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Old 21st October 2011, 07:45 AM   #1
Abaoaqu is offline Abaoaqu  Italy
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Default Jordan jx92s GM MLTL-48 boomy bass

I recently built a pair of MLTL enclosures for Jordan JX92S (MLTL-48 by Greg Monfort).
First listening session was disappointing: the speakers produced a boomy one note bass!
Since the project was realized by many DIYers and all their listeng impression were very good I thought I made a big mistake building the cabinets.
But I triple checked inner dimension, port etc.: it's all as in the project. The speakers are driven by a high damping factor SS amplifier in a room of about 7m x 4m x 3.30m with large and irregular openings to other rooms.
I tried to increase stuffing (density and extension) end port lenght but the difference is very little.
I haven't the gear nor experience for accurate measures but I tried to drive the system with a wave generator to see what happens.
So I noticed a very hight (and narrow) peak in SPL at about 55Hz, a second peak at about 80Hz (with a valley in between). At higher frequencies the response seems more regular. To have the speakers listenable I used FeedBack Destroyer function in a Behringer DEQ2496 creating a sharp noch at 55Hz. But, as you can imagine, I am not very satisfied
What is the problem, in you opinion? Room modes? May T/S parameters of drivers be so different from specs that the enclosure is misaligned?
Any help will be appreciated!!!
Davide
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Old 21st October 2011, 09:04 AM   #2
Colin is offline Colin  United Kingdom
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I found the 48 MLTLs sensitive to positioning. Because they are strong in the bass, getting them too close to corners made them boom (the 31s are easier to place in this respect). Two things to try:

1. Move them to another room or at least try a radically different position in your main room. See how they sound. If they're ok, it's the room.

2. Block off the port entirely. (A piece of card sealed over the port with masking tape will do for now.) That will cut off most of the output below 70Hz. How do they sound now? You can try introducing resistance into the port by putting a foam bung (or a clean sock) into it. This may help reduce the bass.

When I first used mine, I didn't use any BSC circuit. Adding one helped balance the sound but it was the mid and HF that was reduced, so in your case may not be useful. Worth a try after the other options, though.

Hopefully, GM will pop in here with some suggestions.
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Old 21st October 2011, 09:43 AM   #3
Abaoaqu is offline Abaoaqu  Italy
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Quote:
Originally Posted by Colin View Post

1. Move them to another room or at least try a radically different position in your main room. See how they sound. If they're ok, it's the room.

2. Block off the port entirely.
1. Sure, i will try!

2. Yes. I tried to fill the port with polifyll: less boom but bass has gone ... and it seems to me that cone excursion is larger.
Maybe I have a chance to have speakers and drivers measured by a professional audio: i will post the results.

Thank you Colin!
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Old 21st October 2011, 01:18 PM   #4
AntM is offline AntM  United Kingdom
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I have recently completed the 31" version http://documents.jordan-usa.com/Encl...31-MLTL-MD.pdf

so I'm interested to hear of you issues.

These kind of designs do take some playing around to get right and can be made worse by increased interaction with room acoustics due to the very low frequency rolloff.

Here are some things that I would try:
- double check that there is no leakage from any of the joints. Even minor leaks from e.g. where the back panel is screwed in or the driver is mounted can change the tuning, affect bass efficiency as well as introducing boominess
- as mentioned by Colin, try different room position or even in free field conditions if you have sympathetic neighbours
- experiment with different stuffing density and volume. In the 31" design, I used long-haired wool in the top part of the cabinet. I found that teasing out the wool to reduce its density made quite a difference. Also completely filling the top part of the cabinet tended to make the bass more controlled. This tends to be quite a heuristic thing: you just need to experiment to get a sound quality you like.
- experiment with different port lengths. Depending on your design, you may be able to lengthen the port slightly by cutting a short section of tube and temporarily sticking it the outside of the box to lengthen the existing tube. One thing I have done quite successfully in the past is to create a telescopic tube by pushing a slightly smaller diameter tube inside the existing port. You can then set the length just by sliding it in and out. You may want to make the original tube slightly shorter to allow the length to be tuned over a wide enough range

There is still a lot of experimentation needed with my current design. The issue I have currently is that although I can get the bass to be reasonably flat, it is slightly "indistinct" which I haven't yet been able to connect to something more scientific. On the other hand, the timing is quite good and the soundstage fantastic, so the design is very listenable.
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Old 21st October 2011, 01:55 PM   #5
Colin is offline Colin  United Kingdom
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OK, if you block the port then yes, the cone excursion will go up. The port not only allows LF extension, it decreases cone excursion - exactly the same as a reflex port.

I'm running the 31 design now, in part because it suits my room better (and I wanted to try it out). It's easier to place for the reasons the OP describes.

The MLTL48 does benefit from BSC compensation. It might bring everything back into balance - certainly worth a try. As Ant says, long haired wool might be worth a go - it is better suited to LF attenuation than BAF or Dacron fibre (which works better at mid). My MLTL48 cabinets produced notes below 30Hz, so there is a lot going on down there.

I used BSC on the MLTL31 but found I was gradually reducing it to nothing. I don't think it needs it - in my room anyway. It is used close to the walls so BSC simply made the bass sound too loud.
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Old 21st October 2011, 03:47 PM   #6
AntM is offline AntM  United Kingdom
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Colin, out of interest, apart from the BSC, did you follow the MLTM 31 design exactly? I'm wondering in particular if you added extra damping in the bottom 2/3 of the cabinet, or if you left it "dry". On mine I created two shelf braces to give me options of stuffing the top part, middle part and/or bottom. At the moment, there is little wall-vibrational damping except for at the top 1/3 , using Wilmslow's acoustic foam laminate panel Acoustic Foam Laminate Panel
Wondering if you also experienced slightly "indistinct" bass? My BSC is currently 1mH and 4.7 ohm. This gave excessive bass, so perhaps I over-compensated by over-stuffing the top 1/3. On the other-hand, the frequency range affected by the stuffing and the BSC will be different, with the BSC cutting in at a higher frequency.
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Old 21st October 2011, 05:26 PM   #7
Colin is offline Colin  United Kingdom
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My MLTL followed the published plans except for the port. I extended the sides to make cabinet legs, then had the port exiting flush with the bottom, inside of the cabinet. So there is a dead space below the cabinet which just contains the 5" length of port. The only other difference is that I inserted six horizontal dowels, three bracing front and back panels, three the sides, and doubled the thickness of the top panel. The double thickness makes a difference but I can't tell if the dowel bracing does. I suspect I should have tied them in pairs (side-side and front-back) to be properly effective. The cabinet is 18mm MDF.

Now, when I was using the BSC I found the bass too thuddy. It took a while to realise this as I have the speaker running from a Teac receiver which has (shock horror) tone controls. They're only accessible via the remote, so I hadn't spotted that the bass was turned down. When it was level, the bass was too much - not low bass. I reckon it was around 100Hz (ish). I was using the same BSC values as you, so experimented with lowering it to 2 ohms, which was better, then took it out altogether. That's works best for me.

I corresponded with GM and he agreed that BSC shouldn't be necessary with this design (the 31) as it sims flat. Certainly near a wall (it's a couple of inches away) it doesn't need it.

Prior to removing the BSC, I was thinking about stuffing the lower part of the cabinet. As it is, I have some of Wilmslow's finest BAF wadding in the top and that's it. FWIW, I've used their acoustic foam laminate panels in another cabinet and didn't think it did much.

Overall, I really like the design. In some aspects it comes close to a much more expensive, commercial active speaker I've been using. But I also liked the 48 when I had it (my OH still pines for it). I only got rid of that because the woodwork wasn't very gorgeous.
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Old 21st October 2011, 06:03 PM   #8
GM is offline GM  United States
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This alignment is tuned to around 35 Hz, so if you’re using a new generation ‘S’ with a higher Fs in the 50s and lower Vas than it was designed for, then it should be ‘one note’ around 35 Hz before vent damping and ignoring any room eigenmodes, should have a ~smooth rising response to above 100 Hz with no damping; so there seems to be an air leak that’s allowing the driver’s Fs to ‘ring’ and one reason why changing the vent tuning has basically no impact on its performance.

Peaks/dips in the ~65-80 Hz BW are almost always room related unless the cab is tuned in this region, making cabs tuned down where the room typically completely dominates much harder to position. That said, in such a high aspect ratio pipe it will have a different resonance structure with an air leak, so not sure if the ~80 Hz peak is pipe and/or room related. If damping quells it, then it’s the room.

The lone pair of 48s I auditioned had to be placed in a large, open doorway between two rooms and heavily toe’d in before the bass became ‘clean’/’tight’ and tonally balanced over ~all their usable BW.

Anyway, once you get them ‘ringing’ at ~35 Hz, adding series resistance will ‘fill up’ the oversize pipe [a cheap ~25 ohm pot to dial it in, then measure it to find a suitable non-inductive power resistor or multiples of smaller ones to get the desired value], then experiment with different by-pass caps to get the desired HF tonal balance. If the resistance required is high, draining off too much amp power, then use a proper BSC ckt. with inductor.

Note that with SS amps I typically pre-load them to at least 1 W with a bit of series resistance to ensure its stability at low power.

WRT damping, my experience is lining walls/braces/rear of driver with R-19 ‘faceless’ [no paper moisture barrier] and 1” acoustic fiberglass insulation depending on cab net Vb and alignment. Any actual stuffing is limited to IB/AS alignments and the closed end of TLs where damping has the most impact over the widest BW. I’ve zero experience using polyfil, lamb’s wool, etc., beyond many folks commenting that my recommended stuffing density based on my perception of the ‘tone’ of fiberglass damping was way too much using MJK’s software [polyfil], with them removing as much as 2/3 of it. Since my simmed damping recommendations were typically as little as ~1/3 of some others well reviewed designs, I’m at a loss to explain the disparity.

All that said, I’ve repeatedly noted that I consider the 48 a too high aspect ratio pipe even after finally getting to audition a pair and the ‘new’ driver’s specs makes it even less optimum, so if your hearing perception is more like mine than others who love it, you may not be able to reach a high enough SQ in-room performance.

GM
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Old 22nd October 2011, 03:22 PM   #9
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Quote:
Originally Posted by Abaoaqu View Post
So I noticed a very hight (and narrow) peak in SPL at about 55Hz, a second peak at about 80Hz (with a valley in between).
I suspect room modes. What are the dimensions of your listening room?

I stuck my Jx92s's in sealed 1/4 ft^3 boxes and added a sub and active crossover and EQ. No regrets. The small drivers definitely benefit from being relieved of duty below the 80 Hz crossover point. I built an extra sub box and I have the passive radiator. One of these days I am going to get around to adding the second sub. Then a few years of experimentation with placement, and I should have 'er licked.
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Last edited by Dave Jones; 22nd October 2011 at 03:27 PM.
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Old 24th October 2011, 02:12 PM   #10
Abaoaqu is offline Abaoaqu  Italy
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Quote:
Originally Posted by Dave Jones View Post
I suspect room modes. What are the dimensions of your listening room?

About 7m x 4m x 3.30m with large and irregular openings to other rooms.
I know ther are on line calculators but i didn't understand too much

I stuck my Jx92s's in sealed 1/4 ft^3 boxes and added a sub and active crossover and EQ. No regrets. The small drivers definitely benefit from being relieved of duty below the 80 Hz crossover point. I built an extra sub box and I have the passive radiator. One of these days I am going to get around to adding the second sub. Then a few years of experimentation with placement, and I should have 'er licked.
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