the diffraction effects, so they don't cancel each other out. I'll get better proof once I do up some thick wooden flat baffles of similar shape to compare. I still don't know how I'll do the black one thick with big roundovers.

JohninCR · « **Reply #167 on:** 15 Mar 2007, 11:42 pm »

Bob,

I hope you're on the stick this weekend. My wooden waveguides will be final sanded in the am, with only a linseed oil rubdown for total finish. My PVC magnet mounts will get filled with concrete early tomorrow and paint in the afternoon, so the 167's are only a simple small baffle away from completion this weekend. I'm already listening to all, but I may have 4 projects in finished form in the next few days. The ball is back in your court.

Here are the PVC mounts with a 1/4" steel plate for what will be a super stealthy base, and 1x1 angle iron that will be imbedded in the concrete. I did cheat on the metal and welding by outsourcing that, but for $40 including the materials, it's like DIY. For another pair, I'll use pre-bent chromed muffler pipe, so after the concrete they'll be ready (no finish work at all aa). No, the clear tubing isn't for beer cooling the voice coils. That's to run my speaker wires inside the mounting structure.

Russell Dawkins · « **Reply #168 on:** 16 Mar 2007, 12:25 am »

Quote from: JohninCR on 15 Mar 2007, 11:27 pm

A prize goes to whoever figures out the function of the black one first.

it's a very short transmission line with the tuning frequency distributed over a small range (like 2-10 kHz) for some reason!

Semi-aperiodic, like.

JohninCR · « **Reply #169 on:** 16 Mar 2007, 12:33 am »

Quote from: Russell Dawkins on 16 Mar 2007, 12:25 am

Quote from: JohninCR on 15 Mar 2007, 11:27 pm
A prize goes to whoever figures out the function of the black one first.

it's a very short transmission line with the tuning frequency distributed over a small range (like 2-10 kHz) for some reason!
Semi-aperiodic, like.

Interesting but nope. Gotta think like an OB guy.

Russell Dawkins · « **Reply #170 on:** 16 Mar 2007, 12:52 am »

Seems to me the ultimate shape to eliminate edge diffraction, or at least distribute it over a wide range of frequencies would also be pretty unacceptable in the average room. That would resemble a star fish with many (say, 30) slender, long tapered legs. If I were in an experimenting mood, I would even try some sort of fringe around the edge to break up the sharp transition.

It works for animals where the aim is to decrease turbulence caused by air flowing over the sharpish edge of the ears of the likes of cats and dogs. Same effect on Inuit parka hoods. Did you think the fur trim around the hood was for looks? It's to cut down on wind roar.

I'll think 'em up, you build 'em, eh, John?

JohninCR · « **Reply #171 on:** 16 Mar 2007, 01:43 am »

There's no fun for me in building others' ideas, and I wouldn't want a sunflower baffle anyway. The function of the black one has nothing to do with diffraction. In fact, none of those test baffles address edge diffraction. The info that JohnK provided is that there is almost no noise from air movement over the surface area, turbulence if you will. Instead, edge diffraction is more like a very early reflection. The sound waves expand along the baffle, which is constraining it to half space expansion. At the edge, an abrupt change in pressure (since the baffle no longer constrains it), results in a secondary sound source at that point.

A flat circular baffle is the worst because the problem occurs over a narrow frequency range. On the otherhand, Olsen's work demonstrates that a large sphere is the optimum shape, which makes sense because the pressure change would be very gradual. The realistic optimum is something in between. I just know it's not a square edged rectangle that manufacturers typically sell since they are easier to construct. All that does is spread the problem across a broader frequency range.

Russell Dawkins · « **Reply #172 on:** 16 Mar 2007, 04:56 am »

Bob in St. Louis · « **Reply #173 on:** 16 Mar 2007, 05:11 pm »

Quote from: JohninCR on 15 Mar 2007, 11:42 pm

I hope you're on the stick this weekend. ....... The ball is back in your court.

Good to see 'ya talkin some smack there John.

I wish I had half the time to devote to the build process as you John. (Actually, a wife half as tolorant as your is would help too!

)

Nice bass/support, I like it.

The beer fed voice coil cooler is a nice idea also.

I'll get back into the swing of things Saturday morning.

Bob

JohninCR · « **Reply #174 on:** 16 Mar 2007, 07:53 pm »

I don't spend much time, just an hour or so most days. No smack, I just try to be efficient, such as building 2 speakers at a time instead of just one. Pics of the all wood waveguides coming today, while I wait for the concrete to cure in my magnet mounts.

DanTheMan · « **Reply #175 on:** 16 Mar 2007, 08:12 pm »

Sweet John, I've been dying to see the new WGs. Post ASAP please--not to rush you. Could you also go through sound quality(s) too please?

Thanks again,

Dan

JohninCR · « **Reply #176 on:** 16 Mar 2007, 09:15 pm »

Here you go Dan. These are obviously still in raw form. I'm not ready to discuss SQ yet as I want to try a few different drivers in them first before tuning with foam inside the WG. I will say that the dispersion and directivity characteristics work as planned, and the natural open OB sound is retained. Tuning is what will bring the imaging into focus, as there are reflections, HOMs, or whatever you want to call them that are holding them back from the "disappearing act". In the end, I want to cover the ply, but keep the nice mountain laurel roundovers exposed.

Bob in St. Louis · « **Reply #177 on:** 16 Mar 2007, 09:31 pm »

Looks pretty sexy from here John, very nice!
It'd be right at home in a log house.

Bob

DanTheMan · « **Reply #178 on:** 16 Mar 2007, 09:47 pm »

whew! Those look freaking sweet! Much better than the styrofoam/concrete thing. looks like they need some more sanding before finishing, but if your gonna cover them, what they heck, leave'em be. Excellent start! I can't wait to hear listening/tweaking tests! Keep us up to date as time permits!

Dan

Bob in St. Louis · « **Reply #179 on:** 17 Mar 2007, 06:23 pm »

Update:

Things going well (but slow)

Baffle #1; "Great Stuff" Expanding foam doesn't mind being sanded with a belt sander (first), then a random orbital sander. I was afraid it might melt, or otherwise make a mess, but as it turns out it's a rather friendly material to deal with. Sanded entire structure with 60 grit paper from R/O sander. The fiberglass resin on the other hand is the hardest material I've ever tried to sand. It's like trying to sand diamonds with a "Q-tip". Gave up on the R/O sander (60 grit) and went back to the belt sander (180 grit). Much faster. Then back to the R/O sander. My biggest enemy is myself, I lack patience with things like sanding. I think baffle #2 will have a sawdust filler to aid in sanding (recommended by a few forum members).

Baffle #2; Assembled baffle, Gorilla Glue is drying, next step is wire mesh.

A thought occurred to me while I was sanding...Instead of painting the finished product (requires meticulous sanding....) I could have them coated in truck bed liner??

Still unknown what happens to expanding foam when resin is applied. I think I'll go do that right now (if I'm never heard from again, it's because I died from the chemical reaction {fire / toxic smoke}).

Back to work

Bob