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Discussion in 'Waveguides and Horns' started by SpeakerScott, Jan 21, 2013.
More pics on how the pattern jig works on the table saw.
More pics. Also a note on how I get the pieces apart.
I tack them together using two brad nails. I've cut a channel in the template just wide enough to fit my 1/2" Irwin chisel. The chisel is just wedged enough to get the pieces apart.
I've had plenty of woodworkers tell me never to do this with a chisel.
My antique and modern Stanley 750 chisels? Or my fancy Japanese chisels? At $20 -$100 each. No way!!!
An $8 Irwin? Sure. Does the least damage to the work piece.
It's a great tool for the job, and if I smack a brad nail, I won't cry!
(P.S., that $8 Irwin tuned up does almost as well as the Stanley 750's. They won't hold an edge as long, and aren't very good for paring by hand. When sharp though I don't really notice much of difference with any operations requiring a mallet...in fact the extra heft seems to be an advantage.)
Next up is cutting the 14.5 degree bevel on the edges. I marked the jig with a permanent marker to help me find the edges. I thought I would need it...but it wasn't as useful as I thought.
After that I made a set of jigs to cut the bevel on the "B" piece. Try as I could I couldn't figure out a way to do both sides without using a plunge cut. In fact I made a jig that I had to toss...because...it wouldn't work for anything!
THIRD TIME typing this up. I had an attachment too large and this forum loses the text!!!
Six Jigs (Plus 1 in the trash)
A Piece 14.5 Degree Bevel
B Piece 14.5 Degree Bevel
B Piece 14.5 Degree Bevel, Plunge Cut
A and B Piece 30 Degree Bevel
Table Saw Pattern Cutting Jig
Circular Saw Zero Clearance Sled (Already worn out and in the trash, saving scrap to make another)
I think the secondary flare is going to take two patterns and at least 2 or three more jigs. Then, I'll probably need another jig or 4, or 10 for drilling holes, making the compression driver mount...etc...
One thing I've learned is that these jigs always make things turn out nicer. The almost always make things go faster...especially when I need to make more than 2 or three of any one part. In some cases they are the only *safe* way to do it at all...like the 30 degree bevel on the long side of the A and B pieces...there's no way to do that safely without the jig.
About the plunge cut.
Some folks say don't do it.
Other folks say do it.
I agree with those folks...with the following caveats:
1. Clamp all pieces down hard to the table saw top. I don't care how long this takes, binding up a motor and possibly damaging a bearing or belt is way better than any alternative. It's not shown fully clamped in my picture.
2. Figure out which way the piece is likely to fly if it does leave the table top. In my case it would have been mostly straight back. For larger sheet goods they tend to rotate more in kickback off to the left. (Don't ask me how I know. )
3. Get out of the way of where it would go. I actually sit on the floor under the level of the table top. If it does take off and smack into the back wall of the shop (Actually it would hit my band saw first most likely.) I don't care. Even if I were watching, if something were to happen it usually happens so fast you can't react. So...I don't care.
I don't care.
That's better than hitting me.
4. Go. Slow.
5. Go. Slow.
It took me better than 45 seconds to raise the blade for the plunge cut. That's a very long time to crank a 10" saw blade from retracted to full height. I had one hand on the crank and the other on the power button the whole time.
6. If you have any concerns about it...don't do it. Figure out a better way. Even with all of those precautions I was still a wee bit puckerd.
Plunge cut pic....
Clamping board removed. Backup corner clamp board still in place.
In case anyone doesn't know what a plunge cut is....you start with the work piece over the retracted saw blade. The work piece is clamped firmly to the table...and your hands are well away from anything with spinning tendencies (the saw blade). Slowly raise the blade into the piece. With MDF or plywood it's a pretty easy operation. With hardwood you run the chance of relieving stress in the wood as you cut...and the wood warping into the side of the blade. Best case it's a bit blackened from rubbing...worst case it binds the blade to a halt and whatever would happen with a seized motor or a work piece that want's to move rapidly towards where an operator stands happens...
I actually used to work with a trim guy who would loosen the belts on his contractor saw, and do a drop cut...start with the blade up high and guide the piece down into it, by hand. If things got out of hand the belts would slip. He did it for cabinet face frames. I could never watch, it drove me nuts....
Great progress and very nice shop! Mine has to fit in my garage and before I run something through the table saw I always look back over my shoulder to see if my car out in the driverway is in the way of any kickback. I imagine you don't allow any young helpers in the shop while you are working.
re the issues in my last post.
I simulated both the HiVi BN4 and the PA130 on the same horn, same conditions, same throat and chamber parameters (with just a little tuning to optimize each) and the responses came out virtually on top of each other, except the PA130 was about 3 db more efficient due to its size. the HiVi caught up SPL wise at the low end because it has larger xMax. I've run simulations at a wide range of L12 - from 6" down to 2" - and the high frequency cutoff kept rising - indicating that the mass load corner wasn't taking over. 2Fs/Qts is 376 for the Dayton but only 215 for the HiVi yet both were responded quite similarly out to almost 2 Khz as simulated. I don't know what to make of this in light of the conventional wisdom abou the mass load corner but I hope I can trust the simulations...
I think I have the explanation. I just reread Keele's paper and saw two things:
1. the analysis if for an exponential horn
2. the formula depends only on driver parameters
The difference between an exponential horn and a conical is that the throat resistance of the exponential horn is relatively constant above cutoff while that of the conical horn is constantly rising. The derivation of the mass load corner depends on the throat resistance being constant and resistive so the predicted mass load corner doesn't apply to conical horns!!!! Apparently, the rising throat resistance of the conical horn apparently cancels out the drop off effect. This shouldn't be surprising since efficiency is proportional to load resistance. A graph on p90 of Olson's Fundamentals of Acoutistical Engineering shows the throat impedance reaching 90% of its asymptotic value at 2 khz so I wouldn't be surprised to find it increasingly difficult to get response much higher than 2 khz.
One thing to keep in mind (I think someone mentioned it) is if the port is offset from the mid driver center. I've got a set of Paralines (another Danley design) on 60x40 conicals playing right now. Same basic deal though, compression driver with mids flanking it working through ports/compression chambers. I designed the mid port position for a 1500hz roll-off (using the 4" closed back Celestions), but in reality I can only get to 1100hz due to the mid port being offset from the driver center.
Also, for the "traditional" Synergy it isn't all about getting the mid port as close to the CD as possible. Closer proximity can get a higher cutoff for the mid, be we also want it to be in a position in the horn where the CD can't "see" the ports due to the CD expansion within the horn itself, determined by the high pass of the CD. This will help keep the CD response smooth as well as allow the mid to enter in its optimal position. JLH explains this very well in the Synergy midrange thread on DIYA.
That said, I'm really digging this project......good work!
Yeah, I can lock the door from inside the shop. It was pretty funny actually, when we took the adoption classes we filled out all this paperwork and they had us swear up and down about the number of guns we had, recreational vehicles etc.... I had to offer up:
"So I have this woodworking shop and there are literally hundreds of really sharp things in there....or heavy things....or power tools...." It's actually attached to the house and has a full glass door so I can look in and see the whole thing and if there is a munchkin in there. I can lock them out as well. When they first came to us they were sooooo scared of the room (good and not good at the same time), then they started wanting to play in there. I unhooked the first air hose without them knowing about it...psssst....now they listen when I say. "Ladies...it's time to go inside."
I'm hoping to get to measurements on mine soon. The girls are going to spend next weekend at the in-laws house. I may not make that much progress this week. Two business dinners and Mom is leaving me solo one night to go to the opera with some friends.
I found the plot you're talking about. My copy is from a former manager who gave it to me when he retired. He had neglected to return it from the U.T. Austin engineering library after he checked it out. In 1978.
I wonder what the statute of limitations on library books is?
I'll have to ponder what you're saying a bit...but it makes enough sense on first read for me to agree.
Okay...so there's two very important ideas regarding the Synergy concept in this, and I'm going to restate to make sure I understand what you are saying.
1. If the midrange port is offset from the driver center this causes a reduction in high frequency response.
2. It is possible to be too close or too far from the right spot for the midrange entry point.
So...for #1, this is a new concept for me. I hadn't seen this before...or if I had, I'd forgotten it. It would make sense conceptually (again, I view Danley designs as a pressure operated system), air is a relatively lossy system there would be a fair bit of lossy mass in between the port exit on one side of the cone that wouldn't be present on the other.
My question is this...I'm planning on two holes one on each side of the cone...Danley often does this in his own designs...and it effectively centers up the exit by having a mirrored system. The center of the cone sees the same port length the edges would in a center ported design, so it's an effective mitigation of the issue right? In addition I've seen pictures of what I believe is the SH-25 where each of the mids is offset the same amount and the port is centered. It appears that either could be desirable.
For #2, I've seen that thread as well. I'm hoping my projected placement (roughly 2" is about right). Based on what I can see from pictures of the SH-50 and SH-60 it appears about right. I'm looking at 8 ~1/2 " holes or smaller, 2 each mid, at 2" from S1. The question is where is S1 with the compression driver I have.
I'm leary about two holes at 180 degrees. I see that Danley does this for woofers, where, if anything, he wants increased HF roll off, but I haven't seen it done for CDs- except in Paralines. You can see it in Fig9 of the paraline patent. The two holes aren't 180 degrees apart but fall fairly close together on the edge of the paraline bend
I'm afraid symmetry just means the outputs from the two holes are identical. I drew the attached to visualize the problem and see how hard it would be to do an analysis. When one does an analysis like this, one ignores one hole and "calculates" what comes out the other hole. Since you are ignoring the other port hole, it doesn't affect the result. By symmetry the output from the two holes are identical. Of course, the two holes must be placed equidistant from the apex of the cone, which tells me something about how you were planning to mount the mids. As Nate testifies, its clear you get cancellation; the question (if you can't change your plan) is how much. Pehaps you can get an decent approximation by implementing the process in my figure in a spreadsheet. It may be adequate simply to determine where the effect begins simply by relating the maximum path length delta to a fraction (1/4?) of a wavelength.
this morning I'm thinking you can measure this effect without having built the horn. Just screw the speaker down to a scrap of plywood with a port hole drilled through it near the edge of the surround and run a sweep. Move the speaker so that the hole is centered and run another one....
To quote Guinness beer: "BRILLIANT".
I'm ashamed I hadn't thought of that. I'll also take impedance sweeps, and possibly measurements with and without the volume plug/frustrum. This might yield some insight into the performance of the whole system.
I did a little reverse engineering through photo's last night. In pictures of the SH-46, SH-50, SH-60, SH-69, SH-64, and SH-96 it appears that there are 8 midrange port holes in every one of those products. Curiously there are anywhere from 4 to 6 midrange drivers in those. (Also curiously, the compression driver entry is square in every picture.) That would mean, unless there are "fake" holes for the 6 driver horn, that some drivers have 2 port holes while other's have 1 port.
Good stuff. Would it make sense to build a simple test box say quarter volume sized? Then measure upwards with the movable test baffle.
I was gonna mention that..... It's what I did after I had everything built and it wasn't working like I thought it should. It would be interesting to see how 2 offset ports would act.
Nate, can you post any of your measurements?
I'm not sure a single driver, not horn loaded measurement of 2 ports would be all that accurate, since there is no way to simulate the contribution of it's theoretically identical neighboring ports on either side. Still trying to wrap my mind around this one.
Probably the first of several direction changes....
So last night I molded the first "compression plug" for the midranges using some bondo.
1. It's stinky as all get out. (I knew that going in.)
2. It generates a fair amount of heat during curing. ( I knew that going in)
3. It does not stick at all to plastic wrap, even without a mold release agent. (That was news to me.)
4. It made a pretty good mold of the midrange cone. (Phew.)
5. It took way more time to prep and do than I thought. (Hmmm.)
I'm tempted to shape this piece to the size/shape I want, cut it in half, and use it as a template for turning plugs on the lathe. They won't come out identically on the lathe, but I don't think they'll turn out identically using the molding method. The Bondo is too thick and too light to effectively push into all corners and cracks created by the plastic wrap. I'll post a picture when I get a chance.
That said, poplar turns easily. So does MDF and the plug ended up being ~.75" deep.
This weekend I'm going to give the turning method a try.
Got some testing done today....still digesting what I'm seeing. Very little difference between offset and centered, and it appears that the difference is a high-Q notch.
I tested 5 options:
No Frustrum, .5" port, centered
Small Frustrum, .5" port, centered
Small Frustrum, .5" port, offset
Small Frustrum, .5" port, offset, volume/compression plug.
Small Frustrum, .75" port, offset, volume/compression plug.
The highest frequency response was the last, which has the area of two .5" ports, which is what I planned on doing for my build.
More Plugs, and the frequency measurement jig.
Yet more pictures of wooden plugs.
Very interesting results, thanks for sharing them.
You measured by just firing the driver through a hole on the baffle, correct?
Any enclosure on the rear of the driver? How large was the baffle?
No rear enclosure, the baffle was roughly 2'x2'. I measured at a distance of about 4", which allowed me to gate out any diffraction and floor bounce impacts. The idea was to get the output of the port only since that's what will be firing into the horn.
Thanks for the pictures. I wanted to see those plugs and how the sound gets past them. .I guess you'll just suspend them in the cavity.
Your measurements sure proved the value of minimizing the air trapped in front of the cone. Eliminate the air and you minimize the amount of low pass filtering. I was surprised it was such a big difference. I need to go back to HornResponse.
I think this low pass filtering is masking the difference between centered and offset ports. I do expect some but I guess it comes in at higher frequency than the acoustic low pass filtering due to the cavity volume and throat port.
I shouldn't even try to account for that notch in your offset test and I'm not, just sharing an idea. One could make an analogy with a close mic/nearfield woofer measurement. The MIC is equivalent to the throat port in that analogy. D'Appolito (following Keele) calculates a maximum frequency above which the nearfield measurement isn't valid and says you can get effects due to reflections from the rim of the cone/woofer frame in a nearfield measurement that aren't there in the far field. You can see why - the further away you move the MIC, the shorter the center vs edge path length difference becomes. This doesn't explain why there is no sign of a notch at a higher frequency in the centered hole meaurements.