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Why I Rejected Ported/Passive

11K views 74 replies 15 participants last post by  davepete 
#1 ·
Sorry for hijacking Ilkka's thread. terry j was asking why I rejected a ported design for a sealed design. The short answer is my tuning frequency is too low. I've copied the hijack posts here so the discussion can continue as necessary. I'm in italics. terry j in between.

I rejected going LLT in favor of sealed, with IB not an option. For my design goals, namely flat in-room response to single-digits, the ported and passive radiator models didn't turn out very well. Particularly because of the port volume needed to be so large coupled with the rapidly increasing group delay. It would have been extremely difficult to build an enclosure with a port that was larger than the enclosure, so a passive radiator would be more feasible. But the group delay gets even worse with a PR. And it would be harder to find a PR that would provide the necessary mass to fit by itself on a sonotube endcap; I originally resigned myself to a box enclosure, but the calculated weight would have been like 200 pounds per unit, or something like that, for a PR design.

The trade off is I need to pump in much more power and the overall cost is increased, but I am going with sealed sonotubes now.


Joshua, have you written up your sub anyplace, if so I'd love to be able to have a look. I will readily admit that I'm not technically 'savvy' enough to really follow your reasoning above, but can't you use eq on the LLT to get the same in room response as a sealed?? Presumably you came to your conclusions via a modelling program, which one did you use? Could your decision have been different if you'd modelled with different drivers?

I'm still in the processing of building the subs (someone tell me why it is so difficult to get an endcap to fit into a sonotube?!) and plan to write things up and post photos when it's done. Probably next weekend.

I use WinISD Pro (alpha) to do my modeling. I wish there was a decent Mac OS X program to do it, but doesn't seem to exist. My overall conclusion wouldn't depend a lot on the drivers, because all you'd be doing is moving the max SPL level up and down and moving the driver Fs left or right.

So I just modeled a 15" TC-2000 with two SA-PR15-1400 passive radiators. Maximum cone excursion is reached at 7Hz with 120W of power. So you see, applying a boost here would blow everything up. Again, ported won't work because the port volume is just too great for such a low tuning. Ported and PR designs fall off at 18dB/octave and 24dB/octave (IIRC) at the tuning frequency. The tuning frequency of the design I just modeled is 16.55Hz. I can use EQ cuts to flatten the response up to the tuning frequency (SPL at 30Hz is about 30dB greater than SPL at 7Hz), but I can't do anything to boost the low end.

Sealed designs roll off much more gradually, so I need more power but I can use EQ boosts to make it flat without sacrificing the higher frequencies. My sealed design, using two 15" drivers instead of a 15" driver and two 15" PRs, and boosting the low end, reaches maximum excursion at 7Hz with about 2200W of power, but is about 20dB stronger at 7Hz, and the high end is much closer to that point; e.g. SPL at 30Hz is about 10dB greater than SPL at 7Hz.

Also, I'm not talking about max SPL. Just the flatness of the SPL across the spectrum. Max SPL >20Hz is still greater with less watts with a ported or passive design.

As a side note, group delay is not so bad in the PR I just designed. Only a little worse than the sealed and probably negligible.


Just so I understand, you mention the port volume required at the frequencies you're talking about as 'just too great'. Does that mean to compensate by increasing the size of your tube it gets too big, or are you practically saying the port is HUGE and simply not practical. Again, from vague recall of the LLT philosophy, most are knida tuned around 15 hz or so, is yours different in being tuned much lower which then leads to the problems you've encountered??

sorry Josh, just re-read your post, and see that you are tuning to 16.55 hz, so you're not tuning lower. Will have to re-read the LLT again to work out for myself why most can use an LLT and you can't, went and got myself all confused again!!
 
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#70 ·
I don't mind thread hijacking so much. :p Anyway, USB 1.2 speed should be more than sufficient so that shouldn't be a problem.
 
#71 ·
Josuah, did you resolve the problem on your end? I don't recall from your previous post if you were seeing this when you measured using automatic or manual? As I mentioned, I'm only seeing it when I use auto. The manual method stepping through the frequencies one at a time with REW grabbing the SPL data from the SPL meter seems to work fine. It's just a wee bit slower than the auto sweep...
 
#72 ·
I used my Mac's line in and line out and it looks okay now, using the automatic sweep. I loaded both the meter and soundcard calibration files when doing the automatic sweep.
 
#73 ·
Hmm, I didn't load a meter calibration file. Maybe that's the problem. I did see that you could do it, I just didn't know what to load in there. Where do you find that file (assuming you're using the Rat Shack meter)? Or failing that, how do I create one? I've read through much of the help files but don't recall running across anything on meter calibration.
 
#74 ·
There's a link to a forum post to download the calibration files somewhere off the Room EQ Wizard installation instructions. So they're somewhere in this forum. The instructions aren't the easiest to read through and find out everything to need to know, unfortunately.

Here's what you need: Downloads.
 
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