Why I Rejected Ported/Passive

[Josuah]

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!!

[Sonnie]

Well I'll certainly be looking forward to our photos and building process. Personally I'm still trying to understand the various sonotube options... they intrigue me.

[Josuah]

The 15" TC-2000 SVC with two 15" SA-PR15-1400 passive radiators in a 5 ft^3 enclosure gives you a natural tuning frequency of 16.55Hz. My 2 x 15" TC-2000 SVC sealed in a 5 ft^3 enclosure also gives a natural tuning frequency somewhere up there. Probably higher. I am only "faking" a low tuning frequency in the sealed sub by boosting below 20Hz. The point is it's no longer possible to boost the low frequencies in the passive design because it's already hitting max excursion at 7Hz. Can't give it any more power down there.

So you can't boost the low frequencies in a ported or passive design whereas you can with a sealed design.

What I meant by the port volume being too big is exactly that. The total volume of air that's inside the port is so much you're going to be exceeding the enclosure volume. You'd have no non-port volume left! You need a wide diameter to deal with port turbulence/velocity (because SPL is a function of air displacement at a given frequency) and subsequently you need a really long port. Like a dozen feet long. Roll that all up inside a 4' tall ~20" sonotube and you don't have much left.

Construction was completed today. I don't know how people normally lift those drivers into position for a side-firing design. I wasn't able to. I had to always drop the driver in from the top. Watching out for crushed fingers. :-S

Now I'm just waiting for the amp and equalizer to arrive. Everything should be hooked up and calibrated by end of the week.

[Josuah]

Oh, the other possibility is to stick that dozen feet of port outside the enclosure. Like a pipe organ. I think it'd fit into any modern steampunk decor.

[Sonnie]

I'd say you at least make it look pretty cool.

[---k---]

Josuah wrote:

What I meant by the port volume being too big is exactly that. The total volume of air that's inside the port is so much you're going to be exceeding the enclosure volume. You'd have no non-port volume left! You need a wide diameter to deal with port turbulence/velocity (because SPL is a function of air displacement at a given frequency) and subsequently you need a really long port. Like a dozen feet long. Roll that all up inside a 4' tall ~20" sonotube and you don't have much left.

You're confusing me. The TC-2000 driver is very very similar to a SS RL-p15. Several people have successfully built LLT style sonosubs with these. The typical design is 260L tuned to ~15hz. I've never seen anyone complain that they couldn't fit the port inside the box. But this is a very large sub. Are you really saying that you're unwilling to build a HUGE sub? I'm not getting that from your post, but I don't really understand your post either.

If you're unwilling to build a HUGE sub, then yes ported and tuned to ~15hz isn't going to work. The smaller the volume, the higher the tune usually needs to be. If this is unacceptable, then sealed is the way to go.

[Exocer]

Limited space is certainly a good reason to go sealed. This is the one reason I haven't gone vented yet.

When you factor in room gain, large and low tuned subs seem to be able to do almost as well as sealed subs thanks to the low Fb.

[Josuah]

---k--- wrote:

The TC-2000 driver is very very similar to a SS RL-p15. Several people have successfully built LLT style sonosubs with these. The typical design is 260L tuned to ~15hz. I've never seen anyone complain that they couldn't fit the port inside the box. But this is a very large sub. Are you really saying that you're unwilling to build a HUGE sub?

I'm saying if you wanted to tune to below 10Hz. And correct, I'm not willing to build a ported sub the size of a small walk-in closet. My sealed sonotubes are also about 260L but will have the capacity for a "fake boosted" tune below 10Hz. The actual "fake boosted" tune will not be measured, as I'm just going to calibrate in-room. Then verify it's safe to run with that boost without bottoming out in WinISD.

If I was tuning into the high teens, and okay with the fast roll off, I would also have chosen a ported design. My current SVS 16-46PC+ subs are tuned to 12Hz, and I have a flat response to 12Hz (assuming the sub is not pushed had) with a steep drop off below that.

[terry j]

I think that has been what confused me too, I thought you were tuning reeaall low, and so asked the questions on the other thread above. But then re-read your post in which you said """""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.""""

The 16.55 hz didn't seem much lower than other LLT designs I'd seen, and got confused.

But post above seems to indicate a tuning to below ten, which maybe would account for the massive port required, rather than a massive port aith a tuning of 16.55 hz.

For now, at least, I feel less confused. This will surely change when a new post arrives ha ha.

Seems my rather innocent statement on the quality or otherwise caused a little stir, was unintended on my part. Good to see the replys though.

lots of love

terry

[Josuah]

Everyone should just jump to this post and ignore all my previous posts, I guess. There were two questions.

1) Why can't you boost a ported/passive sub the same as a sealed (where all three types have a higher tuning frequency), to get low frequency reproduction?

Because you can't boost low frequencies in a ported/passive sub. Below the tuning frequency the drivers are already reaching excursion limits without much SPL.

2) Why can't you build a ported sub that is tuned <10Hz to begin with?

Because the port volume would be huge.

Now if you want details, read the above posts.

[SteveCallas]

So are you trying to get usable output below 10hz?

[Josuah]

Yes. As low as possible. My previous thread asking about required SPL < 10Hz says 7-8Hz might be the best I can do, as I would need 90dB at 8Hz to feel anything. But I'm going to go as flat as low as possible. 5Hz is the golden target.

The SVS 16-46PC+ subs already give me usable output to 10Hz, although it's pushing the subs a bit hard. Not to their extremes, but almost there.

[SteveCallas]

Just out of curiosity, what levels do you typically listen at? If limiting the sub's output potential to that of it's <10hz capability (or suffering from substantial compression on the way down), you're giving up a LOT for material that will be reproduced maybe 5% of the time.

[drf]

Sonnie wrote:

Well I'll certainly be looking forward to our photos and building process. Personally I'm still trying to understand the various sonotube options... they intrigue me.

Ditto, I was always under the impression tubes with drivers in the ends were second only to symetrical cubes for bad speaker design, However, like sonnie, I am interested in how sonotubes sound and what precisely makes them a desireable sub design.

[Exocer]

One reason Sonosubs are a desirable design is they can provide more internal volume than cubes while taking up similar or lesser amounts of floor space. Also, they're more rigid than cubes and don't need internal bracing, making them relatively lighter than a properly built cube enclosure of the same internal volume.

[bossobass]

Since I've read the general statement that tubes are more rigid than cubes more than a few times, I thought I'd chime in with some thoughts on the subject.

The popular tube is the paper concrete form type. The walls are relatively very thin.

I think a good illustration would be to turn a cube on it's side and sit on it, then turn the tube on it's side and do the same. Certainly, the cube would show no signs of stress under this test, but how would the tube fare?

The tube is only as rigid as the end caps make it so, but...as you increase the length of the tube, the cap has less and less of an effect towards the middle of the tube.

The simple truth is that thin-walled tubes are definitely not more rigid than thick-walled, properly braced cubes in the context of a very large diameter paper based tube that is 5-6' long.

The tubes flex rather easily in comparison.

Also, as SteveC brings up the 'why sacrifice part of the BW to chase single digits?' argument in every place he visits, I'm compelled to comment on that subject as well...

Whether you use a Bi-quad filter or a simple shelf filter to apply a boost curve to a sealed sub's input signal, you get the same effect below the resulting new F3 of it's anechoic FR...a second order roll off.

When using a Bi-quad, you get the added benefit of choosing the F6 BW, or knee, as it may suit your own listening tastes. You may even easily design more than one shape of knee in a user-selectable Q feature. Below that knee, whatever the F6 BW may be, the result is a second order roll off.

A second order roll off just happens to work well with an enclosed listening area to result in a 0 order roll off, or a flat response, so, there is no 'chasing' of anything. It's simply a positive consequence of the alignment.

You give up nothing to achieve in-room response to single digits with a sealed alignment. The in-room response is...flat...to single digits. A simple 2X15" sealed, EQ'd system is plenty of low end for the vast majority of listeners and the average satellite system. If 'compression' (assuming we mean power compression) is limiting the system at 20Hz (because it's an atypically large room, or the listener prefers a boosted low end FR curve, etc.), more system is needed. The system's in-room capabilities below 20Hz have nothing to do with compromising the design.

OTOH, the currently popular LLT design definitely compromises output in the 30-50Hz region to chase lower frequencies, as well as real estate, which ties into the first comments in this post, because, as I said, as you incrtease the length and diameter of the tube, you also compromise it's rigidity.

Bosso

[SteveCallas]

bosso wrote:

I think a good illustration would be to turn a cube on it's side and sit on it, then turn the tube on it's side and do the same. Certainly, the cube would show no signs of stress under this test, but how would the tube fare?

That's an interesting concept, but I think you're forgetting that force in a circle (or in this case a cylinder) is applied uniformly in all directions. In order to deform the tube, you would need enough force to essentially burst the tube. High excursion drivers can create a lot of force, but I've yet to hear about any sonosubs bursting. It's not like turning a tube on it's side and sitting on it at all.

In a box on the other hand, the force is not spread equally, as some points will be weaker than others, and they will deform if the box isn't stout enough. This is why pipes, pillars, and hoses are round and not square.

bosso wrote:

The tubes flex rather easily in comparison.

Now tell me, is this from experience or are you guessing?

[bossobass]

Yes, I've built a couple of 'sonotube' type cylinder subs, very early on. I soon abandoned the method for a much thicker walled, denser cylinder (actually, hourglass shaped).

It's not a circle, or sphere...it's a cylinder. If you put your finger at the end of a straw and suck the air out the other end, it very easily collapses the straw vs trying the same experiment with a ping pong ball that has a hole in it.

If you want the straw to resist collapsing, you need a thick enough walled straw ( or cylinder) to resist the amount of pressure you induce by the amount of vacuum you cause.

Most people have no idea how much pressure is created inside a sealed sub with a couple of high excursion drivers doing their thing. When the drivers are in the out position there's little doubt that the thin-walled sonotube is losing integrity vs a braced MDF box. The bigger the diameter and longer the cylinder, the more it becomes out of proportion to the wall thickness as well as the center (weakest point) becoming further away from the stabilizers at each end.

Opposing drivers reduces vibrations, but does nothing to alleviate the pressure that causes flex, which robs output because it's sympathetic to the frequency being played.

I haven't gone as far as using an accelerometer to measure the distortion of the tube, but I just think that to state as a matter of fact that the thin walled cylinder flexes less than a braced, thick-walled box is baseless gossip.

Cutting a few extra rings to glue inside, properly spaced, shouldn't be viewed as a waste of time, as many have suggested in the past, especially as the diameters and lengths get as huge as they have recently, in proportion to the wall thickness. If you look at drain pipe that's made to resist pressure, the wall thicknesses increase as the diameter of the pipe increases, and that isn't so that it will weigh more.

Just wish to see more evidence and less blanket parroting.

Bosso

[SteveCallas]

bosso wrote:

If you put your finger at the end of a straw and suck the air out the other end, it very easily collapses the straw vs trying the same experiment with a ping pong ball that has a hole in it.

Again, this is another interesting concept - you like to stack the odds. First, replicate the shape of the celluloid ping pong ball with thin thermoplastic and attempt your experiment again - the ping pong ball will collapse. Second, the length to diameter ratio of the straw is much, much larger than any sonosub. Third, the force created by sucking on a plugged straw in relation to it's size is many multiples greater than any high excursion driver can create. It's just not applicable.

Fact is that these thin walled tubes support thousands of pounds of wet concrete. A high excursion driver is moving what, maybe 2.5" peak to peak? Were these sonotubes to flex in the manner you have described under subwoofer duty, owners would see small visible wrinkle lines right around the middle.

bosso wrote:

Just wish to see more evidence

Tell me about it.

[Brian Bunge]

To play devil's advocate for a minute:

1) The tube can't flex outwardly due to inner forces, so it's not the same as pressures exurted on the outside of the wall.

2) A tube sub's endcaps can flex quite a bit and should have some bracing once their diameters get really large.

3) To quote (or paraphrase) ThomasW: "A tube sub has no advantage over a properly built box sub." Also, note that "properly built" means thick walls and extensive bracing.

My own personal opinion is that I prefer flattest in-room response down low over max SPL from a single driver enclosure. That's why I'm building a quad sealed enclosure. Oh, and another quote from ThomasW: "Power is cheap these days."