>I want all the juice down to the cutoff flare.. Tractrix can't give me that..
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Sure it can. To load from 80-500Hz, set the flare rate to 80/2^0.5 = 56.57Hz, throat at 500*2^0.5 = 707Hz and run the numbers. A square or rectangular mouth sitting on the floor needs to be at least 80*2 = 160Hz in area for decent performance.
WRT calculating the rear chamber volume, a close first approximation is to design a sealed cab with an Fb = (80*500)^0.5 = 200Hz using a box program.
To keep compression reasonable, a 10-12" low Le driver is preferred, though a 15" will work if not driven at high power, but you'll probably need to use EQ to limit the signal below 80Hz to keep from overdriving it regardless of the size.
The trade-off to doing it this way is that it becomes ~ a hypex, losing the benefits of the low distortion throat and mouth, but technically a tractrix and still a decent overall performer in this BW.
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>Edgars exponential horns have around 520 sq.cm throat, 2900 sq.cm mouth and 55-60cm length.
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This is way too short, with too large a throat to load all the way from 80Hz to 500Hz, though sticking them in a corner will gain you a little extension as well as several dB of gain.
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>I'd like to know how the vertical positioning of the horn throat (relative to the horn mouth) affects performance.
====
I've built horns with three flat sides and all the expansion on one side to keep them simple and get the driver up to ear level/close to the mid/HF horn and they sounded fine. Even if there are subtle differences to the super keen of hearing, the audible benefits of LF/HF spacing and getting the LF up to near/at ear level far outweighs them IMO, but then I prefer a 'front row, center' kind of presentation, so as always, YMMV.
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>As for total length, I think you can get away with 80-85cm.
>1) I want full juice down to the cutoff-freq.....
====
Path length determines how low it will go with any authority, so for 80Hz it will be long enough to have to fold it to meet your max length requirement. An "L" bend is easy and works well for alignment at the XO point.
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>2) I know the mouth area I am likely to use, but how big should the chamber between the driver's cone and the horn throat be? How do I calculate this pressure chamber, and what is the effects of it?
====
I use Marshal Leach's formulas to calc horns, which does a great job, but you can use the guidelines from by Tractrix response for a starting point to fine tune them in Hornresp.
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>I wonder, is purely exponential a good choice for midbass?
====
I prefer expo in this BW. You trade some length for less loading, smoother response, and a much larger mouth for a given Fc.
GM
====
Sure it can. To load from 80-500Hz, set the flare rate to 80/2^0.5 = 56.57Hz, throat at 500*2^0.5 = 707Hz and run the numbers. A square or rectangular mouth sitting on the floor needs to be at least 80*2 = 160Hz in area for decent performance.
WRT calculating the rear chamber volume, a close first approximation is to design a sealed cab with an Fb = (80*500)^0.5 = 200Hz using a box program.
To keep compression reasonable, a 10-12" low Le driver is preferred, though a 15" will work if not driven at high power, but you'll probably need to use EQ to limit the signal below 80Hz to keep from overdriving it regardless of the size.
The trade-off to doing it this way is that it becomes ~ a hypex, losing the benefits of the low distortion throat and mouth, but technically a tractrix and still a decent overall performer in this BW.
====
>Edgars exponential horns have around 520 sq.cm throat, 2900 sq.cm mouth and 55-60cm length.
====
This is way too short, with too large a throat to load all the way from 80Hz to 500Hz, though sticking them in a corner will gain you a little extension as well as several dB of gain.
====
>I'd like to know how the vertical positioning of the horn throat (relative to the horn mouth) affects performance.
====
I've built horns with three flat sides and all the expansion on one side to keep them simple and get the driver up to ear level/close to the mid/HF horn and they sounded fine. Even if there are subtle differences to the super keen of hearing, the audible benefits of LF/HF spacing and getting the LF up to near/at ear level far outweighs them IMO, but then I prefer a 'front row, center' kind of presentation, so as always, YMMV.
====
>As for total length, I think you can get away with 80-85cm.
>1) I want full juice down to the cutoff-freq.....
====
Path length determines how low it will go with any authority, so for 80Hz it will be long enough to have to fold it to meet your max length requirement. An "L" bend is easy and works well for alignment at the XO point.
====
>2) I know the mouth area I am likely to use, but how big should the chamber between the driver's cone and the horn throat be? How do I calculate this pressure chamber, and what is the effects of it?
====
I use Marshal Leach's formulas to calc horns, which does a great job, but you can use the guidelines from by Tractrix response for a starting point to fine tune them in Hornresp.
====
>I wonder, is purely exponential a good choice for midbass?
====
I prefer expo in this BW. You trade some length for less loading, smoother response, and a much larger mouth for a given Fc.
GM
Rocky said:
I wonder, is purely exponential a good choice for midbass? It'll save me 12cm from a hypex one.. What is it's benefits and drawbacks compared to a hypex flare (still talking in the 80Hz-500Hz area...)
Edit; LOL..
Pure expo is what I use 60-150cycles then hand off to a 10" JBL 2123J for the 150 to 1K range. - Expo is lower lower distortion and IMO better bass near the flare frequency, drawback is the horn is longer.
I am still leaning towards the straight horn, but are also somewhat facinated by the LaScala due to it's superb ability to exploit almost every cubic cm of space in the cube... Could one expect to better the LaScala concept by recalculating and building it with curved inner surfaces? Magnetar?
I have revised the project a little. I will not use a basshorn, but instead go for twin JBL D123s (12 inch, 98dB/W/m, 16 ohms), sealed box, combined with the TAD tractrix in a MTM setup (the horn will be built-in to the heavily braced box). The JBLs will get around 45 liter volume each, resulting in an F3 of 75Hz (subwoofers are needed). Twin 12-inchers should also provide a reasonable polar pattern for integration with the compression driver at a low XO (500Hz), and with their 16 ohms impedance, they will provide a very high efficiency (102dB/W/m +) and a steady 8 ohms load, the same impedance rating as my Radian diaphragms.
The setup has no supertweeter included. The TAD will cover "full range" from it's crossover point, and I will probably find it "beaming" in it's upper range from around 8kHz (Fo*24?), resulting in a very small sweet spot at higher frequencies.. I will add supertweets to battle this later on (as an "external add-on component").
Crossovers will be passive, but I plan on active equalizing (I need to.. because of my Radian diaphragms... Rodd had a good tip on using a 4.7uF filter cap though. I'll try that too). Only reasons for passive XOs is ofcourse that I can drive the MTMs with a single stereo 2A3-calibre amp. This passive XOs will probably be subject to revisement in the far future, but they should make a good starting point.
In my case, I think this is close to the best compromise between sound and WAF. No horn in the midbass is ofcourse a big compromise, but I trust the JBLs to do a fairly good job in the 75Hz-500Hz range.
I attached a picture of the design. If you think the TAD is placed too close to the ground, think again. This is not a floorstander (!!!)
No more thinking for a while.. I'll use my spare time on construction instead
The setup has no supertweeter included. The TAD will cover "full range" from it's crossover point, and I will probably find it "beaming" in it's upper range from around 8kHz (Fo*24?), resulting in a very small sweet spot at higher frequencies.. I will add supertweets to battle this later on (as an "external add-on component").
Crossovers will be passive, but I plan on active equalizing (I need to.. because of my Radian diaphragms... Rodd had a good tip on using a 4.7uF filter cap though. I'll try that too). Only reasons for passive XOs is ofcourse that I can drive the MTMs with a single stereo 2A3-calibre amp. This passive XOs will probably be subject to revisement in the far future, but they should make a good starting point.
In my case, I think this is close to the best compromise between sound and WAF. No horn in the midbass is ofcourse a big compromise, but I trust the JBLs to do a fairly good job in the 75Hz-500Hz range.
I attached a picture of the design. If you think the TAD is placed too close to the ground, think again. This is not a floorstander (!!!)
No more thinking for a while.. I'll use my spare time on construction instead
Attachments
No links, just emails, but here's a pic of a guy I met at AA that uses 2 - 2202's per side (JBL 12" double the eff. of the D123).Rocky said:Thanx for the info Rodd. Any link?
I chose the JBL D123 because I needed 16 ohm and high sensitivity.. And ofcourse great sound
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