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The Othorn tapped horn - Page 3

post #61 of 132
Quote:
Originally Posted by mwmkravchenko View Post

I have some good news for you Josh.


1 pascal = 0.000145037738 pounds per square inch.

Your pressure is not that high. Not all that bad.

2.08 PSI peak. Surely that overgrown cone can handle that!

Mark

2psi integrated over the 260sq-in radiating area comes out to 520lbs of force....no way in the world the motor structure or diaphragm is strong enough to push that.

F = BLI, so 32.57 Bl x 120V / 8ohms = 488 Newtons or 110 lbs of force at 15amps current through the voice coil.

Those are DC calculations and I can see it being able to drive more in a steady state resonant condition (pushing in sync with a swing), but that's still a lot of force to deal with.

Interestingly though...that could very well explain the soft limiting behavior instead of clipping.
post #62 of 132
Thread Starter 
Quote:
Originally Posted by MBentz View Post

Interestingly though...that could very well explain the soft limiting behavior instead of clipping.

Indeed sir...
Do you know of any research into this area of performance in horns? I will be doing a few things eventually, but if someone has already done the leg work that would be great. No need to plow the same ground.
post #63 of 132
Quote:


2psi integrated over the 260sq-in radiating area comes out to 520lbs of force....no way in the world the motor structure or diaphragm is strong enough to push that.

It doesn't.

You are missing some of the finer points required to calculate the true pressure in the enclosure.

Given your thinking as stated no cone structure could survive the pressures listed. But they can and do. Therefore there is an error in your application of the calculated loading.

Josh there are examples of papers written.

Some are listed in the Horresp thread at diyaudio.

When I get them I will be happy to forward them over.

Mark
post #64 of 132
"2psi integrated over the 260sq-in radiating area comes out to 520lbs of force....no way in the world the motor structure or diaphragm is strong enough to push that."

i doubt it is anywhere near 2 psi.

here is a chart that matches spl to psi.

http://www.audiogroupforum.com/csfor...37&postcount=4

i don't know if his equations are accurate. if we go with them...then:

for a rough guess, start with the mouth spl and find the psi. then multiply that by the size of the mouth / size of the thoat because psi is proportional to cross sectional area.

say ricci's horn is 136 db at the mouth and has a 5:1 mouth to throat expansion. 136 db is 0.02 psi, so at the cone it might be 0.10 psi. that is a massively violent change in pressure, but no where near 2.0 psi, which as you note would likely destroy the system.
post #65 of 132
Quote:
Originally Posted by mwmkravchenko View Post

It doesn't.

You are missing some of the finer points required to calculate the true pressure in the enclosure.

Given your thinking as stated no cone structure could survive the pressures listed. But they can and do. Therefore there is an error in your application of the calculated loading.

Josh there are examples of papers written.

Some are listed in the Horresp thread at diyaudio.

When I get them I will be happy to forward them over.

Mark

You do realize that you provided the 2psi spec? I also don't know of any documented paper cone diaphragm surviving those kinds of SPLs. Care to elaborate on any examples, or provide the different mathematical solution?
post #66 of 132
Mike you have been in similar debates more than once.

This is what I will say.

The pressure statement was under a specific condition discussed in the post.

It came from a simulation from a well respected program.

Your application of the air pressure on a diaphragm fly's in the face of a reasoned understanding of the true loads on a moving speaker diaphragm in the enclosure under consideration.

I am not going to go into the math required to explain the air loading.

I'm not a math wiz.

I can and do apply mathematics where I need to.

But my high level math is almost 25 years old.
Dusty cobwebs would be the correct assumption.

But understand how these puppies work I do that well.

Josh has already confirmed that the driver stands the power and resulting pressures in his enclosure.

So what is there to argue about in the end?

I'll say this much.

air pressure on the front of the cone is not the only factor to consider. The front is actually the lower pressure side in this particular enclosure. The rearward side is the area where the pressure is higher.

I think I picked the wrong sides of through driver in the last paragraph.

Memory serves correctly the driver proper sits in the horn mouth. So the greater pressure is indeed on the conventional front of the cone.

Mark
post #67 of 132
Thread Starter 
post #68 of 132
Nice intelligent box design.

Those pocket hole screws make life easier to.
post #69 of 132
Nuts!
post #70 of 132
I am glad to see this thing coming along. Finances changed for me so I have to wait on the building supplies for my build. I will be using this sub for my backyard listening.
post #71 of 132
Will you be able to post some info your databass site for this sub in the next month or so?

And what is a reasonable quote for CNC for this horn?

I know my quote will be higher due to being in Australia but I was just curious. I think I am going to just cut all the measurements myself and dial the angles with a magnetic angle finder.
post #72 of 132
Thread Starter 
I still dont have the finished cabs...When I do there will be an update.
post #73 of 132
Interesting. i will keep it locked in here for the results as they become public...
post #74 of 132
Thread Starter 
I finally have my pair of cabs. Once again the cabinet shop has done a remarkable job with them. These have been coated with Duratex, are very solid and should support the weight of a truck with no problem. I still have to think about what hardware I'm going to use on them. I have mounted one of the 21's and checked an 18" with the adaptor plate and both mount up perfectly.








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post #75 of 132
Thread Starter 
Here are a few more pics showing the adaptor plates for 18" drivers. These bolt onto the existing baffle and fit down into the 21" driver cutout. The plate bolts into the holes for mounting the 21" driver plus a few extras.



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Unfortunately I have not had a chance to actually do much with them yet. Hopefully I will get a chance soon. All I have so far is one driver mounted and a very quick close mic response measurement in room and no damping in the cab. Overall it is shaped a little different than the simulation but is fairly smooth up to 250Hz (For a TH ). There is a bit of a dip at 90Hz. As usual the actual corner is a little lower than the simulation. Driver excursion minimum occurs at 28Hz so 25Hz extension should be well met. The SPL level in the measurement is uncalibrated and at an arbitrary level. I am not sure what is causing the small notch at 42Hz. I haven't had a chance to think about it too much yet. The waterfall also shows that there is some sort of a issue at 42Hz and an odd one with output decaying rapidly for the first 20dB but then ringing a little at that point. Perhaps reflected energy from the backside of the driver? Other than that the decay rate is remarkably clean for a horn with very rapid loss of energy. In particular I expected there to be a notable ring at 108Hz and a couple of the other response spikes which is usual with other high Q response features, but instead the energy drops 30dB very rapidly. I suspect that after developing EQ to flatten the response above 100Hz and the addition of some damping in the enclosure that this will improve even further. I have access to the throat, 1st corner, last 2 corners and mouth, so I will try adding some damping in these places when I have a chance. An actual outdoor quasi-anechoic response measurement might show slightly different response features than the extreme close mic also.




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post #76 of 132
Is the adapter plate in your adobe file for the TH?

The cabinets look awesome. I just got my quote back from the CNC guys here and it looks like I will stick with building it myself.
post #77 of 132
Interesting. I noticed that dip/peak thing around 40hz and to me it would suggest some form of cancellation at the mouth but then maybe some internal ringing? I'm not sure but it is odd.

That being said, the response looks pretty damn good and wow is it clean! I'm sure it will go super loud too. Good job, Josh!
post #78 of 132
Thread Starter 
Bump for the impedance measurement...

Working on checking a couple of different damping strategies now.







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post #79 of 132
Not bad, Josh. This thing is a beast!

What do you think of it? How does it compare wrt SQ and SPL to say... your vented B&C 21 cab?
post #80 of 132
Thread Starter 
I don't know Scott...It has had a total of a few minutes of pink noise and a couple of test signals sent through it so far. I hope to get there soon. I can tell they are going to be capable. Should have another 4 or 5 dB on the vented cab if all goes well.
post #81 of 132
The ability to switch between 18s and 21s is fun.
post #82 of 132
Thread Starter 
Quote:
Originally Posted by WiSounds View Post

The ability to switch between 18s and 21s is fun.

Definitely...I have been doing the removable baffle thing for awhile. It makes trying new stuff easier.


Unfortunately neither the Zv3 or UXL18 matched up with this cab. I tried. The 21LW1400 works though. Actually the 4 ohm last version Solo-X 18 looks workable. Got a Pro5100 that will get tried too.
post #83 of 132
Did the Z have a cabinet resonance issue?

Need a SoloX to test with? Lol
post #84 of 132
Thread Starter 
Quote:
Originally Posted by WiSounds View Post

Did the Z have a cabinet resonance issue?

Need a SoloX to test with? Lol

I thought about it. It actually looks decent in the Gjallerhorn too but I would have to cut into the baflle for the square driver and I just can't do that to try something which "may" work.

Z looked like this....Bleck...



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post #85 of 132
Thread Starter 
Back from the dead again...

Here is a bit of an update on this 27Hz TH design. I finally had a chance to get outdoors with it. Unfortunately it was a bit of a bitter sweet event involving both good and bad.

First the bad part...I have damaged one of my 21SW152-4's during the testing which prevented me from getting the full data set and it will need reconed. Truthfully the fact that they have both survived unscathed everything they have been through thus far is impressive to say the least so I'm not discouraged by that.

The good is that I did get some measurements and much of it was better than I had hoped for. The raw response shape is what I would call pretty good for a TH and the 40Hz response blip I encountered during the initial close mic measurements is gone so that was obviously some sort of in room acoustic artifact. Additionally the response decay is much much cleaner than I had expected with very little ringing exhibited. I have only seen this type of data for a few TH's, pipes and FLH's but usually due to the nature of their operation there are some prolonged resonances which can ring and color the sound somewhat, usually associated with high Q notches or peaks in the response shape. Most higher order systems seem to exhibit these to one degree or another.

The 1w/1m sensitivity result is also good and I was able to test the cab with and without damping in a couple of places along the internal pathway and also with a rigged up throat compression plate which attempts to compensate for the large volume of air contained inside of the large 21" cone. This test was inspired by some discussion that occured in a TH related thread at DIYAudio a few months back. This extra volume of air is not usually accounted for in the horn models very well and small differences in the throat section between the models and the real world system can account for significant response differences. Anyway I made a very simple plate and installed it under the driver which did improve the top end response and sensitivity somewhat compared to the measurements without it. Unfortunately this plate is likely the cause of the damaged driver. This cabinet is already out there beyond recommended conventional practice with a 4/1 compression ratio on a 21" driver. The addition of the plate further decreased the throat volume by a significant amount and truthfully I did not verify with certainty whether physical contact of the driver cone with the plate was impossible prior to letting it rip. The result was a damaged cone with some buckling in a few spots and separation from the surround at one point. I suspect that this is caused by simply too much pressure on the cone rather than physical contact but it could be either.

Now...Let me explain what the system was being subjected to at the time that the driver cone broke down. This was during maximum output burst testing with a Powersoft K10 run on 240V and bridged into a single cab. The amplifier was being driven into clipping with a 6.5cycle duration shaped burst of 63Hz. I'm not exactly sure how much voltage or power that is but let's just assume it is just a smidge more than recommended or would be used as "program" power. I usually take 3 or 4 readings at least to make sure that the results are repeatable. If the distortion gets out of hand or distress starts to become readily apparent I will stop increasing the level. This cab was actually ok up to amplifier clipping at 125,100,80 and 63Hz but on the 3rd full power burst the output dropped considerably and the tone became different indicating that something had given way. Turns out it was the driver cone. I'm not sure whether it physically contacted the plate in the throat or whether the pressures were simply too great for repeated blasts at that level. The output being recorded was just above 128dB at 63Hz at 4 meters outdoors ground plane which equates to about 134dB 1m fullspace(anechoic) or just above 140dB at 1m halfspace. Higher readings were obtained at 80,100 and 125Hz. (The levels involved require measuring with the microphone at a 4m distance from the cabinet in order to avoid overloading the input stage.) Hopefully with removal of the additional throat plate the second drivers cone will survive this type of abuse. I believe it will.


Pictures and measurements shortly. The discussion on pressures involved with this cab starting on post #43 to post #66 becomes a little more interesting now. cool.gif
post #86 of 132
Dang, you're insane dude - that sucks about the driver. Looking forward to the pics.

Btw, here's the force numbers run at 4000W and 12000W at 63Hz where your impedance is ~4ohm and the phase is 0 (so we're talking real power):
4000W = 31A = 230 lbf (160 lbf with 70% Bl)
12000W = 54A = 400 lbf (280 lbf with 70% Bl)

Btw, that's the accelerating force - Your moving mass is roughly 1lb, and at 63Hz, the amount of acceleration being imparted to the moving mass follows this equation:
43
So the peak acceleration is (63)^2 x displacement (units = in/s^2). With 400W, your simulation is claiming about 0.75in excursion (20mm) at 60Hz (using Akabak model).

Converting between lbf and lbm:
84

We have lbf (moving mass) = 1lbm * 63^2 x 0.75 / (32.17 * 12) = 8 lbf. That means the minimum air load is somewhere in the ballpark of (160 - 8) / 260 sq in = 0.6psi (166dBspl)
At 12000W, the excursion is ~1.7x greater, so you're looking at ~13 lbf for the moving mass. That means the max possible air load should be less than 1.5psi (174dBspl).

Now let's take into account the suspension compliance (which is 0.01 in / lbf). For an excursion of 0.75", the force used up is 75 lbf. That reduces to (160 - 75 - 8)/260sq in = 0.3psi (160dBspl).
At 12000W you're looking at (400 - 127 - 13)/260 = 1 psi (170dBspl)

At those SPL's, it would be interesting to run these numbers:
http://books.google.com/books?id=b_wxNccLhXoC&lpg=PA90&ots=Kg5gbKvL8N&dq=beranek%20horn%20throat%20distortion&pg=PA91#v=twopage&q&f=true
Not sure if you can apply the Fc concept to the tapped horn or not...I think using the tapped horn Fc would give an idea of the minimum distortion level possible.

Using this calculator, I'm figuring at least 400 acoustic watts in the throat with 4000W input:
http://www.sengpielaudio.com/calculator-soundlevel.htm
(multiply by the 0.04 sq m of the throat)

So 1.76 sqrt(400) x (63 / 30) / 100 = 0.7% THD with 160dBspl in the throat.
With 12000W and 170dBspl, it's more like 2.3% THD.
(That's actually quite surprising to be honest, but I guess that's just the nature of low bandwidth horns).


When you break it all down, the air should represent somewhere between 48% and 65% of the load at the SPLs you were pushing...probably slightly less since the suspension isn't linear either. I wonder if they build in a 2x to 3x strength factor on the diaphragms? I would imagine that they'd design the cone to be stiff enough to clip hard into the limiting region of the suspension - so how much stiffness is left over when adding the air load?

As a side point, if the air load ever goes away, then all of that motor force turns into diaphragm motion and your excursions will be way higher. So either you end up with way more SPL or the electrical impedance will be rising (or you get into the power factor stuff)....we're not seeing that happen with the measured impedance. I do think it is interesting to note however that the air load across the diaphragm does change with time as a function of the propagation distances through the cabinet.


So ya, I'm still not seeing any way around the fact that there are extreme pressures inside the cabinet. Maybe we should find a mic that will measure the actual pressure across the diaphragm and take all the guess work out? If that's something you'd want to try, then I can talk to our mic guys at work to see if they can't put together something real quick like. We'd want the mic diaphragm exposed to both sides of the baffle, so there'd need to be room for a small hole through the baffle.

Btw, how much is the recone kit?
post #87 of 132
Thread Starter 
"So ya, I'm still not seeing any way around the fact that there are extreme pressures inside the cabinet. "

Agreed...If you are generating these types of levels from a cab this size large stresses are happening in the throat and on the cone plain and simple. I might be interested in this mic sensor if you want to look into it...I already have holes through the baffle and under the throat that could be used. 4 of them are offset into the corners in order to bolt the 18" adapter plate. I also have a few small screw holes in the board under the driver that the throat plate was attached to. Perhaps one of the Termlab pressure sensors used in car audio SPL comps could be useful under the driver in the throat?

I have found a recone for around $250. Not too bad but not chump change either. Oh well sometimes you have to break some eggs.
post #88 of 132
Thread Starter 
Here are some pics...

Here is the generic throat plate that I added to try and improve the expansion.
450
450

Here are some pics of the driver cone showing the damage. The creases were formed on the tighter end of the throat right at the beginning of the horn path. I do not think that enough excursion was produced to physically contact the throat plate shown above but I am not sure of that. That may be what happened.
338
338



Here are some GP measurements taken at 18.6v (1w into the 3.4ohm impedance min) . I tried a few different tests: 1. a generous amount of damping in the last 2 turns of the horn and in the throat and first turn. 2. The empty undamped cab as designed, 3. With the stepped throat plate added under the driver and 4. with only the damping in the throat. After looking at a couple of different measurement types the damping did nothing that would justify the small hit in efficiency taken by using it. It showed negligible improvements in group delay, decay rates, response flattening, etc and a noticeable cut in sensitivity so I will be using none in this cab. As can be seen the orange trace with the throat plate added increased the top end efficiency somewhat and flattened response a little. This was the reason that I decided to try testing with it. Unfortunately since the driver was damaged in the process I have decided to revert back to using the cabs as designed with slightly degraded top end but hopefully less stress on the driver cone at maximum drive levels.
479


Here is basic raw response of the cab with the phase shown at an 18.6v drive level. Overall I think the response shape and phase is very good and relatively flat for an unprocessed TH cab.
452


As I have found when testing cabs outdoors the sensitivity comes in much higher than the simulations would suggest. My guess is that this is due to directivity and/or mic proximity to reflected energy from a large cabinet. However comparing 10m 18.6v to 1m 1.86v measurement indicates not as much difference as would be needed. Voltage input was almost exactly 1.86v at the cabinet terminals. I've also noticed that some manufacturers seem to end up with a sensitivity some 3dB or so higher than would be expected with no explanation. Anyone have any ideas?
476

Waterfall and group delay show rapid decay and loss of 30dB of energy with little ringing except for near 105Hz. This is much better than I had expected and indicates that the Othorn should be capable of excellent sound if the distortion is well controlled. (I already know that the response can be made flat without much effort, there is little ringing or resonance and that the headroom for BIG dynamics or response modification to suit to taste is there).
458
438
Edited by Ricci - 6/19/12 at 12:04pm
post #89 of 132
Looks like FUN times.... sorry to hear about the loss of the driver, but that's some pretty impressive numbers for a single cabinet outdoors.... eek.gif
post #90 of 132
Pics of the recone process please biggrin.gif
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