2 hz tuning - pros and cons - Page 2

I think Mr. Data Bass beat the hell out of his. He posted the results.

I did model this is WinISD, as I said it doesn't look as pretty but I'm not sure why (I listed 3 possible reasons but not sure which if any are correct).

I can model this in Hornresp and Akabak and it will look slightly different than both MJK's mathcad and WinISD, but Hornresp and Akabak will agree closely with each other.

MJK's mathcad is the only one I can model stuffing with (Akabak will do it but I haven't bothered to figure out how to make Akabak do it yet). Also, afaik, MJK's mathcad is the most accurate model available for this type of thing.

If there's something specific you want to see let me know exactly what it is, I'm not going to simulate the same thing with several programs just to see if they agree, and then post all the results. I've already done it in 2 different programs but I'll show you whatever you like, assuming I have the program you want to see it done with.

I can also compare to other designs (already done that in post #1 where the blue line is IB response). But again, I already know what a variety of other designs are going to look like (for example 5 hz tuning, 10 hz tuning, 15 hz tuning, 20 hz tuning all in ported boxes, all those tunings also simulated in a reverse taper tl and some quick flh and tapped horn simulations too, as well as a variety of sealed box sizes. All with this driver. So let me know what you want to see, but if you are just suggesting I do it for my own benefit, I already did it.
Edited by diy speaker guy - 12/9/12 at 6:47pm

Again, this conversation has be completely confused. Can you (or Mark, or anyone) explain how MJK's transmission line mathcad worksheet can't simulate long ports? From every account I've seen this software is amazingly accurate in all respects that people actually measure. I'll admit it's hard to measure port velocity at home but I've never once (in all the years I've been reading about simulation software) heard anyone claim that this software (or even any of the less softwares available) is this far off in predicting port velocity.

You guys seem to be getting all caught up in the fact that the port is long and narrow but this is EXACTLY the reason MJK made this software - to accurately simulate different shapes and sizes. (And there was Akabak 20 years ago.) Unless I'm missing something (and I freely admit I might be) this is just a math equation and the simulation software I used adresses all the important aspects. It seems too good to be true (I was surprised to see this myself) until you think about it for awhile. As you tune lower, you can get by with an increasingly small diameter port and still maintain the same port velocity, I hope we all realize this. If you don't believe that you can model it yourself in your favorite simulator, I can clearly see it every time I tune a design lower and lower. By the time you get down to 2 hz you can get by with a very small diameter port with acceptable velocity, I'm not sure why (or at what point) you guys think the model breaks down, but I can't see any reason why it would not be accurate at all frequencies.

Yes, thank you, everyone has made that incredibly clear. But can I ask you something? Did you guys give the guy with the fan sub (member here) this much negative feedback on his decision to have a (very expensive) device for just this purpose? Did you give Imagic (member here - Dub King) this much hassle about making music that plays this low? Who harasses the guy that makes the LF content movie lists and posts spectrograms? If I recall correctly, these were all very cool things and universally and wholeheartedly embraced on avsforum until the exact moment that I posted this thread. WHAT IS GOING ON HERE? I realize Krypto took a bashing for doing what he did, but what he was doing was not really practical by anyone's standards. And even so I don't think anyone could argue that he probably would have been an interesting guy to know and despite the overzealous nature of his projects I'm sure they would have been incredibly cool to experience in person.

This is an intellectual excercise, a conversation about simulation software and the validity of it's predicted results and one design in particular. I may not ever build anything even similar to this but I find the exploration of the idea INCREDIBLY interesting. For the sake of arguement let's say that if I did build this thing I would make 2 of them (4 drivers total) and I would put them directly behind my couch, about 12 inches away from my head. In this situation there is no need for crazy high output above 16 hz but it would be SUPER COOL if I could get down to at least 5 hz flat with a single Behringer EP4000. My simulation shows it is more than just possible, it is now affordable even for me. Why would I not want to do that? (Please don't answer that - this is not supposed to be a conversation about tuning preferences.)

This is very much similar to asking about a specific tractor schmatic and being told tractors are stupid, cars is where it's at.

4000 you say? I say they could probably handle 6000 before they failed. But that's all irrelevant unless you measured compression. The ratio of power to compression is important, power handling by itself a useless specification. That's what I'm asking, not because I want to know how much they can take before they burn up. I realize this info is on the databass site but it's a different application and I don't really want to extrapolate without further information. Besides, I'm not really comfortable feeding any single driver 4000 watts and can't afford to do that for several individual sealed subs.
Edited by diy speaker guy - 12/9/12 at 8:05pm

Yeah, see, I agree with you completely that ports can only handle so much velocity. But PLEASE look at the velocity graph in post #1, shown at 1000 watts. You can clearly see port velocity is below 10 m/s until below 5 hz. And please explain why this is not accurate. Several people now have said it's not accurate but no one can give a reason why this might be so. This has never ever been questioned before as far as I know. Why is it different now? Port velocity is port velocity, the model doesn't break down. If you think it does, please explain why.

My take is what I see in the simulation software. The software I used and WinISD show dramatically different velocity but in WinISD the peaks are very narrow, like 1 pixel wide, and peaks like that are normally absorbed and suppressed by internal losses. (Extremely high q peaks are never what they seem since there are no losses in simple software.)

MJK's software is a more complex model, it can simulate stuffing (there's .25 lb/ft in the first half of the "line" in my simulation) and that will greatly suppress the peaks as well. This software might even account for some internal losses, I'm not sure and it's not really important.

But I will say this, in WinISD it's not just the higher frequency peaks that are huge (but only 1 pixel wide), it's also the bump down by tuning that's much much larger, but since it's so low in frequency (below 2 hz) that hardly matters.

Anyway, I'll find (or redo) the WinISD model and post it up. The trends in both softwares are the same but there are differences.

Aside from the simulations, my intuition says velocity is a simple math equation and MJK's mathcad sheet takes everything important into account. But with no experience in this area I decided to ask avsforum what it thought.

Yes of course that's ok, this is why I posted this in the first place. What do you want to know?

I'm not sure that's exactly the same thing but way better than close enough for these purposes. This is dual SI HT 18 in 700 liters (probably a bit bigger than the first sim - I didn't do the math) with 1000 watts, 1.8 hz tuning done in WinISD with port resonances turned on. Like I said, all the trends are the same in general but there are some differences, mainly the q of the port velocity and excursion. The port is 1.85 inch diameter (a bit smaller than the 2 inch port I posted in the other simulation) and it's 88.72 inches long.

The difference in excursion might be due to MJK using RMS deflection, I'm not sure and it doesn't really matter.

(Please excuse the big blank spot at the top, my MS Paint skills are not what they should be. In other words I was too lazy to fix it.)

Is there anything else anyone would like to see? Shall we go through each program one by one to make sure they all agree? I'd prefer to skip that part. I hope we can agree that the simulators show no port velocity problems until you get below 3 hz.

So the trends are the same even if the model is a bit different. We can accept the models or question them, either is fine but if we question them we need to give valid reasons.

And don't forget, nobody ever said this isn't going to chuff like a *__________* but that will only happen below 3 hz (and maybe even 2 hz if you use a good roundover), so low in frequency it doesn't matter, which in part is the beauty of this design routine. That in conjunction with the ultra low tuning is what lets you get by with a tiny diameter port - on paper at least, as I've shown.
Edited by diy speaker guy - 12/9/12 at 9:16pm

Well, I waited with my breath baited (whatever that means) for Mark's reply but he's either busy, gone to bed, or completely bored of this already.

In anticipation of his response, the only thing I can possibly imagine he is talking about is "skin effect" or whatever it's called, where the port walls provide friction so the air right up against the wall doesn't move much, effectively decreasing the apparent diameter of the port at higher volumes. So I'll assume that's what he's going to say and respond to that since I have to go to bed at some point. (Same reason you don't want to have super high aspect ratio slot ports, like for example 1/4 inch tall by 60 inches wide port mouth is bad even if it's an oversized port.)

So if that's what you are talking about Mark, I agree that that is an issue but not one that I have not considered. When you start out at 10 m/s you have a bit of room to play with and I don't see this effect completely blocking the port at any velocity the driver will be able to provide. Again, the main point of this design is that all this stuff happens so low in frequency that it doesn't matter. There is no 2 hz content and even if there was there's no amp that's going to be able to produce it with any authority. (Ok, as pointed out there might be one or two but you'd have to look really hard to find one.) So you can basically cut the graph off below 3 hz (maybe even 5 hz) and not even look at that part of it because it makes no difference whatsoever. No content = no problem.

Above 5 hz there's no velocity problem, it's 10 m/s second at worst and that leaves a lot of room for whatever is not included in the simulation math as far as "skin effect", "surface friction" and whatever else you (and the physics of reality) want to throw in there.

But I still want to see your opinion on all this.
Edited by diy speaker guy - 12/9/12 at 10:52pm

Neither of my simulations show anything close to that, both of mine are around 30 m/s and there's not really any content at 3 hz anyway. What software are you using? Are you using manufacturers t/s or DataBass measured t/s?

EDIT - I see the problem. Add 20 inches to the port length to match my simulations and you should see the same port velocity I show in my graphs. Why did you change port length in your simulation? Changing the tuning creates a whole new situation - the exact situation I was trying to avoid by tuning low to push the problems right out of the usable bandwidth.

I think you are talking about sensitvity, which clearly does not show a huge advantage. But xmax limited max spl is a huge advantage according to the simulations in post 1.
This is good to know, I didn't know that, but the only graphs I'm looking at are the long term max spl graphs, so no need to add 6db to compare to my simulations shown at 1000 watts. Of course this isn't fair because there's no compression in my simulations so max spl looks unrealistically high but there's not much I can do about that. It's also not fair that the WinISD sim is shown way past xmax for reasons stated earlier but for the sims I matched input power, not excursion.

Anyway, I'm not trying to take down the Ghallerhorn in any way, it was just something to compare to. I'll be the first to admit that comparing measured results to a simulation is unrealistic and unfair, especially as shown (past xmax).
Edited by diy speaker guy - 12/10/12 at 12:39am

I editted my post, you probably missed it.

Those are not the parameters I used, I used the manufacturer's published specs. It appears you might have used the DataBass measured t/s but you left Le at zero. (I admit, I used the specs that I did because I liked them better and this is just an intellectual excercise anyway. No way to tell which is going to most accurately reflect the finished product anyway.)

But the 20 inch shorter port in your simulation makes a big difference. You need to change that, but as it is I agree with you the velocity looks bad in your sim.

I was originally simulating a 200 inch port (same diameter) and tuning down nearer to 1 hz but I backed off since there does seem to be a point of diminishing returns. But you can't go shorter than 90 inches or all the advantages disappear, the problems get up into the bandwidth where the equipment non linearity won't hide them and the whole design routine doesn't make sense anymore.
Edited by diy speaker guy - 12/10/12 at 12:58am

Ok, so I poked around a bit at the link you provided. It's not the first time I've been there but it is the first time I've downloaded the Flare It program.

So here's what I found, a direct quote:

And here's a picture of the Flare It program. This pic is telling me core velocity is fine at 10 m/s below 20 hz. I don't know how to change the graph to show 2 hz, but it appears the red line probably extends horizontally below 20 hz, in other words my core velocity is fine above 5 hz in my design.



This is my first time using this program and I didn't read the instructions but it seems pretty simple, the only thing that affects core velocity is port diameter, which seems fine at 2 inches.

Your quote above says something very different, so either I'm using the program wrong or misreading the results or you are incorrect about acceptable core velocity.

I probably should have thought to do this simulation in the first place, but now that I have it seems to support my POV and not yours.
Edited by diy speaker guy - 12/10/12 at 2:35am

Yeah, I had to change WinISD tuning to 1.8 hz to match the other simulation. But my WinISD shows 33 m/s at 3 hz, so at least we are in the same ballpark now. I'm using the new WinISD.

How do you know that's the proper thing to do? It looks like a horizontal line below 20 hz to me. In fact in the text in the green box below the graph it says so. Not directly I guess, but that's how I read it.

How do you extend the chart to see more of it?
Edited by diy speaker guy - 12/10/12 at 2:40am

See the blue line in the same graph? It's leveling out to flat as frequency decreases. I'm assuming once it levels off flat it also will stay flat down to DC.

I have no idea what that means but I do agree it does sound interesting, all of this is very very interesting. In fact it keeps me up at night sometimes.

But I really have to go to sleep. Hopefully we can continue this tomorrow.

Thanks for discussing the actual details of this weird design. This is by far the most stimulating and on topic part of the conversation so far.

Yeah, John and Mark said the same thing (at least I think they did, who is John?). But the software clearly says otherwise so I guess there's no point in going any further with this conversation. No one can give any reason why the software might be wrong (I'm talking about both the acoustic simulators AND the Flare It program.

I'm not educated in fluid (or gas) flow but I do know there are similarities and I do know that pipelines carrying water and oil have been and are being built that are several thousand miles long. If what you guys were saying was even reasonably true, these pipelines would have to be almost infinitely large to be effective at all.

I expected more from avsforum than a bunch of guys dropping by to drop the "you are wrong" bomb without providing any proof at all besides what they think might happen in a situation they have no practical experience with. To be fair I have no experience either but I do have the common sense to trust software that is much better at predicting this stuff than anyone who has commented here.

Mark has not posted here again, I have to assume he has no rebuttal to the Flare It program output, so unless ANYONE can give ANY reason the software might be incorrect I have to assume I am correct in trusting it but there's not much point in discussing it further I guess.

Thank you for this.

In this case the primary motivation was simply to model this driver to see what it takes to extract it's full potential within the rated power handling spec. It takes a very low tuning to do this. Like I said, I'm not married to the 2 hz idea and probably won't build this anyway but I'm still interested in the concept. And the concept is sound, as I've shown here. I'm satisfied with that and I don't really care that I haven't convinced anyone. It wasn't hard to do though, since the only arguement put forth is that all the software is all wrong.

The point of this concept was to use only the top half of the port resonance, the other half (the problematic half) was ALWAYS supposed to be buried in signal chain rolloff ON PURPOSE. This idea was not ever about playing 2 hz tones at full volume, or ever even "hearing" 2 hz at all. It's not meant to be providing effective output until around 5 hz or higher, where there is actually some content in some material. The concept uses a very small port but buries the consequences of such so low that it doesn't matter. It won't chuff because it won't ever see the frequencies that might cause chuffing.

So I'll leave you guys (and this topic) alone now. Thanks anyway.
Edited by diy speaker guy - 12/10/12 at 12:27pm