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Old 16th November 2004, 01:57 AM   #1
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Default Tumult Horn or TL?

Currently I have my adire Tumult rocking in a bass reflex enclosure, but I've gotten bored and have decided to do an entirely new project. I am considering either a TL or horn loaded enclosure for my new project.

Project Goals:
Flat response to 10hz in room at the highest SPL possible.

With that in mind I am strapped on the decision between a transmission line and a horn loaded enclosure. From my understanding the horn should give me higher SPL and if designed properly, lower frequency response. I also had an idea for a horn which I havent yet seen on the internet. Give some ideas/reasons to go with one enclosure or the other. Size is really not a factor, though I'd like to stay under 4 4'x8' sheets of MDF due to weight issues and material cost. My horn idea is shown in the attached picture below. Could these loadings give me even lower frequency extension with proper tuning? I know impulse response would be comprimised, but I'm willing to do that.
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File Type: jpg horn loadings.jpg (35.0 KB, 456 views)
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Old 16th November 2004, 11:19 AM   #2
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Neither configuration will work as the acoustic load of the horn will swamp the tuning of the reflex system in a horn long enough to work to the sub bass. Forget flat to 10Hz, that would take a 28 foot horn, and room lift makes it unnecessary anyway. A horn flat to 30-35 Hz or so is plenty, room lift will supply the rest.
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Old 16th November 2004, 06:20 PM   #3
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Room lift can't do miracles, he should aim for 20 Hz, but as you say, it will take a very long horn to be flat.
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Old 16th November 2004, 11:48 PM   #4
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6cubic feet tuned to 10hz gives me a very shallow rolloff of 6db/octave anechoic. 10hz is -12db. How would a big horn compare to that? How would the horn's acoustic load "swamp" out the tuning frequency. I'm talking flat to 10hz in room, not anechoic. Typical horns use a horn loaded sealed enclosure, my design would use a horn loaded bass reflex enclosure. I dont see how the tuning frequency of the box would be changed as the port length area and box volume are not effected by horn loading. I think the horn would just provide higher dampening?
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Old 17th November 2004, 12:21 AM   #5
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[QUOTE] I dont see how the tuning frequency of the box would be changed as the port length area and box volume are not effected by horn loadingQUOTE]

But they are, and radically so. For one thing a horn with enough acoustic impedance to give even a 40Hz fc can lower the effective fs by 1/2, so if you start with a driver fs of 20 Hz it can well be lowered to 10Hz when mated to a horn. If the system f is allowed to remain that low compared to the horn fc the horn will not work properly. This phenomenon is usually addressed by using a sealed rear chamber, which if made small enough can raise the system f back to where it will function properly. You can successfully horn load a reflex box as you depict it with a horn with an Fc as low as 80 Hz or so; I first did so in 1997 with my Snail; James Novak first did it in 1953, and someone probably did so before him. But to my knowledge no one has successfully done so with a horn that operates to the low bass, myself included.
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Old 17th November 2004, 12:44 AM   #6
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Despite no one having done it before, do you think it could be done?
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Old 17th November 2004, 12:55 AM   #7
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No. If I did I would have already done so. When I prototyped my Tuba 24 I tried it, since it does work in my DR horns (that go to 80Hz fc). It took me a while to figure out why it didn't work. Even with my DRs I found that with an fc below 100 Hz you can't put the vent at the horn throat, as the mass of the air column in the horn dominates the system, and the vents have to exit into the horn at least 1/2 way down the pathway. With a large bass horn the air column mass is so great that using a vent to tune the system lower than a sealed rear chamber is counter-productive; if anything raising the system resonance is required.
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Old 17th November 2004, 01:53 AM   #8
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Could you use a rear vent instead of your front firing vent?
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Old 17th November 2004, 04:49 PM   #9
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Good idea Simon, but who knows what effect that may have on the horn's response, except maybe Bill... Have you tried this before Bill? I do see how the horn could add apparent length onto the port. I suppose there arent a whole lot of standard equations and/or modeling software for such a situation to play around with. So basically if I did have a port exit into the compression chamber or throat it would effectively lower the tuning frequency by some unknown amount? If the horn were loaded with a reflex port that exited outside the horn and not into the horn I'd imagine the port tuning wouldnt be effected, but since the horn changes the response of the cone of the driver it would also change the output of the port.
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Old 17th November 2004, 05:23 PM   #10
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You can exit the port outside the horn but if the horn is making the driver run at 1/2 its normal fs anyway then the port is moot. A ported rear chamber is fine on a short horn to extend bass response below fc, as in an A7 for instance, or one of my DRs, but when the horn is sufficiently large, as is the case in any true bass horn, the system f is too low for the port to be of benefit and a sealed rear chamber is the better way to go.

To see how it models imagine your rear chamber is 2 cuft, and the horn loading has lowered the system f to 10 Hz. Now try modeling your driver in a 2 cu ft enclosure tuned to 10 Hz and then try to figure out where to put all that duct length.
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