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Old 13th June 2005, 10:07 AM   #1
maxro is offline maxro  Canada
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Default MLTL driver/port placement, floor reinforcement etc...

Okay, first post, here goes:

I'm planing a MLTL using the Jordan jx92s and have many questions. I would like something a little shorter than GM's 48" design but still want the driver at around ear level when seated (36"). Maybe around 40"H x 10"W x 6"D (internal) with a 2" dia port 2.5- 4" long depending.

After playing around with MJK's worksheets, I've figured out the general gist of driver and port offset values to best squelch unwanted overtones. Now, I noticed that the driver and the port are interchangeable with respect to distance from the ends of the box. For example, driver down 8" from top and port 3" up from bottom on 42" long, 60sq" box graphs the same as driver 3" down, port 8"up. (playing with the box length and port length will keep the driver at ear level and the graphs similar.) So, would not the lower port gain more floor boost, thus disrupting an optimally flat graph? I've noticed that most designs are using ports at or near the floor. Many fire straight at the floor. Is floor boost integral to their design calculations? How does carpet vs. hardwood affect the bass absorption/reflection? It seems to me that the higher port option would reduce this variable and realise in-room closer to mathcad models.

The width of the box is 1/4 the length. Am I going to have any surprise port output from standing waves along the width, not accounted for in the MJK ported box worksheet?

ps. Has anyone tested the T/S parameters for the jx92s? I'm working off Jordan's specs as I have yet to order the drivers (and wouldn't have the means of measuring them even if they were in front of me).

I found one website listing different figures here:

http://www.spectrumaudio.de/

But their Qm and Qe values don't even calculate properly to the Qt they provide. Looks like rounding error, or something.

More questions to come...

Max
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Old 13th June 2005, 12:05 PM   #2
MJK is offline MJK  United States
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Quote:
Now, I noticed that the driver and the port are interchangeable with respect to distance from the ends of the box.
You are correct, the currently available versions of the worksheet do not take into account the driver in the room or the baffle step loss at lower frequencies. So you have to use some judgement in assessing the actual geometry used in your design and how the floor, the front baffle size and shape, and the distance between driver and port impact the idealized response.

The assumptions in my worksheets are consistent with most other freeware modeling tools. I do have newer versions of the worksheets, that include all of these effects, that I am currently using but have not decided how or even if I am going to make them available.
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Old 13th June 2005, 09:51 PM   #3
maxro is offline maxro  Canada
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Thanks for the reply Martin, and also thank you very much for the worksheets.
Anyone else care to chime in?

Max
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Old 14th June 2005, 03:27 AM   #4
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My own experience gives me a preference for the port at
the floor, firing toward the rear wall. It seems to load well,
you hear fewer of the higher frequency artifacts, and it
maximizes the apparent external distance between the port
and the driver.

As an alternative, I've also had good luck with a QWTP (with no
bottom) firing down into thick carpet. You can alter the port
dimension by raising and lowering the enclosure.
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Old 14th June 2005, 04:03 AM   #5
maxro is offline maxro  Canada
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Quote:
Originally posted by Nelson Pass
My own experience gives me a preference for the port at
the floor, firing toward the rear wall. It seems to load well,
you hear fewer of the higher frequency artifacts, and it
maximizes the apparent external distance between the port
and the driver.
With the proper port/driver offsets and stuffing, it seems that there won't be too much high frequency output from the port. Unless there are problems over 1kHz, which is where the MJK worksheets cutoff, so I can't predict this.

Why would you want to maximise the distance between the driver and port? Multiple drivers, according to common practice, should be as close as possible to each other (point source, line source). So, why not driver and port? What about phase issues of a rear firing port?

Max
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Old 20th July 2005, 11:21 PM   #6
gamma is offline gamma  Sweden
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Default baffle design idea

I m read some threads about baffle step in this and in other audio forum. This is new staff for me..

Anyway, for two weeks ago, I loaded down a "baffle step"-simulation program - "edge".

I get the best result with this baffle shape you can see at the picture. I made the picture in paint so its not perfect...

The intention is to build an ordinary box and later add "the rest" off the baffle. What do you think about this idea?
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File Type: png baffel3.png (2.1 KB, 596 views)
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Old 21st July 2005, 12:17 AM   #7
gamma is offline gamma  Sweden
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Sorry..i forgot the picture
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File Type: png baffel4.png (2.0 KB, 598 views)
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Old 21st July 2005, 12:24 AM   #8
GM is offline GM  United States
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Greets!

Don't see any pic, but WRT your proposed design, Altec, etc., thought it was a pretty good idea, adding 'wings' to their theater basshorns to get the desired BSC........ http://www.lansingheritage.org/html/...s/lf-horns.htm

GM
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Old 21st July 2005, 12:43 AM   #9
Dumbass is offline Dumbass  British Antarctic Territory
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Why not just build GM's MLTL-31 design?
http://www.ejjordan.co.uk/diy/

As long as you keep internal dimensions the same, you can custom-design the cabinet for your desired driver height.

You could even put the thing on stilts, like in this concept drawing:
http://www.stat.ucla.edu/~erickson/jpgs/trapmltl.jpg

Building a wide-and-shallow enclosure, and adding wings, are decent ideas IMO. Since you can never entirely predict the response of the speakers in-room, however, the process is more trial-and-error than science.
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Old 21st July 2005, 04:25 AM   #10
gamma is offline gamma  Sweden
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"Building a wide-and-shallow enclosure, and adding wings, are decent ideas IMO. Since you can never entirely predict the response of the speakers in-room, however, the process is more trial-and-error than science."

Ok... otherwise I got a REALY good result when the shape of the baffle come close to samething that looks like a logarithmic spiral...well I have to find out an "easy modified" material for the "add-on-baffle"
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