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Discussion Starter · #1 ·
When modeling an enclosure, you can model your drivers and estimate that you will get the best response with "x" cubic feet when tuned to "y" with a port of "z".


You may or may not choose to include a dampening material, such as polyfill, which can result in your enclosure giving a response of an enclosure up 1.4 times the size (depending on the amount of fill per cubic foot).


I am trying to work out my design in reverse and am not exactly sure how to do it.


Say my design calls for a cabinet that is 24"x12"x12" (interior volume). That is two cubic feet. Then I have to subtract the displacement of the drivers and bracing and the volume of the port. I am unclear as to whether or not I should include the effective increase in enclosure volume from the polyfill.


The problem is that the volume of the port is dependent on the volume of the enclosure. So if I start off with 2 cubic feet, subtract .2 cubic feet for drivers and bracing, that is 1.8. If I plan on using a polyfill amount that increases effective volume size by a factor of 1.2, I now have 2.16 cubic feet. If I figure out a port for this size, I get a port with a volume of .3.


So now what? Do I subtract the .3 from the 1.8, giving 1.5, x1.2 for polyfill, which results in 1.8 again. However, if I model this enclosure size, I get a different port size, which then sends me back to the beginning.


In other words, everything is interdependent.

Am I missing an easy way to figure this out?


Am I correct that the port volume is entirely subtracted from the enclosure volume?


Am I correct that polyfill's influence on apparent enclosure size will in turn alter the tuning of the port and therefor should be included in the model?
 

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If you have a size in mind start modeling with a little bit smaller box volume (say .25 cubic feet smaller) then you have the rest to work with for driver and port volume.
 

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Presuming this is a vented subwoofer application, I wouldn't suggest stuffing the enclosure at all. -Some may disagree with my opinion in this regard.



Fully stuffing a vented enclosure (the only way to gain the apparent volume you indicate) will reduce the port output, ultimately turning it into an aperiodically damped enclosure.


I suggest the increase in voice coil resistance under power will likely affect the tuning far more than lining the walls with stuffing. To partially compensate for this, I feel it is better to design with a little lower tuning than to optimize the enclosure assuming no voice coil changes at all. Try changing Rg to say, 0.4 ohms in your box modeling program, and see what it does to the response


IMO, the most important thing is to ensure the enclosure is adequately braced. Subtract the volume of the bracing, port volume, and a rough guesstimation of the volume displaced by the sub. Select your port based on this volume. Realistic variations in Qa (stuffing) will do little to affect the tuning of the system.


C
 

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The solution to your problem is very easy. It is called "prototyping". Build a box clearly too big. Tune the volume by adding books or some known displacement. Keep adding books and stuffing playing with the port until you are happy. Measure the result and build a pretty box.
 

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+1 on Unibox, but like any tool, you need to know how to use it. Damping, fill and port end options give you a reasonable idea what to expect, but there are a lot of other variables to consider.


Damping material comes in 2 kinds, surface treatment and volume fill. Vented boxes only use the surface absorber so the volume will resonate at the desired frequency, but higher frequencies will be absorbed. In subs, this will also have to be quite thick. Fill is common in sealed boxes, and modifies the effective volume as you note, but it's rare to see it in a ported box for the reasons Curt noted. Resonant structures like sono-subs are a notable exception.


For a given driver and box size, optimization in Unibox is straight forward.

- find the driver in the database, or add it (what I did)

- page down to vented designs and enter your desired interior volume (sans port and driver as you note)

- select your options (walls only, no leaks, port flaring per your your intent)

- enter your intended port diameter, or let it tell you the minimum

- I normally let it optimize for flat response (no peak)

- I'll then play with Fb, box tuning frequency, just to see sensitivities to port length


The itterations are needed as you adjust port parameters and box tuning to get a good fit; a 24" port is a tough fit in a 24" box. Fundamental limitations creep in as you reduce port size to get a port length that fits at your desired tuning frequency, only to find you'll exceed port velocity at max power and low frequency, causing audible chuffing.


I also don't use full rated power, since I'll only hit that on the peaks and any degree of bass extension will predict dire over-excursions. However, I'm building full-range speakers, not subs; I leave my subs sealed as that meets my needs.


In HT use, you may want to build bulletproof subs, which means modeling full power+ to fall within cone excursion and port velocity limits, and planning a sub-sonic cut-off filter to remove the extreme low-end. Sealed sub designs are a lot easier but less efficient, and that's OK in my small room.


Ultimately, it's why we DIY - we get what fits our needs.


HAve fun,

Frank
 

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Quote:
Originally Posted by theater_lover /forum/post/17014336


When modeling an enclosure, you can model your drivers and estimate that you will get the best response with "x" cubic feet when tuned to "y" with a port of "z".


You may or may not choose to include a dampening material, such as polyfill, which can result in your enclosure giving a response of an enclosure up 1.4 times the size (depending on the amount of fill per cubic foot).


I am trying to work out my design in reverse and am not exactly sure how to do it.


Say my design calls for a cabinet that is 24"x12"x12" (interior volume). That is two cubic feet. Then I have to subtract the displacement of the drivers and bracing and the volume of the port. I am unclear as to whether or not I should include the effective increase in enclosure volume from the polyfill.


The problem is that the volume of the port is dependent on the volume of the enclosure. So if I start off with 2 cubic feet, subtract .2 cubic feet for drivers and bracing, that is 1.8. If I plan on using a polyfill amount that increases effective volume size by a factor of 1.2, I now have 2.16 cubic feet. If I figure out a port for this size, I get a port with a volume of .3.


So now what? Do I subtract the .3 from the 1.8, giving 1.5, x1.2 for polyfill, which results in 1.8 again. However, if I model this enclosure size, I get a different port size, which then sends me back to the beginning.


In other words, everything is interdependent.

Am I missing an easy way to figure this out?


Am I correct that the port volume is entirely subtracted from the enclosure volume?


Am I correct that polyfill's influence on apparent enclosure size will in turn alter the tuning of the port and therefor should be included in the model?

the equations are all known, thanks to thiele, small, and others.


if you could define "best response", you could readily solve for the other parameters.


such a task could become complicated though.


as a result, as noah noted, most folks around here iterate to a solution "close enough" to specs and therefore worth building, then measure and modify as required.


remember also, that winisd is 2pi space. put in the corner of your room will change things greatly. put in a small room will change things greatly.


in an average sized home room, you will get almost 12db per octave "free boost" from about 25hz down to about 12hz and a little less below that.


put in a corner, you will get about 2-3db over winisd calcs from about 100hz or so down.


all this might even bring you back to the decision to go with a ported box vs. a sealed box. so, while i hate to suggest that vas may be correct, he may be correct in his suggestion for a sealed enclosure.


maybe if you could provide some more details, more folks could weigh in with suggestions.
 

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Discussion Starter · #11 ·
Here is where I am currently in my design and I feel fairly comfortable with the numbers. This was calculated with WinISD.


SPEC:

Dual ACI SV10s

Passive Design

Enclosure of 2.85 cubic feet.

I will likely be using polyfill to get an effective 25% size increase.

Port will be 4" Diameter at 41.5" long.

Drivers, bracing and port are roughly .45 cubic feet.

This should result in a 3.0 cubic foot enclosure tuned between 16-18Hz.

I will have two of these.

This will ultimately be EQ'd.


At MAX POWER for the drivers (500 Watts for two), here is where I stand...


SPL:

94 dB at 10 Hz

106.6 at 20 Hz

111 at 30 Hz

112.6 at 40 Hz

113 at 50 Hz (with 1 dB up from there)


GROUP DELAY:

15.2 ms at 20 Hz

9.8 ms at 30 Hz

6 ms at 40 Hz

4 ms @ 50 Hz (drop from there)


PORT AIR SPEED: (I will have large flares on the ports)

19.6 m/s at 20 Hz

9.7 m/s at 30 Hz (continues to drop)


DRIVER EXCURSION:

XMax + 10%


Any thoughts/improvements on the design are welcome, but I think this should give a great result in not much space. Hopefully when combined with room gain and the equalizer, it will give quite the quality bang for the buck.
 
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