Why I Rejected Ported/Passive - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-01-06, 09:04 AM
Senior Shackster
Ben

Join Date: May 2006
Posts: 343
Re: Why I Rejected Ported/Passive

I don't think that should be mentioned on a public forum.

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Old 12-01-06, 01:12 PM
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Steve

Join Date: Apr 2006
Posts: 2,398
Re: Why I Rejected Ported/Passive

Quote:
Ilkka wrote:
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.
Indeed.

Bosso - here's an example you might like. Take a piece of sonotube - let's say any diameter larger than 6". Now cut it into two pieces, one that's only 4" long, and one that's 6' long. Now try with your bare hands to compress the 4" length. I don't know if you have sonotube laying around, but I do, so I can tell you it's very easy. When I try the same with the 6' length, it's nearly impossible, it won't give at all. This is because the force I am creating is being spread over a larger surface area.
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Old 12-01-06, 03:17 PM
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Bosso

Join Date: Jul 2006
Location: Charlotte, NC
Posts: 57
Re: Why I Rejected Ported/Passive

Quote:
SteveCallas wrote: View Post
Indeed.

Bosso - here's an example you might like. Take a piece of sonotube - let's say any diameter larger than 6". Now cut it into two pieces, one that's only 4" long, and one that's 6' long. Now try with your bare hands to compress the 4" length. I don't know if you have sonotube laying around, but I do, so I can tell you it's very easy. When I try the same with the 6' length, it's nearly impossible, it won't give at all. This is because the force I am creating is being spread over a larger surface area.
Yes, now...try the same experiment and increase the diameter of only the 6' long tube, with end caps installed on both examples. Which tube gives more? The thought that a smaller box or tube is weaker is incorrect.

The first incorrect assumption here is that the tubes are perfectly round to begin with, which they aren't.

The second incorrect assumption is that I'm somehow talking about huge flexing.

The third misconception seems to be that either method of enclosure design is flex-free. We're talking about the least amount of flex, not which one has zero flex. And, we're talking about very small distances of flex.

Ilk...when you say that the tube expert says that if a 200 pound guy sits on a tube and it only would flex 'a bit', what specifically would that mean in mm? If he has that info, we could extrapolate the amount of flex with 2 opposing drivers, which would stay on topic with the OP's design. I would imagine that what the guy means as 'a bit', in terms of this discussion would actually be quite a bit.

Then again, if the same 200 pound guy would sit on a braced box of the same internal volume, would it not flex even less than the tube?

From Dan Wiggins from back when his company was called Avatar:

: Now, when a driver begins backward motion, into the box, it will create a
: compression zone (pressure above ambient). This zone will travel as a wave.
: When the driver moves forward, it will create a rarefraction zone (pressure
: below ambient). Likewise, this zone will travel as a wave. This is a basic
: fact of wave mechanics and physics, and will occur for all cases.

My argument is that when the pressure zone wave travels through the tube, it, at the very least, would correct the tube to perfectly round where it isn't so to begin with. When the rarefraction zone wave is traveling through the tube, it would, at the very least, return the tube to it's imperfect shape.

I would also say that my argument goes beyond these least case scenarios to a case where the rarefraction zone distorts the shape of the tube beyond it's original shape.

This thought is what Dan seems to be saying in the same conversation:

"While this is true theoretically [that a tube cannot flex], in practice, one will often have flexing
in a tubular enclosure, simply because the enclosure is not a perfect tube."

He also brings up a point that goes with the very large tube enclosure that most have not addressed:

" Well, you forgot about internal reflections. Realize that when the
wavelength is equal to 4 times the length of the tube, one will have
cancellations from reflections inside the tube."

Using a 6' tube, that would be around 45Hz, no?

Bosso
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Old 12-01-06, 03:48 PM
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Steve

Join Date: Apr 2006
Posts: 2,398
Re: Why I Rejected Ported/Passive

Quote:
bosso wrote:
Yes, now...try the same experiment and increase the diameter of only the 6' long tube, with end caps installed on both examples. Which tube gives more? The thought that a smaller box or tube is weaker is incorrect.
It's not that a small tube or box is weaker, the strength - let's say resistance to deformation - per surface area is actually higher than that of a larger tube or box with walls or endcaps of the same thickness. However, because the larger enclosure has much more surface area, the force applied per surface area isn't as much. If you really want to get detailed, Ilkka's numbers are only one half of the equation. We'd also need to know the resistance of each enclosure type to deformation. Then you can compare the force per surface area of each enclosure to its resistance to deformation. I'm not particularly interested in calculating that

But to get back to your earlier comment about pipe, and the wall thickness increasing when diameter increases, is that not because the pressure in the smaller tube and larger tube is going to be the same? Moving x amount of water at some rate in tube of y diameter = some pressure (pressure = F/A). In the larger tube, with a larger cross sectional area, in the same amount of time, you're moving more water, so the force is much higher. If we assume the cross sectional area is multiplied by a factor of 3, and the force is multiplied by a factor of 3, then yes, the wall thickness of the larger tube needs to be increased compared to the smaller tube, because the relationship of tube wall thickness to tube diameter would have to be the same, as the pressure is the same. HOWEVER, in a large and a small subwoofer enclosure using the same driver, the force is a constant, not a variable, and thus, if enclosure surface area of a tube increases, the pressure is reduced, and wall thickness does not have to increase.
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Old 12-01-06, 04:13 PM
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Steve

Join Date: Apr 2006
Posts: 2,398
Re: Why I Rejected Ported/Passive

As for your comments on sealed vs ported and giving up headroom, we've had these discussions before, and you seem to keep forgetting the outcome. Keep in mind that with a LLT you only need to be amp limited to some point above tuning, usually ~20hz. With sealed, you need to amp or enclosure limit based on how low you want to reach. If we assume you want something close to a flat response, then headroom in the higher bass frequencies is going to take a big hit. Assuming you want to delve deeper than ~20hz, how exactly is the LLT giving up headroom to it? An LLT is only giving up headroom to a higher tuned ported sub with a highpass, not a sealed with as much flat extension. The sealed will NEVER be able to keep up assuming the same driver. If you use a highpass on a sealed as opposed to trying to get relatively flat, usable output to say 5hz, you can gain more output capability in higher frequencies.
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Old 12-01-06, 04:24 PM
drf

Join Date: Oct 2006
Location: Somewhere else.
Posts: 1,249
Re: Why I Rejected Ported/Passive

Quote:
bossobass wrote: View Post
...snip...

He also brings up a point that goes with the very large tube enclosure that most have not addressed:

" Well, you forgot about internal reflections. Realize that when the
wavelength is equal to 4 times the length of the tube, one will have
cancellations from reflections inside the tube."

Using a 6' tube, that would be around 45Hz, no?

Bosso
I understand that a 45Hz signal would be 180degrees out of phase by the time it returns to the driver, however If the signal starts at 0 then at 180degrees the signall will be at 0 again, not causing cancellation, however if the signal started at 90 degrees then when the wave returned to the driver it would be at 270 thus 180 degress out of phase from peak compression to peak rarefaction, causing cancellation. Now a driver/enclosure can't start at a peak compression, it must start at zero and create it first, therefore I don't understand how a tube 4 times longer than the sound wave will cause cancellation.

Have i got this right? If I have then the wave form would have to be only twice the length of the tube to cause cancellation i.e 22.5Hz.
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Old 12-01-06, 04:48 PM
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Will

Join Date: Sep 2006
Location: Matthews, NC
Posts: 521
Re: Why I Rejected Ported/Passive

Yo Bosso...want a 24" diameter 48" long piece of Sonotube to experiment with?

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Old 12-01-06, 04:51 PM
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Steve

Join Date: Apr 2006
Posts: 2,398
Re: Why I Rejected Ported/Passive

A bit more.

Quote:
bosso wrote:
The first incorrect assumption here is that the tubes are perfectly round to begin with, which they aren't.
You stick a perfectly circular endcap in the top and a perfectly circular endcap in the bottom, and assuming a tight fit, you're basically forcing the tube to take the shape of the endcaps. Could there be some imperfection along the way? I guess, but I'd have to think it's negligible.

Quote:
bosso wrote:
The second incorrect assumption is that I'm somehow talking about huge flexing.
Quote:
bosso wrote:
The third misconception seems to be that either method of enclosure design is flex-free. We're talking about the least amount of flex, not which one has zero flex. And, we're talking about very small distances of flex.
I didn't think we're even talking about the discrete amount of flexing, we're comparing tendency to flex from one enclosure to the next.

Quote:
bosso wrote:
My argument is that when the pressure zone wave travels through the tube, it, at the very least, would correct the tube to perfectly round where it isn't so to begin with. When the rarefraction zone wave is traveling through the tube, it would, at the very least, return the tube to it's imperfect shape.
Why do you assume the tube would change shape as opposed to the air just getting compressed? In a small sealed, when a driver is displacing 2 liters of air, the enclosure walls don't flex on a 1:1 ratio with the driver, correct? In fact, they move very little. And that's working with a pretty small effective volume, so the resistance to allowing the air to compress is high. So what do you think is gonna happen with a 300 liter + enclosure?

Last edited by SteveCallas; 12-01-06 at 04:56 PM.
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Old 12-01-06, 04:53 PM
Senior Shackster
Will

Join Date: Sep 2006
Location: Matthews, NC
Posts: 521
Re: Why I Rejected Ported/Passive

Oh, just for kicks, I laid my sonosub on the floor and sat on it. It didn't budge/flex one bit, not that I expected it to.

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Old 12-01-06, 04:56 PM Thread Starter
Senior Shackster
Josuah

Join Date: Apr 2006
Location: San Jose, CA
Posts: 636
Re: Why I Rejected Ported/Passive

Quote:
SteveCallas wrote: View Post
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.
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.
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