# How do additional subwoofers in the same chamber affect the port calculations?



## ARCuhTEK (Dec 22, 2008)

I am off on an experimenting tangent tonight....sooooo...here goes.

I have designed single subwoofer enclosures, sealed and ported. I have designed two sub enclosures, sealed. But when i start trying to figure out ported boxes where multiple (3-4) subs are involved, I get lost in the requirements. I understand displacement, volume required, area of the port, length of port. I used various software calculators and I use the mfr. specifications. Here is what I am trying to do ...with questions after the post.

I have a maximum box size and shape, based on where it will be located in my vehicle. I have this craving to experiment with four 6" subs (mainly because nothing larger will fit the face of this enclosure) instead of just two. I have modeled this box (see pics) for you to look at... the perimeter CANNOT change. It is so tight I will likely barely get it to fit.

Note that everything is designed EXCEPT the port, port length and vent area (vent area shown but not correctly sized)

*SA requires the following for ONE subwoofer:*
specs, if you care are here: X v.2 SERIES
MUST be ported. Sealed is not acceptable for this subwoofer.
0.33 net free clear volume (cf)
4 square inch port area
Port length not yet calculated.

Questions:

1. The mfr. states cf req's for a single sub. When I add another sub to the same chamber, does the net free and clear volume (cf) have to double?

2. When I calculate the port, the calculators I used only allow one sub to be input. Does the port area have to double if I add a second sub to each chamber? Same question about the tuning port length...does it change with the addition of another sub?

Pics:




































Any help is appreciated...


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## Jheitt142 (Dec 7, 2011)

You can run into port noise if the port area is too small. Now I'm not sure if you need to double the port area for the second sub but if you do you know you won't have any chuffing. 

For length you are just tuning for the volume of the box, As long as you are tuning net volume it doesn't matter how many drivers are in it. 

Sent from my SM-G930W8 using Tapatalk


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## porscheman (Jan 1, 2012)

a quick and dirty look at using 4 of them says you cant get enough port length and displacement in your max dimensions. its around 2.2 cubic feet with the port and not counting the displacement of the drivers


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## ARCuhTEK (Dec 22, 2008)

porscheman said:


> a quick and dirty look at using 4 of them says you cant get enough port length and displacement in your max dimensions. its around 2.2 cubic feet with the port and not counting the displacement of the drivers


I was afraid of that... Part of this post/thread was to determine (for me) how you arrived at that answer? The 0.33 net free clear volume does include consideration for the driver displacement, so that part I understand. I understand the port area. I also understand port length. I just dont understand those three items when another sub is added... If you have a moment and the willingness, some clarity would be appreciated.

Thanks you


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## ARCuhTEK (Dec 22, 2008)

street.terror said:


> You can run into port noise if the port area is too small. Now I'm not sure if you need to double the port area for the second sub but if you do you know you won't have any chuffing.
> 
> For length you are just tuning for the volume of the box, As long as you are tuning net volume it doesn't matter how many drivers are in it.
> 
> Sent from my SM-G930W8 using Tapatalk


I have plenty of spare area to achieve the port area for one sub per chamber and in fact I have enough spare area to achieve doubling of the port area. I am speaking only of cross sectional area....not port length. It is the length I fear I do not have enough room to achieve in either of the chambers (for one sub per chamber ...let alone two).

Can you elaborate on the "just tuning to the volume of the box." comment? Do I need 0.33 PER DRIVER? So in one chamber the smallest volume I can get away with is 0.66 cf net free area. The port itself and the driver displacement must be in additional (thus the term net free area).....is that what I should be doing? If so, this box will not yield that much net free area and I will have to stick to one per chamber.


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## ARCuhTEK (Dec 22, 2008)

I played around with the calculator and the port size/length when only considering one subwoofer per chamber. Amazing how much smaller in length a port can be if it is round.










Round with new cross section area of 3.98":


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## Jheitt142 (Dec 7, 2011)

Net free area is the amount of space you have after you subtract driver displacement and port displacement. 

Eg. You have a 1cuft box. You put in a driver with .05 displacement then say you take up another 0.1 with port displacement. This means you have a box with a net volume of 0.85cuft. 

The smaller the box the harder this becomes, especially with slot ports as they take up all kinds of space in a hurry. You often run into situations where the port won't fit in the box, this is when you end up building the port around the box. 

Sent from my SM-G930W8 using Tapatalk


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## ARCuhTEK (Dec 22, 2008)

street.terror said:


> Net free area is the amount of space you have after you subtract driver displacement and port displacement.
> 
> Eg. You have a 1cuft box. You put in a driver with .05 displacement then say you take up another 0.1 with port displacement. This means you have a box with a net volume of 0.85cuft.
> 
> ...



Thank you, but I already understand net free area, displacement and subtracting for the port in order to achieve the net free area. I also understand the port's cross section area required and the length. What I am still not able to get to the bottom of is if you have to either add a second port of equal volume/length/area when you add a second subwoofer. I also do not know if you have to double the net free area a mfr. spec's for one subwoofer if in fact you are installing two per chamber.

I guess it really does not matter in this case because it seems that the box is not going to yield the right internal net free area if I have to go from 0.33 cf nfa to 0.66 cf nfa when adding the second subwoofer to the chamber.

Thank you for your assistance with it.


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## Jheitt142 (Dec 7, 2011)

Download WINISD alpha, see what it tells you to build as a box for 2. You may up only adding 50% more volume and achieving the results you want. You'll also be able to directly compare the response and given output of the 0.66 to say a 0.5 with 2 in it. 

Sent from my SM-G930W8 using Tapatalk


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## porscheman (Jan 1, 2012)

port length is a function of enclosure volume and port area. the smaller the volume the longer the port, the larger the area the longer the port. ie. if vb is 1 and you have a required area of 15 square inches to not chuff your port is 36", if you keep the same vb and increase your area to 20" your port goes up to 48 inches.. that same box and tune if dropped to .5 cubic feet lengthens your port to 75" and 101". this is tuned at 32hz, it gets worse at lower tunes. so that .5 vb has a internal port displacement of 1.17 cubic feet.

for the 4 sundown x6's you'd be looking at a net volume of .64 and a port area of 16 square inches and 61.75" long, that gets you tuned a 32hz. get adventurous with a little fiberglass, eliminate the bit one hole, one port on the side of the box and you "might" be able to squeeze them in. 

the 16 square inches is borderline, I'd probably go with 20" but that runs your port length up to 77 5/8"


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## ARCuhTEK (Dec 22, 2008)

porscheman said:


> port length is a function of enclosure volume and port area. the smaller the volume the longer the port, the larger the area the longer the port. ie. if vb is 1 and you have a required area of 15 square inches to not chuff your port is 36", if you keep the same vb and increase your area to 20" your port goes up to 48 inches.. that same box and tune if dropped to .5 cubic feet lengthens your port to 75" and 101". this is tuned at 32hz, it gets worse at lower tunes. so that .5 vb has a internal port displacement of 1.17 cubic feet.
> 
> for the 4 sundown x6's you'd be looking at a net volume of .64 and a port area of 16 square inches and 61.75" long, that gets you tuned a 32hz. get adventurous with a little fiberglass, eliminate the bit one hole, one port on the side of the box and you "might" be able to squeeze them in.
> 
> the 16 square inches is borderline, I'd probably go with 20" but that runs your port length up to 77 5/8"


Thank you for the input and explanation. I did download WINisd Alpha and came to pretty much the same conclusion. Unfortunately, I cannot get rid of the Bit One hole and I have zero else place in the entire truck to mount this DSP. No room under either front seats, rear wall is already full and glove box is full...even the ashtray has the DRC in it...lol I am not sure how fiberglass would help as it will not gain me any space, unless I am missing something. This design consumes 100% of the available space I have to work with in the truck.

Since my last post, I have tweaked the design a bit but I am not through with it yet. I do know I will be reducing the drives from four to two. I may actually place them in one of the chamber and use the rest of the length (and remove the divider to open up more cf of clear free area) to gain cf and also create a much longer port. The box is approx 52" long so if I put in a bend or two in the design I might actually get up to 70" of port length. 

Again....Since I have the ability to work with many different cad and 3d tools, I often get off on tangents like this...its too easy and too tempting. I started using SketchUp 13 years ago and was friends with the original inventors...so modeling a speaker box is something I can do while sleeping. 

Thank you again.


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## porscheman (Jan 1, 2012)

glassing the bottom of the enclosure will net more volume, both from a thinner material and by matching the contours of the cab exactly.


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## Viggen (May 2, 2011)

I would consider finding a different sub that can be used in a sealed box... then possibly get 4 of them if needed


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## ARCuhTEK (Dec 22, 2008)

Viggen said:


> I would consider finding a different sub that can be used in a sealed box... then possibly get 4 of them if needed


I already have that box designed and subs picked out (it is my existing install). In this train of thought, I am simply exploring what could be done in my vehicle if I chose to go a route other than sealed.


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## Theslaking (Oct 8, 2013)

Why not use 3? You have enough space for them.


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## ARCuhTEK (Dec 22, 2008)

Theslaking said:


> Why not use 3? You have enough space for them.


I do have room for three. It comes down to ohms and port length. It is possible...I think. I just need some more desk time to work through it. Probably post more this weekend on this....


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## Justin Zazzi (May 28, 2012)

If you're interested in learning more about the equations involved and how to calculate the values for port length and cross-sectional area vs all the other variables mentioned, pick up a copy of Vance Dickason's Loudspeaker Design Cookbook. The following is my interpretation of what he writes in the vented enclosure section.

You can trick any simulator into modeling a pair (or more) of woofers into a single vented cabinet by doing the following (this also works for sealed enclosures):

-Enter the same Fs and Qts and Xmax for multiple drivers as you would for a single driver.
-Multiply the Vas and Sd for each woofer you are using. For example if you want to use a pair of woofers, then double the Vas and Sd.

This should result in the net volume of the enclosure being multiplied by the number of drivers you're using. So for example, double the Vb if you are using a pair of woofers.

Then multiply the minimum vent cross-sectional area by the number of woofers you're using. The minimum recommended port area depends on both cone area (Sd) and excursion (Xmax). The next largest size port you can. If the minimum turns out to be a 2.68" diameter pipe, use a 3" pipe. There are slightly different minimum area formulas based on if you're using a square-edge port flush mounted, or a flared port on one end, or flared on both ends, etc.

To calculate the dimensions of the port for a pair of woofers, double the port's cross-sectional area. This does not mean double the diameter of the port however, since the area is related to the square of the diameter:
_(cross-sectional area of a circle) = pi * (diameter/2)^2_

The equation for the length of the port is more complicated and you'll need to rely on a simulation program to find it for you. It seems like the port length will be similar in length, but not the same.

I hope this is what you are looking for?

---

If you're still considering multiple smaller subs, maybe consider mounting them in pairs facing eachother with one woofer's polarity reversed. This would cancel out mechanical vibrations of the box, and you'll reduce the tactile cues that pull your attention towards whatever is physically vibrating.

You could also mount half the woofers facing inwards to the box and half facing outwards like normal. I hear this can cancel out some of the harmonic distortion, but I have not listened to such a setup and I'm not sure how worthwhile it would be.


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## ARCuhTEK (Dec 22, 2008)

Jazzi said:


> If you're interested in learning more about the equations involved and how to calculate the values for port length and cross-sectional area vs all the other variables mentioned, pick up a copy of Vance Dickason's Loudspeaker Design Cookbook. The following is my interpretation of what he writes in the vented enclosure section.
> 
> You can trick any simulator into modeling a pair (or more) of woofers into a single vented cabinet by doing the following (this also works for sealed enclosures):
> 
> ...


BEST answer yet! This is what I was looking for... THANK YOU! I am off to see the WIZARD!


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## Justin Zazzi (May 28, 2012)

I put on my robe and wizard hat....


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