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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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#14 ·
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.
 
#3 ·
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.
 
#6 ·
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.
 
#4 ·
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. :p
 
#7 ·
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.
 
#9 ·
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
 
#10 ·
Everyone should just jump to this post and ignore all my previous posts, I guess. :p 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. :p
 
#12 ·
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.
 
#40 ·
I'm hoping for -10dB from reference level at the listening position. That's sort of my cut-off point for keeping things flat. But I'll see what's possible at reference as well. Reference meaning LFE peaks at 115dB at the seating position.

So far, I think I'm more than okay. But I'm having a really hard time finding the right LP shelf filter setting to use and applying EQ. I can, however, say that it is possible for me to get usable bass at 7Hz. Last night I just popped in Black Hawk Down and went to the Irene scene, which has the helicopters at 7Hz, IIRC. And it was a serious SPL level. With the 12Hz tuned SVS 16-46PC+ subs, there was nothing here.
 
#15 ·
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.
 
#16 ·
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
 
#17 ·
bosso said:
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 said:
The tubes flex rather easily in comparison.
Now tell me, is this from experience or are you guessing?
 
#18 ·
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
 
#19 ·
bosso said:
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 said:
Just wish to see more evidence
Tell me about it.
 
#20 ·
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."
 
#21 ·
You're both talking about pressure pushing out of the cylinder.

To repeat, when the 2 drivers are in the out position, the cylinder distorts to ovoid, when the drivers are in the in position, it returns to round.

Of course, I'm not talking about a visible flex, anymore than you'd suggest the cube's flex is visible, but flex is not good in any amount, no?

Noah Katz has posted something about his tests of box flex. I don't recall if he actually measured the flex or used math formulae, but if he did measure a box, I wonder if he included a sonotube in his testing?

Bosso
 
#22 ·
bosso said:
Noah Katz has posted something about his tests of box flex. I don't recall if he actually measured the flex or used math formulae, but if he did measure a box, I wonder if he included a sonotube in his testing?
Send him an email, I'm sure he could simulate it. I think you are underestimating the strength of the tube. Something else I don't think you are taking into consideration is that you feel the larger tubes are more prone to flexing as compared to smaller ones or small stout boxes - since they have more internal volume though, it's going to take a lot more air displacement to reach the same internal pressure forces.

But hey, if you really want to put your money where your mouth is, you could always redo your plumbing with some of this stuff :)
 
#23 ·
Steve,
I think you are giving the tubes more credit than they deserve. I've been thinking about this recently myself, but as I have no direct experience, I have decided to keep my mouth shut.

I am a forensic structural engineer. I am very familiar with concrete and tubes. I am also very experience in making finite element models of complex buildings. When I was constructing my sealed subs, I was a geek and made a quick model of the box using SAP2000. There were some posts between me, noah, and a few others discussing it. I was shocked at how little flex the box model was showing. Next to nothing due to the pressure of the driver.

However, when I built the box and turned on the tunes, yes I could still feel slight vibration. But that is exactly that, it was vibration - not movement due to pressure.

And that has had me wondering ever since, how does a tube structure prevent vibration? Vibration isn't equal pressure being applied to all surfaces, which would take advantage of the tube. No, it unbalanced impact. Like the example Bosso mentioned, if you strike the side of the tube, you will crush it.

Now, I'm sure that you have placed your hand on the side of your tube while playing it and not felt vibration. I wonder though if this makes it universally true.

Next time I'm at Jon's I'll have to feel his sub up. :)
 
#24 ·
One more thing to add,
I've often been confused by listening to people's discussions of bracing subs. I often even hear people discussing bracing a ported sub, because of the pressure inside. Huh? How can you have pressure inside a sub with a big hole in it? Air moves through the port, thus relieving the pressure and creating sound.

Once again, bracing in a ported sub, I believe is to reduce panel resonance effects.

A driver in a ported sub will cause vibration. This vibration will not be a uniform pressure on the tube. Sound (back) waves will also be created, and again these won't produce a uniform pressure on the tube and result in pure tension. No, I think the result would be similar to striking the side of the tube.

I guess though one great thing the Tube has going for it, cardboard has a very low stiffness. Therefore, when you strike it, it doesn't resonant like wood does.

And my final argument against tubes, women dig the look of real wood, not tube socks. :bigsmile:

I could be all wrong. Its happened once or twice before.
 
#27 ·
One more thing to add,
I've often been confused by listening to people's discussions of bracing subs. I often even hear people discussing bracing a ported sub, because of the pressure inside. Huh? How can you have pressure inside a sub with a big hole in it? Air moves through the port, thus relieving the pressure and creating sound.

I could be all wrong. Its happened once or twice before.
Actually the pressure inside of a ported sub (around the tuning frequency) is larger than inside of a sealed sub due the port loading. The port acts like a hole only below the tuning frequency of the enclosure.
 
#25 ·
---k--- said:
And that has had me wondering ever since, how does a tube structure prevent vibration? Vibration isn't equal pressure being applied to all surfaces, which would take advantage of the tube. No, it unbalanced impact. Like the example Bosso mentioned, if you strike the side of the tube, you will crush it
Vibration caused by the actual driver movement? A thick, dense baffle (bottom cap) and an overall heavy sub with a baseplate should minimize any vibrations. Not sure how the vibration relates to striking the side of the tube though :scratch:

---k--- said:
Next time I'm at Jon's I'll have to feel his sub up
So you're saying you didn't get touchy feely the first time? :R

---k--- said:
I've often been confused by listening to people's discussions of bracing subs. I often even hear people discussing bracing a ported sub, because of the pressure inside. Huh? How can you have pressure inside a sub with a big hole in it? Air moves through the port, thus relieving the pressure and creating sound.

Once again, bracing in a ported sub, I believe is to reduce panel resonance effects.

A driver in a ported sub will cause vibration. This vibration will not be a uniform pressure on the tube. Sound (back) waves will also be created, and again these won't produce a uniform pressure on the tube and result in pure tension. No, I think the result would be similar to striking the side of the tube
You might be misunderstanding how the port actually works - check out this. At and for a little above tuning, there should be high pressure forces in a ported sub, otherwise the port wouldn't work. The air in the port is constantly moving from high to low pressure when it's in use. I still don't get where this idea of striking the tube is coming from.
 
#26 ·
Interesting scientific discussion. If you don't mind, I will chime in. :nerd:

Not long time ago I thought about this internal pressure thing. It came up during the construction process of the Finnish sonosubs. The brother of the builder of the second sonosub is a manager of a sonotube factory here in Finland, so he knows a thing or two about sonotubes. He let us know that they make several types of sonotubes: 6mm, 8mm, 10mm and 12mm (wall thickness). When I explained where we would use these, he said that the 6mm tube would be strong enough for this application (although the guys went for the 12 mm tube, not because they needed, just because they could). He said that if a 100 kg (220 lbs) man would sit on a 6mm tube (no end caps), it would only flatten a bit. A 12mm tube would need around 1000 kg (2200 lbs) to flatten it. As we remember, pressure is defined as a weight against an area, so the larger area the weight is distributed, the less pressure there is.

Well how much pressure there is inside a subwoofer box? It's not difficult to calculate if you know some basic physics. I save you from the trouble by making some calculations. The example woofer used is a 15" (800 cm^2) with a 25mm max excursion, so the Vd is 2 liters. That is the amount of displacement that causes the pressure inside the enclosure (or the external pressure when the driver moves outwards). As I explained before, the more area there is, the less pressure there is. That of course also means that the more enclosure volume there is, the less pressure there is.

I'll list some enclosure volumes and internal pressures:
600 liters: ~1.5 mbar
300 liters: ~6.0 mbar
150 liters: ~25 mbar
75 liters: ~100 mbar

1 mbar can be defined as ~10 kg per 1 square meter or ~2 lbs per 1 square foot. So for example 6 mbar would equal with 60 kg per 1 m^2. Well how about our 100 kg guy sitting on a tube (area around 30 cm by 30 cm)? That's around 1100 kg per 1 m^2, meaning around 18 times more pressure than what the woofer can produce. Although in a 75 liter box they would pretty much equal.

Based on this, I'd say the internal pressure isn't a big problem, especially with large enclosures. The smaller the box gets, the more thicker (and braced) the walls need to be.

Enclosure resonances are a whole another discussion. I don't know the internal resonance frequency for a sonotube, but I'd guess it's much higher than what the MDF has ("MDF and chipboard resonate at averagely 150-400 Hz, with the strongest resonances usually at 250-300 Hz"). And that is a good thing since we want the resonance as far away as possible from the working range of the enclosure. Here's a quote from a page I found: "A single-piece cabinet, like a concrete tube, will usually have less vibration than a cabinet that is assembled of separate parts."

I haven't touched a DIY sonotube (yet), but I can say that an SVS tube vibrates much less than the box version using the same woofer.

What is the best material to make speaker boxes ?

An ideal speaker cabinet material would be very stiff, so that it would not tend to move with variations in box air pressure. It would also be very well damped, so that if it ever does deflect from air pressure, it will come back to the original position without resonating. It would also have a very high resonant frequency (supersonic), so that low frequency box air pressure would not cause it to resonate. An attractive material is preferred, and additional credit is given for a material which is easy to cut, glue, and finish. A great material would be cheap, too. Finally, it would be nice if the material were light, because we all have to move our speakers sometimes, and it's hard to appreciate good speakers with a sore back.
With all of those attributes, it would seem that no material is perfect. However, there are many materials that have enough of the above good attributes to make excellent speaker cabinets. Yet each has advantages and disadvantages.
 
#28 ·
In speaking with ThomasW about this, here's what he had to say:

"Bosso has a point, when tube subs become HUGE they should be reinforced. My comment you quoted was made regarding smaller more 'normal' sized tube subs.

When tubes get to the size of ~300+ liters or so, they should be reinforced with additional rings and vertical 'stays', because yes the walls can flex.

Steve's position that internal pressure supports the walls is true, but that situation is based on having massive amounts of internal pressure equalizing the walls (think concrete).

That massive internal pressure is not available with the any alignment (sealed or ported) the size of an LLT. So Steve's stance that LLT's don't need reinforcement is incorrect. And it's particularily in error given the 600+ liter size of his personal LLT.

Take care,
Thomas"
 
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