MLTL problem - behaves like 1/2 wave instead of 1/4 - diyAudio
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Old 8th December 2015, 03:41 AM   #1
JIftw is offline JIftw  Canada
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Default MLTL problem - behaves like 1/2 wave instead of 1/4 [SOLVED]

Hi !

I've build an MLTL prototype and have had unexpected results.
The line is 60 in long, which should, I think, boost around 56 Hz, or lower because of the restriction from the port.
The driver is 8 inch Hi-Vi M8N
The port is 3" diameter and 4.5" long.

I don't know why, but I ended up with something which has no deep bass boost, but a **huge**, overwhelming boost at 100-120 Hz instead.
There was *no* significant output from the port; I was even able to block it, and it made little change to the frequency response...

Is it because of the port orientation ? Must the port be longitudinally aligned with the length of the line or can it be at an angle like in my design ? (Is my enclosure behaving like it is closed at the end ? Like a half-wave resonator instead of quarter-wave ?)

Or is because there are too many bends ? Is the rightmost part acting as a resonator in itself, with 2 open ends (1/4 wave) but at half-length ?

Thanks for your help !
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Last edited by JIftw; 14th December 2015 at 04:06 AM. Reason: Solved
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Old 8th December 2015, 06:11 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

I'd say far too many similar dimensions in the line.

rgds, sreten.
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Old 8th December 2015, 11:59 AM   #3
xrk971 is offline xrk971  United States
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What does a bass reflex sim with same volume and vent say? The MLTL will give you better come control and smoother bass if done right but it will tune similarly to a BR of same volume. You may just have too small a box for that driver and a classic symptom of too small a vented box is higher bass bulge. Also check for air leaks in internal divider. But if drawing is to scale, I think Sreten is right about channel aspect ratio too "square".

If you have ability to do impedance sweep that will tell you problem.
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Old 8th December 2015, 03:30 PM   #4
JIftw is offline JIftw  Canada
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Yes, the drawing is to scale.
BR simulation was good. (I had modeled it after reading the "accidental MLTL technique".) The sim tells me it is tuned to 27.9 Hz.

Also, there was a MLTL sim done here:
Good for transmission line?

The enclosure specs were :
L = 59.1"
CSA = 113.726"^2
driver down 21.33"
vent near/at the bottom
rp = 1.5"
Lp = 4.5"
density = ~0.2lbs/ft^3
(Taken here: Enclosure for Jordan JX150 )


From this design, what I did was round the length to 60", reduce CSA to 88 sq in, reduced driver offset and folded the line. The internal net volume is 86 liters.

Before I put the middle panels, it behaved like a very good BR. Everything was fine. To my knowledge there are no air leaks. I can't do impedance sweep.

Too square / similar dimensions ? I though it was necessary not to reduce CSA too much; and ok to keep CSA constant along the length (no taper). Perhaps I misunderstand you, sorry.

Is it ok to have put the driver in the bend ? Could that be the problem ?

Your help is very much appreciated
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Old 8th December 2015, 04:55 PM   #5
xrk971 is offline xrk971  United States
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To debug try removing as much stuffing as possible and see how that helps. Put stuffing only on section from closed end up to driver. It puzzles me that it sounded good as a BR. So you had a BR and you opened it up to add the internal divider and it sounded worse?
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Old 8th December 2015, 11:44 PM   #6
pkitt is offline pkitt  United States
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Essentially what you built is a vented box with some solid braces; the dimensions of the dividers you used to create the line are too short to actually form a line. You'd be far better off by using a single divider that splits the depth of the box into equal halves, making a single-fold line. The divider could be attached to the top of the box and other end stopping the same distance above the bottom of the box as the depth of the two halves. With line being ~60" long, an optimum location for the woofer's center would be around 12" below the top of the box. An optimum location for the port's center will likely be around 3-4" below the top on the back panel. You need to model the line with a program that takes into account the 1/4-wavelength resonant frequency of the line, which is ~56 Hz as you calculated, and the correct port dimensions for tuning the system lower to optimize it for the woofer's Fs and Qts. The first half (front half) of the line should contain polyester fiber stuffing with a density on the order of 0.5 to 0.75 lb/ft3. Last, if you didn't, you need to model with actual T/S measurements for your woofer, which aren't necessarily the same as those published by the manufacturer.
Paul
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Old 9th December 2015, 02:09 AM   #7
JIftw is offline JIftw  Canada
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xrk971: Removing the stuffing doesn't make much difference other than a muddier midbass/midrange. I have to specify, the 100-120 Hz boost I hear is not accompanied with an early roll-off at lower frequencies, like one would get with an undersized box. There is still output down to the 50s. It really sounds like an 100-120 Hz or so EQ boost... easily +6 dB !
As a BR, with 86 L, there is ample box volume (60 L would be sufficient) and, I assure you, it sounds very good (I've reverted it back to BR). I'm thinking it's either due to the way I folded the line and/or the driver being "in the bend" ?

Paul: Thanks a lot for these recommandations. So I understand port orientation was ok; it's because of how I folded the line - then I am curious, how could a solid brace have such an effect ?
Would you know of a good program for 1/4 sim ? I have read about one (forgot the name, Akabak maybe), I think it's script-based :S
And hearing how much line geometry plays a role, I'd be totally awesome if we could make 3D models of enclosures and have the software compute the response. Does such software exists, at a reasonable price for DIYers ?

Thanks again everyone, much appreciated !
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Old 9th December 2015, 02:38 PM   #8
pkitt is offline pkitt  United States
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I don't know the specific answer to your question. How did you determine the required port dimensions? Were they calculated based on the box starting out as just a ported/BR box before adding the line dividers? If so, the tuning was based solely on the box's volume, whereas in an ML-TL the tuning comes from two contributors, the line's 1/4-wave resonance and the box's (line's) volume. Maybe the line dividers you added created just enough of a (not so good) line that the bump in output around 100 Hz was created by the two tuning contributors?

I'd be happy to model an ML-TL for you that would likely work in your box height of 30" and be a single-fold line like I described. I use Martin King's software. It's also possible that a single-fold, tapered TL would also work well in the same box, and I can model that, too. An important requirement, though, is to have appropriate T/S measurements for your woofer; I can certainly model with the published values but wouldn't have 100% confidence in the outcome. If you can't measure T/S values, maybe some other forum members have and can provide them. With the published values for Qts and Fs, a system tuning frequency of 25-28 Hz would probably result in an optimally flat response from just above the knee in the response curve on up to ~1 kHz.

Quote:
Originally Posted by JIftw View Post
xrk971: Removing the stuffing doesn't make much difference other than a muddier midbass/midrange. I have to specify, the 100-120 Hz boost I hear is not accompanied with an early roll-off at lower frequencies, like one would get with an undersized box. There is still output down to the 50s. It really sounds like an 100-120 Hz or so EQ boost... easily +6 dB !
As a BR, with 86 L, there is ample box volume (60 L would be sufficient) and, I assure you, it sounds very good (I've reverted it back to BR). I'm thinking it's either due to the way I folded the line and/or the driver being "in the bend" ?

Paul: Thanks a lot for these recommandations. So I understand port orientation was ok; it's because of how I folded the line - then I am curious, how could a solid brace have such an effect ?
Would you know of a good program for 1/4 sim ? I have read about one (forgot the name, Akabak maybe), I think it's script-based :S
And hearing how much line geometry plays a role, I'd be totally awesome if we could make 3D models of enclosures and have the software compute the response. Does such software exists, at a reasonable price for DIYers ?

Thanks again everyone, much appreciated !

Last edited by pkitt; 9th December 2015 at 02:47 PM.
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Old 14th December 2015, 04:11 AM   #9
JIftw is offline JIftw  Canada
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Remade the enclosure into a single diviser as you suggested - Problem solved
Thanks a lot !
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