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post #2611 of 2619 Old Yesterday, 04:26 AM
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Quote:
Originally Posted by HopefulFred View Post
Oh, I forgot to say: some of this (even a lot?) may be in fact due to the subs. Look at Mike's graphs. Do they look like chrapladm's?


I don't know what features of the horn or its construction contribute to the variations from predicted behavior - but maybe you have for whatever reason an example that maximises those variations?
Construction of the horn could surely throw the tune off. The measured response of the sub is showing ~6dB between peak and dip (assuming 80Hz xover).. the room is showing ~15dB.

Also a good reason to measure the subs in free air If you're really hard core about getting the room right, drag one of them outside and do a measurement.

Good to have the data going in. Which is why I say the room modes should be calculated What are the exact measurements of the room?

Tim
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post #2612 of 2619 Old Yesterday, 07:00 AM
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Quote:
Originally Posted by Mr.Tim View Post
Good to have the data going in. Which is why I say the room modes should be calculated What are the exact measurements of the room?

Tim
IIRC it's 18'8" by 32'4" by 9'. (Huge) the way I figure, room mode calculations should be verified; that's what I am trying to get J_P_A to do.
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post #2613 of 2619 Old Yesterday, 07:03 AM
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Hey JPA.. Rooms is coming together nicely..

The room simulator in REW 5.0 has always amazed me..

Not necessarily as a "point A" to "point B" map but more as a compass to help get me going in the right direction..

Once you pick the Mic/MLP position, You can easily move subs without back strain to see a generalized result.. and play with absorption coefficients on individual walls..

I think I have your rough room dimensions below... the simulator predicted a dip close to what you are seeing..


Playing with the simulator I think you'll see:

-there are only minor improvements available by moving the subs along the front wall.. (not saying you wont see some improvement).. My guess is since the 2 subs will co-locate.. exhibiting single sub behavior, the changes are minimal.

- as you mentioned earlier increasing the rear wall absorption will bring down the ~ 40 & 70 Hz peaks and bring up the 50Hz dip.

- Option 3 for smoothing, although possibly not viable in your situation, would be a mid wall sub.

Anyway.. the simulator is free and not very taxing from an time/effort standpoint..

And yes.. I did stay in a Holiday Inn recently..
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post #2614 of 2619 Old Yesterday, 07:48 AM - Thread Starter
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WOW!!! You guys have been busy while I was away! I like it

Depending on where/how you measure, the room is about 29.8'x18.4'x8.7' plus another 2.4'x10.6' area at the back. There's the wainscoting, the soffit, the stage, and riser that all affect that.

HF

Sounds like a great idea to me! I have another thread started where I want to track progress as I add treatments (linky), and one thing that I'm trying to decide on there is choosing an array of measurement points. The catch is they need to be repeatable over the several months it will take me to actually add the treatments. In addition to that array of measurements across the seating positions, I'll also plan to walk around the room and try to get an idea of what the modal distribution looks like as you suggested. That's a good idea!

I don't recall reading anything about a recommended sweep duration. I'm guessing adding to the duration will increase the spectral resolution, which we may or may not need, but hadn't considered the impact on measuring ringing. I'd like to get the measurement parameters set early on so that I can repeat my measurements consistently as I add treatments! Any suggestions on where I can look for guidance on that?


Mr. Tim

I was planning to drag one of the subs out and test it in the yard, but after emailing lilMike a bit he didn't think it was necessary. Short of something wrong with my build, he felt pretty good about the measurements that have been made already. Maybe I wimped out, but it sure didn't take much to talk me out of dragging that behemoth out to the yard! If an opportunity presents itself, I may still do it. We'll have to see.

HF/Mr.Tim

Part of the reason I like this forum is all the different viewpoints make me reconsider the things I just accept at the first past. These dips are an example. I immediately jumped to room modes being the cause of it, but after what you guys are saying I think I need to dig into it a bit just to make sure.

KNKKNK

I didn't know REW would do that! You may have mentioned it before, but I've slept since then. That's almost scary how good that is. It's pretty commonly accepted that to get any sort of accurate prediction of the room in the modal region requires some pretty fancy analysis software. For REW to do that out of the box is astonishing. I'm going to have to play with that a bit and see what I come up with. It will be interesting to see how it holds up to testing throughout the room.

Thanks guys. Keep the feedback coming!

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post #2615 of 2619 Old Yesterday, 09:10 AM
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Too much going on here to comment but some good conversation and progress it looks like.

Paul,

I would focus on getting the room near 100% and fully measured, treated, and all that. Then you can tackle the nitty gritty with tuned absorbers and all that.

If you need help with a helmholtz resonator let me know. But my gut feeling is that it's not worth the trouble to go with tuned bass treatments. Some of the horn wonky is just a horn being a horn, some of the other is going to be your room being a room. My gut tells me when you get everything settled in you should be able to treat it in other ways.

I think in this day and age I am of the belief that more functional and variable is an electronic tuned bass treatment system, rather than a physically passive one. (Although I do not have totally sufficient personal experience with this yet to confirm it's true) ^

My thinking is that a tuned bass treatment can work great when (A) you really understand the problem and solution. (B) you build it properly to treat that specifically. There is a journey to get there first, so my advice is basically make sure you arrive before you start with all that. The problem is that if you are off in the build, or the calculation, it won't work as well as intended, and the other side of the problem is it's mostly a one trick pony.

An electronic treatment could be a very modest sealed sub, located properly, with DSP to fix the phase or cancellation or whatever. Yes this is quite challenging too, but you have a lot of options and upgrade paths. You can try many different settings and locations and really settle in on a great solution, and if you ever undo it, or decide to go another route the worst case is you have a couple cheap subwoofers as spare. You can use EQ and DSP to run a sub out of phase as an active bass treatment. While this takes understanding, it's not beyond you at all, and if it does not work you are not fried just yet. You can move it, or try something else, and continue to learn and progress.

MiniDSP board and a ($60) sealed infinity 1260 12" sub in a $20 box is all you need. That is $60 for a woofer, $20 for wood and glue, $80 for DSP board (or get a amp plate with cat5 and remote into that, or just buy a basic DSP enabled amp...etc...etc...) You can get digital DSP amps that do well for this for a cheap price. I would try to get something with network or USB function so you can remote into the DSP from laptop while doing the set up and settings.

You might find a smoothing sub makes a good enough improvement. If it does not, convert it to an active bass treatment system. It can work in both directions.

But - again get all your other treatments and set up stuff situation before you worry about this. These are calibration things you typically should do after your theater is "done"

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post #2616 of 2619 Old Yesterday, 09:22 AM
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And I should mention that my thinking is that $300 or so spent on a very high value sub/DSP to be used as an active bass treatment, or a balancing sub is worth the cost in comparison to the effort to design, and build, test, install- and validate a tuned bass trap. I do not get the sense the budget is a huge problem for you, so the time spent might be a bigger problem? If so, at least you won't waste any efforts if something goes amiss.

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post #2617 of 2619 Unread Today, 06:48 AM - Thread Starter
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Time is definitely a limiting factor. I've got time to do hour projects here and there, but I can't set aside enough time for a major build right now. Heck, right now I'm rushing this post b/c my work is piling up behind me!

At any rate, I like the idea of a smoothing sub, but I'm seriously limited on placement options. I did play around with the REW simulator, and found something quite interesting. It looks like my MLP is much better suited to a front back setup of my subs. Once I've had a chance to do a little measuring, to verify that this is modal issue, I may try to wrestle one of my subs into the back of the room. Getting that thing up and over the wainscoting and then back into that corner is going to be tough to say the least. However, if this simulator is correct, this would be a big improvement on that dip, and it dealing with those peaks is much easier with EQ than dealing with the dips.



My back hurts just thinking about this! It could be worse, though..... I could be trying to build a laminated LVL in-place overhead Know anybody crazy enough to try that, Mfusik?
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post #2618 of 2619 Unread Today, 09:34 AM
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I'm not sure that's too much better. There's a 30dB swing in the modeled response.

I would try flipping or moving L/R one of the subs. Seems to be the path of least resistance. See what happens. Also you could try firing it up or forward. Won't fix a length mode, but may even it out a little better.

Filler subs are great, but you have to be careful mixing designs. IIRC horns may be 180 out of phase with a portion of the response of a sealed sub in a lot of the cases. (edit: mike pointed this out above).

I mixed horns and sealed subs but I used the sealed subs to fill in under 20Hz.

I even out the response of my horns with DSP.

Tim
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post #2619 of 2619 Unread Today, 10:21 AM
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The good news about balancing subs - especially in a case like this where output is not stretched to be adequate - is that they can be band-limited. Phase is pretty stable in the horn across this band, I think, so as long as the other sub's are also phase-stable in this band, there should be no issue.
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