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Old 30th April 2013, 03:20 PM   #841
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Default Back to the CD lower cutoff frequency for a moment

Quote:
Originally Posted by JLH View Post
That's not correct unless you are using a compression driver that is smaller than 1". A 1" compression driver has a throat area of 5.067cm^2. Calculate the flare rate that it sees from what I wrote above.



S1 is less than 5.067cm^2 because he is accounting for the internal structure of the compression driver. I'm not sure if he actually measured the acoustical path length, or made a physical measurement of the driver. The most accurate method is to use speaker measurement equipment and send a pulse to the speaker. The time delay (or time of flight) can be used to calculate the real acoustical center of the driver. This is different than the physical measurement of the diver's pathway. The acoustic center is usually more forward than what the driver physically measures.

The 6.7cm distance for the BMS4550 comes from an actual acoustical measurement. The internal flare is of no concern. You want to know what your horn is doing. Start with 5.07cm^2 and perform the flare rate calculation based on the distance it takes to expand to 10.14cm^2.
I had to step away from the conversation on the weekend but caught up last night. I ran the 5 to 10 sq. cm. flare rate numbers for a range of horn angles, getting roughly 4 Khz for a 90 degree horn and 2 khz for a 60 degree horn. I found TS parameters for the BMS4550 in an akabak script posted by JLH (thanks) and simulated it on those horns. I did some research and found Bill Waslo's 90x60 Cosyne crossing right around 4 khz. OTOH, several 40 and 60 degree horn threads showed crossovers at/around 1 khz, half the calcuated cutoff.

As you can see in the attached chart and no doubt expected, the cutoff is quite gradual in a conical horn. Its apparent both why Bill pushed his crossover as high as possible and why in a narrower horn you can get away with crossing over under the calculated cutoff frequency. The sim was run at just over 100W into the CD. The driver is at 1 mm excursion at 800 hz or so on the 60 degree horn and 1 khz on the 90 degree horn trace.

I'm now torn between going with the 256x90 horn that fits my intended corner placement better and with a 60x60 which would be an easier design and not suffer from pattern flip.

Thanks to JLH and PB and all for the enlightenment of the last few days.
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File Type: gif Compression Driver Cutoff Frequency Sim Screenshot.gif (92.4 KB, 348 views)
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Old 30th April 2013, 04:26 PM   #842
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Quote:
Originally Posted by nc535 View Post
As you can see in the attached chart and no doubt expected, the cutoff is quite gradual in a conical horn. Its apparent both why Bill pushed his crossover as high as possible and why in a narrower horn you can get away with crossing over under the calculated cutoff frequency. The sim was run at just over 100W into the CD. The driver is at 1 mm excursion at 800 hz or so on the 60 degree horn and 1 khz on the 90 degree horn trace.

I'm now torn between going with the 256x90 horn that fits my intended corner placement better and with a 60x60 which would be an easier design and not suffer from pattern flip.
256x90 horn ?

1mm excursion would sound totally gross, the 4550 has around 8% distortion at 111.7 dB (one meter equivalent) with a dual sine wave tone of 1046 &1865 Hz in to a 13 x13 degree conical horn.

The 13 x13 degree Maltese horn has a much lower cutoff frequency and much higher on axis sensitivity (around 115 dB one watt one meter) than your sims.

My measurements suggest Xmax probably on the order of .2 mm or less for the BMS 4550. Nice sounding driver, but it won't sound nice at 100 watts at 800 Hz on a 60 x 60 conical horn!

If you want to actually hear how the driver sounds at high drive levels on a conical horn, check out this thread:

High Frequency Compression Driver Evaluation

A shorter version (only 3000 rather than 8000 words, and a lot less pictures) can be found here:
High Frequency Compression Driver Evaluation

The sound files in the soundforums.net posts do not have the low frequency portion of the music mixed in, listening to the HF horn alone makes it easier to hear the difference in sound quality and distortion between the various drivers all tested under the same conditions.

Art
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Old 30th April 2013, 04:34 PM   #843
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Thanks, Art.
256x90 = typo, horn model was 90x60 big enough to keep pattern control down to 256 hz

I wouldn't dream of listening at that excursion either. It was just wherer I happened to do the simulation. I think I'll be down in the 1-10W region, certainly RMS wise, don't know about the occasional peak. And I'll push the xover up to 1.5 or 2 khz, if I can.
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Old 30th April 2013, 05:49 PM   #844
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Quote:
Originally Posted by nc535 View Post
Thanks, Art.
256x90 = typo, horn model was 90x60 big enough to keep pattern control down to 256 hz
90 x 60 pattern control down to 256 Hz will require a horn with a mouth around 4.4 feet (1.6 meters) wide in the 60 degree dimension, that's rather large.
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Old 30th April 2013, 07:35 PM   #845
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Sorry for speaking loosely. The 256 hz pattern loss was horizontal. vertical pattern would be 50% higher. Not quite so big at 3' across the mouth and not overbearing when tucked tightly into a corner.
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Old 30th April 2013, 09:11 PM   #846
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Quote:
Originally Posted by nc535 View Post
I ran the 5 to 10 sq. cm. flare rate numbers for a range of horn angles, getting roughly 4 Khz for a 90 degree horn and 2 khz for a 60 degree horn.
I think you might have some miscalculations. When determining the flare rate in the throat of these conical horns, you have to start with a 0.7071" by 0.7071" throat because from corner to corner it is 1". You then sand out the throat to make it 1" in daimeter. The actual starting area for your calculation would be 3.226cm^2 and not 5.067cm^2. Try re-running your numbers with this in mind. You should see those flare rates drop to much lower numbers. I know on a 60 X 40 horn the flare rate the compression driver sees is around 1300Hz.
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Old 30th April 2013, 09:41 PM   #847
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Thanks for pitching in, JLH.

that is where I get lost - those little details - because I haven't seen and worked through all the math that they are based on. I understand that construction detail but I'm having trouble relating it to the flare rate calculation. I'm embarrassed to admit that I keep getting this calcuation wrong because you didn't give me explicit directions as to precisely where to click like you did for the mid calcuation.

In hornresp, I just set S1 to 5, S2 to 10, set the FTA to the desired angle, then calcuated the length. Then changed S1 - S2 to Exp and read off the F12. I'm not surprised that is wrong given I was guessing as to the method. But if they are wrong, why do they seem to agree with the hornresp sims I did using YOUR driver model? I know I seem to be a poor student but give me credit at least for trying to validate those results
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Old 1st May 2013, 12:23 PM   #848
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Quote:
Originally Posted by nc535 View Post
that is where I get lost - those little details - because I haven't seen and worked through all the math that they are based on.

Its best to draw out your horn throat and take a ruler to measure it. This way you can be sure what the actual flare rate will be. When you machine out the throat to 1" in diameter, you in effect decrease the flare rate. The throat becomes more like an exponential contour. See attached diagram. Measure the length it takes to go from 5.07cm^2 to 10.14cm^2. For a 60 X 40 horn this should be on the order of about 1300Hz.
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File Type: jpg horn throat.JPG (27.7 KB, 312 views)
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Old 1st May 2013, 03:12 PM   #849
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AHA!
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Old 2nd May 2013, 03:11 AM   #850
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Quote:
Originally Posted by natehansen66 View Post
I'll try to explain what I was getting at with the Summa reference. I understand the reasoning that the comp driver won't exert any pressure on the horn walls once its output is past the point in the horn where the circumference is greater than the wavelength of the lowest freq we want to achieve (1200hz in your example). This makes sense since as you've been hammering home the conical horn has a variable flare rate.

Where my confusion lies is with e.g. the Summa's 15" waveguide. From pictures I'd say that 15" includes the radius transition to the baffle. Taking a guess, the radius begins where the waveguide is about 10-12" in diameter. For the Summa to hit 900hz, we're looking at a circumference of about 15", which is a diameter of 4.8". Obviously Earl's waveguides are much bigger than this (4.8" dia), to control the directivity to match a woofer at the xo. It stands to reason in my mind then that the compression driver in the Summa is exerting some pressure on the waveguide walls past the 15" circumference, otherwise it wouldn't need to be as large as it is. I also understand that the radius to the baffle helps control directivity at the low end to prevent waist-banding the polar pattern there, contributing to the larger size.

Drawing that line of reasoning to the Synergy horn, it seems to me that the horn will be constraining the comp driver's directivity past the area where the flare rate no longer loads the device (a catch-22 to your line of reasoning, I know). This would be the region where we are tapping the mids in, and I wonder if at off axis angles approaching the horn wall angle (for a 60x60 then we would say 30) if the comp driver would in fact "see" the mid ports? Or does the fact that we are using square horns here mitigate that effect like you say? Or is comparing one of Earl's waveguides to a conical horn comparing apples to oranges?

If that isn't clear as mud I'll make some drawings
Click the image to open in full size.
Click the image to open in full size.
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Click the image to open in full size.
Click the image to open in full size.

Your question can be answered by studying directivity sonograms. When I've studied them, I've observed the following:

1) There are waveguides where the sound seems to 'detach' itself from the waveguide. If you look at the sonogram on some waveguides, the directivity sometimes widens or narrows at an unexpected frequency.
2) Sometimes you can have two waveguides which both *appear* to be carefully engineered, but one of them simply works better. It's not as simple as having it 'big enough'; there seems to be a lot of other factors. For instance, I've seen waveguides that seem to 'screw up' the wavefront in the first inch of the waveguide, and directivity along the entire response curve suffers.

In a nutshell, it's not as simple as making it the right size and shape. There appears to be a lot of details that impact the smoothness of the waveguide's directivity, and the overall smoothness of the frequency response.

The pictures and the sonograms above show this. Though the waveguide dimensions and geometry are similar, the sonograms are completely different.

Paul Spencer did a great series of waveguide measurements here:

Red Spade Audio: Waveguide shootout

In my situation, I'm leaning towards using a 'known good' waveguide, instead of building a conical horn. Back in the day when Lambda Acoustics was around, the owner of Lambda made a subtle change to the throat of the Unity horn, and it changed the frequency response of the horn. By quite a bit, iirc.
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