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What happens?

  • It works normally if you can package it

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Discussion Starter #1
I'm curious what would result from such an endeavor. Take the SI HST-11 for the example below.

From another thread...
I keep hearing that the port would be too long. I want to try wrapping the port around the outside of the box or even some crazy long external port. My question would be, what happens when the port has more volume than the cabinet itself? How does that affect the model or actual performance?
The port would be HUGE. In 1.0 ft^3 tuned properly (around 19 Hz) the 6" round port needed to cope with the displacement would be around 18 FEET (213 inches) long.

Haha, I'm actually getting tired of saying the same thing over and over again about these 11's - SEALED ONLY!! :cool:
I've heard that before and I greatly respect his skill, but other than physical limitations, what happens?

My current living room ceiling is over 20ft high so I could go up 10ft and back down with room to spare. I'm more interested to know how WINisd models are impacted or what happens, real world, when the cabinet volume is smaller than the port volume as indicated above. 1ft^3 cabinet & 3.5ft^3 port.

I'd try it, if I had $400 to spend on a driver that I don't need.
 

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In your 20 ft long port scenario, you would be creating an organ pipe with the pipe resonance tuned to 28hz. Regardless of what the box tuning is.


Ports should generally be shorter than 40" if you want to cross over at 100hz. With 60" or so as the max limit for something that get's low passed at 60hz.


I think you could put that little sub in a 4-8cuft box and get a reasonable port length.
 

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I am sure it starts to act more like a funky horn or transmission line rather then a ported enclosure. The port will have severe resonances within in the pass band of the sub.
 

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Yeah probably like a non-tappered horn or something.

You'd have nasty stuff happening above 50hz, and you'd probably have a massive peak at 25hz (or worse, a massive null...)
 

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If the port isn't made of soggy wet newspapers a long port is sometimes necessary and perfectly performing in order to get the desired tuning in the desired size box. But sometimes you have to put various forms of silencer material inside the ports (ports are so often perfectly symmetrical). You'd probably be better off just making a horn-loaded box if you have room for it.
 

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The port volume is greater than the cabinet volume . . .
Where should I put it?

 
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Discussion Starter #9
I am sure it starts to act more like a funky horn or transmission line rather then a ported enclosure. The port will have severe resonances within in the pass band of the sub.
Some kind of TL was my uneducated guess.

If you have 4.5 cubic feet, why not build a sub to that volume with a driver more suited to it?
I'm just trying to figure out what's happening. I don't have a use for it other than learning.

I've seen it mentioned several times that "The port would be too long" in the sense that packaging won't work. I'm just trying to understand the physics and performance impacts of the scenario.

Here's an example of what I was really thinking for a design.
 

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This is from a different thread, but illustrates the point. This is a transmission line model of a UM18 in an 8 CF box with a 72" long port, tuned to something like 10 Hz. Notice the giant mess starting around 90 Hz? That's from the port resonances. The longer the port is, the lower in frequency those resonances happen.
 

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the real question is, what will you really gain when going from sealed to organ-pipe-ported....?
how much different will the response be and will is really be worth the trouble ? If u have the space, get a 15 or 18 ?
just my $0,02
 

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What happens when port volume > cabinet volume? It's called a transmission line. If the transition from cab to port is abrupt it's called a stepped transmission line.

This is from a different thread, but illustrates the point. This is a transmission line model of a UM18 in an 8 CF box with a 72" long port, tuned to something like 10 Hz. Notice the giant mess starting around 90 Hz? That's from the port resonances. The longer the port is, the lower in frequency those resonances happen.
That "giant mess" isn't nearly as bad as it looks. Internal box losses alone would almost completely flatten the sharp peaks and dips, and whatever is left could be almost completely eliminated if desired by using a small bit of wisely placed stuffing, leaving an almost perfectly smooth curve.

Resonances are no problem whatsoever, even when they reside inside the passband, as long as they are prepared for during the design process. Otherwise transmission lines and mltls would just be a big mess. In fact, a lot of the ported boxes presented on this forum are large enough that they really should be simulated as mltl regardless of port length. The box dimensions alone are sometimes enough to cause issues.
 

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I'm more interested to know how WINisd models are impacted or what happens, real world, when the cabinet volume is smaller than the port volume as indicated above.
In WinISD Pro for W7 click on the Advanced tab, check the box for 'Use transmission line model for port simulation'. The port resonances will show up on the SPL chart as notches followed by spikes, commonly referred to as a phase zits. You want the lowest frequency zit to occur at least an octave above the lowpass frequency, preferably higher, depending how severe it is. If you leave the tuning frequency constant while adjusting the box volume you'll see that the smaller the box volume the lower the frequency of the first zit, the larger the box volume the higher the frequency of the first zit. Make the box as large as required to move that zit high enough so that it doesn't cause a problem.
 

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In WinISD Pro for W7 click on the Advanced tab, check the box for 'Use transmission line model for port simulation'. The port resonances will show up on the SPL chart as notches followed by spikes, commonly referred to as a phase zits. You want the lowest frequency zit to occur at least an octave above the lowpass frequency, preferably higher, depending how severe it is. If you leave the tuning frequency constant while adjusting the box volume you'll see that the smaller the box volume the lower the frequency of the first zit, the larger the box volume the higher the frequency of the first zit. Make the box as large as required to move that zit high enough so that it doesn't cause a problem.
WinISD is the wrong tool to use if the resonances are anywhere near the passband. The "use transmission line model for port simulation" is laughably inaccurate. There are plenty of very good free tools, please don't use WinISD for this.

As I mentioned before, there's nothing wrong with having low frequency resonances even if they are right inside the passband. They can even be helpful. This is how horns work, a series of resonances spaced apart to prop up the entire bandwidth for up to 3 octaves.
 
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