Building a new OB

markC · « **Reply #140 on:** 9 Mar 2007, 02:36 am »

John. I've gotta' ask...When was the last time you sat down and listened to a complete album/disc that you genuinely enjoy? I realize that it's your passion but your scaring me with the amount of "work" your doing.

JohninCR · « **Reply #141 on:** 9 Mar 2007, 04:12 am »

My source is my PC, so I listen to very few albums, but I live at my computer and listen to music all the time. Plus I have a set of line arrays in my garage work area that are connected to B out on one of my amps, so I listen to music the whole time I'm building, which is mostly in 1 to 2 or 3 hour spurts on almost a daily basis. That's my R&R time, because the wife, kids, and customers leave me alone for the most part while I'm there. I get plenty of listening in, but critical listening is reserved for latenight, when there's no one to interrupt.

Bob in St. Louis · « **Reply #142 on:** 9 Mar 2007, 02:57 pm »

Quote from: JohninCR on 9 Mar 2007, 12:07 am

Think of the Earth cut in half at the equator, and then put a life ring for kids around the equator.

Ok John, now I understand.
Thanks

Quote

line arrays in my garage

You're a stud, dude!

Bob

JohninCR · « **Reply #143 on:** 9 Mar 2007, 04:31 pm »

Quote from: Bob in St. Louis on 9 Mar 2007, 02:57 pm

Quote
line arrays in my garage

You're a stud, dude!

Bob

They're just 12 $1.50 drivers per side, so the wood cost more, but they're perfect for listening while up and moving around due to the nature of array dispersion. They are, of course, OB line arrays, the first pair I built almost 3 years ago.

DanTheMan · « **Reply #144 on:** 9 Mar 2007, 05:10 pm »

Quote from John, "Think of the Earth cut in half at the equator, and then put a life ring for kids around the equator. The interior is the oblate waveguide..."

John, are you sure that's the way Dr. Geddes is cutting the oblate spheroid? That seems to me to make a prolate spheroid. It's all just a matter of perspective, but I believe that the quality of sound if the shape is sliced improperly will be dramatic. I will admit that is the way Dr. Geddes WG looks--that's why I was confused when he said oblate spheroid. So then the proper way to make one is to slice the oblate spheriod along its major axis? In any case, it's like a very short parabolic horn.......correct? Have you tried calculating or modeling responses of such a design?

JohninCR · « **Reply #145 on:** 9 Mar 2007, 07:11 pm »

Dan,

I'm not certain which way it goes. If you know of a pic or drawing of Dr. Geddes', please let me know. I've only seen a pic with the plug in place. Regarding oblate or prolate I looked around at reflectors for lights, and prolate seems to be more closed. To me, the waveguide definitely needs to be more open, since prolate reflectors beam. Having the WG and the spheriod with the same axis seems to make sense for the name. The confusing part is that we look at sphere's with the axis vertical, but speakers with the axis horizontal. I don't think any modelling exists. I can't even visualize how the OS waveguide functions, so I wouldn't dream of even attempting the raw math.

Find me a pic or a profile drawing of his waveguide, and I'll take a shot at carving one out of styrofoam today. Then add sawdust filler to my epoxy to create a paste to make the waveguide smooth and rigid. If it works better than my conical WG's, it can be the basis for a mold for duplication.

DanTheMan · « **Reply #146 on:** 9 Mar 2007, 09:40 pm »

Unfortunately the only pics I know of don't look like a parabola at all, they look like a short hyperbolic or actually exponential curve(like a trumpet flare) That's why I've had so much trouble with his white paper. I wish I could help you(and me) more with this problem. I'll do more research and let you know what I find.

JohninCR · « **Reply #147 on:** 9 Mar 2007, 09:44 pm »

Dan,

Someone sent me a drawing, but it appears to just be conical with large radius roundings at the throat and mouth.

JohninCR · « **Reply #148 on:** 9 Mar 2007, 09:50 pm »

Everyone,
Please forget anything good I said about styrofoam as a construction material in previous posts. I don't know the exact properties that make it bad, probably light weight combined with rigidity. Sound waves hit this stuff and it vibrates alot and transmits sound very well, making it a secondary source of sound. I guess that's why pro cabs, where weight is a problem, aren't built with resin over styrofoam. I learned the hard way, so you don't have to.

Bob in St. Louis · « **Reply #149 on:** 9 Mar 2007, 10:13 pm »

John, as I said on Decware, "At least you had the 'nads to try".

Hey John, What's the next idea in your head? I know there's at least a half dozen in there just beggin' to jump out and land on the workbench.

Try the fiberglass. Not expensive, rather solid, it's just a mess. But Darrel explained to me how to do it so it's not so messy.

Bob

DanTheMan · « **Reply #150 on:** 9 Mar 2007, 10:58 pm »

John, have you seen the waveguides at partsexpress? They look doable. Also, do you know how to calculate a frequency dependant directivity index of an open baffle(the bass end)? I've got an idea buggering in my head--I just can't figure out how to let it come out. There are too many pieces of the puzzle missing. Constant directivity OB is possible, but it will take some very careful planning--are you willing?

JohninCR · « **Reply #151 on:** 9 Mar 2007, 11:19 pm »

Quote from: Bob in St. Louis on 9 Mar 2007, 10:13 pm

John, as I said on Decware, "At least you had the 'nads to try".
Hey John, What's the next idea in your head? I know there's at least a half dozen in there just beggin' to jump out and land on the workbench.
Try the fiberglass. Not expensive, rather solid, it's just a mess. But Darrel explained to me how to do it so it's not so messy.
Bob

As long as no one else goes out and tries it, I won't feel bad. At least I have a few new beach toys for the kids.

First thing will be to salvage what I have, so first I'll add a layer of fiberglass cloth and thicken up the epoxy, which is pretty thin. Then I'll cut a hole on the backside of each set of roundovers and pour in some acetone to melt away the styrofoam, leaving the roundover shell. Next, scrape out any loose material and fill them with concrete, including most of that V shape in the center, so for less than $10 of concrete and some wood scraps I'll have nice heavy dead dipole waveguide speakers.

I can hear a big benefit with the big radius roundovers. The surround sound effect coming from the styrofoam was throwing me for a loop until I put my hand on it while music was playing and understood what was going on.

Next, I want to try a mold to cast some round waveguides out of concrete or concrete+plaster, just because it's so cheap. If I have any hiccups with that, it's back to wood. I'll just double up layers of 2" thick wood (I can't work with 4" with my table saw) and do the roundovers as part of the panels before assembly. That requires perfect angled cuts and perfect dimensions of each identical trapezoid, which is why I didn't go that route to begin with. Imagine these in 4" thick wood with big rounded edges. aa

JohninCR · « **Reply #152 on:** 9 Mar 2007, 11:40 pm »

Quote from: DanTheMan on 9 Mar 2007, 10:58 pm

John, have you seen the waveguides at partsexpress? They look doable. Also, do you know how to calculate a frequency dependant directivity index of an open baffle(the bass end)? I've got an idea buggering in my head--I just can't figure out how to let it come out. There are too many pieces of the puzzle missing. Constant directivity OB is possible, but it will take some very careful planning--are you willing?

Dan,
Can't you tell by now that I'm game for just about anything? I can't promise doing something that is someone else's idea though. That's not the same kind of satisfaction. Team effort sounds good though.

Look at the dipole response pattern at Linkwitz's site. That and easier angles is why I shot for +/-45 degrees for my constant directivity waveguides. I really need to get into measuring and take my and measure them outside along with a 15" for bass at different axis angles to see what these initial efforts accomplished.

DanTheMan · « **Reply #153 on:** 10 Mar 2007, 12:21 am »

One thing I was thinking about those waveguides (pictured above this post), was truncating the rear WG by a couple inches and then adding a t-shirt(or2 or3 or......) stretched across the truncated opening. This would dampen high frequency response and slow down the bass wave to make up for the truncating effect. Bringing the dipole null back where you want it. Of course out doors it wont make much difference, but indorrs I think that loosing the mid-HF out of the back may make for a less distorted sound. This would be an awefully careful balancing act--I would be afraid it may end up sounding like an aperiodically damped box of overdone. Between you and Bob, I don't know who's getting me to think more. I better take a break.

BTW, I definitely like the concrete idea and the large radius. What kind of FR are you getting out of those? I like the 45degree angle too. Also for indoors, have you thought aboutnarrowing the vertical dispersion?

JohninCR · « **Reply #154 on:** 10 Mar 2007, 01:11 am »

Dan,

The front of my room is more reflective than the rest of my room, so I typically do some damping of the rear wave to tone down the high frequencies, and it doesn't do anything to the low frequencies. Having the front deeper than the back won't have the effect you are thinking. The area of greatest null is where the front and rear waves have travelled the same distance. If the front is deeper, then the null will shift toward the front and reduce bass response overall in front.

These things aren't all that relevant though because I cross to the woofer before I get down to the dipole response of the waveguide. I wanted an open baffle alignment because I like the natural open sound. I felt that it needed to be dipole to get equal dispersion front and rear. Plus it enables me to flush mount or turn the driver around and rear mount (another thing to discover the effects). It's going to take measurements to see how well the dispersion control worked, especially since the rear radiation of the woofer doesn't beam at the higher frequencies like the front. Going by ear though, there is a big null at all frequencies at the sides, outside of the WG dispersion pattern.

Bob in St. Louis · « **Reply #155 on:** 11 Mar 2007, 01:08 am »

Update on the fiberglass:

Belt sander (with 80 grit) nicely removes the "resin titties" and high spots. Easily, minimal dust.

Amazing how precision a fellow can be with a belt sander

Another coat of resin over the entire structure (except the rear of the WG).
In the AM, I'll flip it over and install one layer of glass over rear WG and final coat of resin over entire structure.

Then: sand, fill minor imperfections.
Coat with primer just to get a feel for what it is I'll be looking at. Too many weird angles, colors, etc.. to imagine what this thing is going to look like finished.

Still have not gotten baffle #2 assembled yet. Waiting on #1 to be 95%+ finished to see what's "going on" with this whole mess.

Things are looking decent as of right now though (sorry Dan, things are looking better for the project).

Nothing worthy of a picture. Wouldn't be able to tell a difference on the 'net anyway.

Bob

Bob in St. Louis · « **Reply #156 on:** 11 Mar 2007, 09:15 pm »

UPDATE:

Some time spent with a belt sander to remove the resin "wavies", then another coat of resin to fill in the low spots brought a little light to the end of the tunnel.
Fingernails tapping on the glass section of the baffle created a distinct "tappy" noise, 'hollow' noise? (for lack of better words). It seemed like an undesirable trait, so instead of more coats of fiberglass cloth/mat, I decided to try a nice layer of expanding foam on the back of the "waist" area of the baffle. Hopefully this will deaden what appears to be a potentially "lively" sounding rig.
Between time spent on "life's little interventions", waiting for the final (final???) coat of resin to set up, and now waiting for the foam to dry, not much has happened this weekend on the experiment.
Certainly not as much as I was anticipaiting.

But isn't that always the case?
When the foam dries, It'll be sanded so it doesn't LOOK like expanding foam was blathered all over the back of the baffle. I'll coat with one last layer of resin* so it can be painted.
I keep refering to the "final" coat. It'll probably take several spot touchups to fill in the inevitable holes. So the final coat may not be an actual coat, more like dabs here and there.

* I coated a scrap piece of wood with the expanding foam so next weekend (after the foam is 100% cured) I can coat it with resin to see if it melts or catches fire

Bob

Bob in St. Louis · « **Reply #157 on:** 11 Mar 2007, 11:32 pm »

UPDATE: (Part II):

By the way - Anybody contemplating a similar fiberglass design, but hesitating due to cost, here's what I've got invested in the experiment:

$17 - 3/4" Plywood (NOT Baltic Birch)
$3 - Paint tray for mixing resin
$7 - Fiberglass Cloth (Nine square feet)
$7 - Fiberglass Mat (Nine square feet)
$2 - (2) Paint brushes {the very cheap disposable one's}
$35 - Fiberglass Resin (One gallon)
$5 - "Great Stuff" Expanding Foam
$5 - (2) "Simpson" brand "U" deck brackets (modified to hold the drivers)
-----
$81 Total

Now, $81 CAN get you TWO baffles (As I have one so far.) But the only things I'll have to purchase again will be (2) more paint brushes ($2) and one more can of "Great Stuff" foam ($5). If a fellow were to assemble and build BOTH baffles at the same time, you wouldn't have to purchase these items again. The brushes would be used on the second baffle before they dry into hard, useless 'paddles' and the expanding foam (which isn't "savable" beyond a few hours) would be enough for a second baffle.

Now, that being said, I've only heard one baffle (the left channel to be exact) and that wasn't with the foam. But, ...so far, ...so good. Taking into consideration what the first one sounded like without the foam, I have nothing but very high hopes for the project.
Taking longer than I thought.......

Bob

DanTheMan · « **Reply #158 on:** 12 Mar 2007, 06:31 am »

No need to apollogize Bob, I knew that is was going to get better. Glad to hear it though--I didn't want you to go through all that for nothing. Hopefully that foam will do the trick, keep us posted.

DanTheMan · « **Reply #159 on:** 12 Mar 2007, 06:32 am »

BTW John, I didn't realize you were crossing those over. HOw's the project coming?