# Box tuning question



## _Cynic_ (May 8, 2021)

If I have a subwoofer enclosure that is 7.4 cubic feet after all displacements besides the port how would I go about calculating how big my port is if the port itself is supposed to be subtracted too? At 7.4 ft cubed my port size 3” by 24” by 13.6” long but that is calculated without the port being subtracted from the volume the subs will be in. So does that mean I have to subtract the port volume then retune the port at 6.82 cubic ft? 7.4- port (0.5666)=6.8 so new port would be 16.22” long at 35 hertz then I’d have to redo the port displacement in my calculations so 7.4- port(0.68)=6.72 then do the damn tune again? I feel like I’m doing circles. So new port length after third try at tuning would be 16.57” long which would mean the displacement changes yet again to 7.4- port (0.69)=6.71 then port changes again to 16.6” long which it seems to me the stupid thing is finally tuned. Is my thinking correct or am I just a dum dum dummy doing circles in my math for no reason and the first calculation was the right one? At first I thought the port was included in the box volume but it doesn’t look like it by what I’m reading on the web. Any input would be greatly appreciated; thanks.


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## _Cynic_ (May 8, 2021)




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## Grinder (Dec 18, 2016)

You're on the right track, just overthinking it a bit. While I'm no expert, I've played around with vented sub alignments a fair bit, and despite precise calculation and construction, I have yet to hit target tuning frequency (Fb). I always end up around 1 - 2 Hz lower than target/calculated/modeled Fb. You're not going to hear a difference if you're off your target by one or two hertz, so don't sweat it too much.


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## _Cynic_ (May 8, 2021)

I’d have it built already but I’m overthinking the design too lol. It’s going behind the back passenger seat of my truck and I was going to have the subs face the door and port face the drivers side in front of the backseat so the port is centralized to the subs but I kind of would like to have the port under the seat on the bottom of the box facing the drivers door. I just feel like that would mess with the sound though. Then I was thinking mounting one sub on the top of the box and the other sub inverted facing the passenger seat but that might be a little too crazy.... I’m going to drink a few more pints and read a some more before I put the trigger yet.


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## Grinder (Dec 18, 2016)

Be sure to provide plenty of space for air movement near the opening of the port. Nearby surfaces can alter tuning (when acting as an extension of the port) and restrict output. Ideally, we might try all practical sub/port orientations (down/up/side/front/rear firing) to see which sounds/performs best in our vehicle. Short of that, others with the same or similar vehicle may be able to point you in the right direction.


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## _Cynic_ (May 8, 2021)

That’s why I was going to have it just in front of the backseat facing the drivers door cause it will have a ton of space for the port but the passenger back door might take a beating from the subs facing it. Wasn’t thinking it would mess with the tuning having the subs close to the door though. Vehicle is a 2017 Denali hd and subs are 2 kicker 15 L7s...I’ve had them in a Tahoe with 6-7 cubic feet per sub and in a Ford Fusion with like 3.2 cubic ft per sub in the trunk till the weight broke the stupid transmission lol... both boxes blew up so hopefully third try I’ll make a box strong enough


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## Theslaking (Oct 8, 2013)

Quite often loading off the back wall is best. If I only had one try I would face it off a back wall. 

It is true though. The box orientation varies vehicle to vehicle.


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## _Cynic_ (May 8, 2021)

I don’t see an option to face it off the back wall without messing with the port tuning but I could try and put the port underneath the backseat... read somewhere on here that a port closer to the floor would help with backloading whatever the hell that is... I just know my first option I might have to wear a hoodie or ear plugs so I don’t blow my ear drums out. Then read everyone likes the subs facing forward so that could definitely be an option; but I might make something to protect the woofers from hitting the seat. I just have never made a box with the port on the side so I don’t know if it’ll mess things up. Always have made the boxes like my option one but the subs directly opposite to the side of the port.


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## _Cynic_ (May 8, 2021)

Damn then there’s some guy on here saying you can put the port on the outside.... I could save .75 cubic feet doing a 10” wide by 8” tall by 15.84” long exterior port that goes under the seat. Anyone ever done this ? Is it recommended? I’m just looking for maximum airspace possible


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## Grinder (Dec 18, 2016)

Yeah, exterior port is a great option. There's no reason at all for the port to necessarily be located within the box. If you can more easily fit the port outside the box, go for it. Exterior ports are not at all uncommon.


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## _Cynic_ (May 8, 2021)

Hell yeah thanks grinder. I’m going to go over my math and finish cutting the port out and kreig jig this bad boy together
Edit; and thanks too slackKing; this way the port will be next to the wall and floor too. Should work out nice


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## _Cynic_ (May 8, 2021)

I honestly never thought about it but would round ports produce better sound waves since the waves are supposed to be round? I mean like my square subs are definitely not that good of sound quality compared to some badass round ones but they work for great for spl. I measured the space under my backseat and I’m limited to around 4.5” tall so I can only do a 3” port at 18.5” wide and that turns out to be 9.95” long. I bought a prefab box way way back and it sounded like there was a monster in the back with all the horrible turbulence and I’m hoping my port is big enough to not make some nasty ass turbulence. But that could be due to the fact the box was like less than 2 cubic feet per sub... way under the recommended space for them. If I did round ports though it might not have the back seat extend the port like a slot port and change the tuning. Idk just throwing ideas out there. I’ve heard clean bass and I’ve heard my loud ass bass maybe the round ports will clean the bass up a bit. Or maybe square subs create square sound waves? Haha.... drinks beer... definitely pros and cons to square subs; more surface area and apparently according to kicker website they work together nicely.


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## _Cynic_ (May 8, 2021)

So if I were to calculate a round port I’d just put 8” round in the calculator and 35 hertz at 7.58 cu ft volume which turns to 8.74” long then divide that in half for two 4” round ports at like 4.37” long. Then go buy some pvc pipe from Home Depot and get er done?


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## Grinder (Dec 18, 2016)

Port area is what is important, not port shape, the only exception I'm aware of being slot ports whose cross-section consists of one dimension that is, say, eight or more times greater than the other dimension (for example: 1" x 8").

I believe round ports are most efficient per port area.

I do not believe that port shape alone colors the sound.

Two 4" round ports have a combined area of 25.14 in^2, while one 8" round port has an area of 50.27 in^2. Here's an area of circle calculator: Circle Calculator

<edit> In your example, two 4" ports are roughly half the length of one 8" port because two 4" ports are roughly half the area of one 8" port. Given the largish cabinet volume of 7.58 ft^3, I highly doubt two 4" ports will be adequate to prevent port noise, let alone port compression (i.e. loss of output potential), which begins far below the port velocity threshold for port noise. If you seek maximum SPL potential, you'll want to minimize port compression by keeping vent velocity below ~22 m/s. If you don't mind some compression and are mostly concerned with port noise, you can go as high as 30 m/s or so, or as high as 40 m/s or so with an adequately flared port such as this: Home

You can use WinISD free modeling software to determine port velocity among a number of other useful things: WinISD - Linearteam

Short of modeling software such as WinISD, here is a handy port area calculator: Triticum's Improved Port Area Calculator V2


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## _Cynic_ (May 8, 2021)




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## Grinder (Dec 18, 2016)

With the make and model numbers of your subs and amp, assuming I can find T/S specs for your subs, I can model them for you in WinISD and get back to you later today or tonight. Modeling will produce much more accurate and reliable vent area recommendation.


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## _Cynic_ (May 8, 2021)

Damn that seems like a lot of work for you... I clicked on the trig icon first would that winISD link get me there to do it myself? I don’t think I’m going to be entering any competitions but I’m running two 08 kicker 15L7s and a kicker zx1500 amp and the birthsheet said 1637 watts. I honestly was just lost on how to calculate a 96 square inch port. I can go get the make and model numbers though if you want me to. Learned a lot already I’m down to learn more... could probably start a subwoofer enclosure business with the amount I learned this morning lol


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## _Cynic_ (May 8, 2021)

I like the trig icon one because it’s in google spreadsheet the winISD I’m a little scared to download lol
Edit; so the square inches of port means how big the hole is not how long? I can do like 18.5” wide by 3” tall which is only 55.5” squared so I need to figure out how to make a larger port opening?
Example 18.5” by 4” = 74” squared
18.5” by 5”= 92” squared which would be close to optimum


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## Grinder (Dec 18, 2016)

Fear not, at the WinISD/Linear Team link you can download the modeling software for free (download "the latest version"). There can be a bit of a steep learning curve initially, but it's well worth a couple/few hours to get acquainted with it. Enter driver T/S specs in the following order, and let WinISD autofill everything else (or any unavailable specs): Qes, Qms, Fs, Re, Vas, Xmax, BL

For the time being, I would be happy to model this for you later today or this evening. It's not an elaborate process, I'm just in the middle of a few things right now...


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## Grinder (Dec 18, 2016)

_Cynic_ said:


> I like the trig icon one because it’s in google spreadsheet the winISD I’m a little scared to download lol
> *Edit; so the square inches of port means how big the hole is not how long?* I can do like 18.5” wide by 3” tall which is only 55.5” squared so I need to figure out how to make a larger port opening?
> Example 18.5” by 4” = 74” squared
> 18.5” by 5”= 92” squared which would be close to optimum


Yes, port area is simply the cross-section (area) of the port. Yes, a port with a cross-section of 18.5" x 3" is 55.5 square inches (in^2). Yes, 18.5" x 4" would be 74 in^2 (and so on...).


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## Grinder (Dec 18, 2016)

Before we conclude that 92 in^2 is optimum, let's see what WinISD says.


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## _Cynic_ (May 8, 2021)

I’m glad I didn’t cut anything last night lol I don’t think the port can go under the seat I’ll just have to do my original plan of having the port in front of the seat in my option 1. I don’t think I have the books for my subs and amp but it looks like I have a 2010 zx1500 watt amp and 2- 08 kicker 15 L7s unless I bought a 2011 one to replace the one I blew up from being a dumb dumb dummy and doing a smaller ground wire than a power wire to my amp. I’ll scour these subs till I find the model numbers


edit the subs are both 11S15L72 
And the amp is a 2010 zx1500.1


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## Grinder (Dec 18, 2016)

This will help you get started with WinISD:


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## Grinder (Dec 18, 2016)

While the only T/S specs I found for the 11S15L72 are Vas, Fs, Qts and Xmax (at: Kicker 11S15L72 Solo-Baric L7 Series 15" subwoofer with dual 2-ohm voice coils at Crutchfield ), WinISD was able to produce some useful results.

Two 11S15L72 in 7.4 ft^3 (net volume, with external port), tuned at 35 Hz, 1,500 watts RMS: Two 6" tube vents (56.55 in^2 vent area) x 12.35" long = 21.3 m/s. Peak output 125.67 dB at 49 Hz (+4.87 dB peak), 30.7 Hz F3 (-3 dB roll-off).

For SQ (flatter response and lower extension), this sub seems to prefer a much larger cabinet and much lower tuning.

For SQ in 7.4 ft^3, I'd tune as low as 26.5. For a practical compromise of vent length, vent velocity (vent noise) and output (vent compression), I'd go probably go with one 6" flared vent at 10.21" (42.3 m/s peak vent velocity) tuned at 26.5 Hz, for an F3 of 28.3 Hz. If I had room for the extra vent length, I might go with two 6" vents x 24.8" (21.1 m/s peak vent velocity), 123.3 dB at 50 Hz (+2.43 dB peak).


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## Grinder (Dec 18, 2016)

PS: You'll want to wire each sub's 2-ohm voice coils in series with each other, and then wire the two subs in parallel with each other, for a combined load of 2 ohms. Like this:










Because of an effect called "back EMF," do not wire each sub's 2-ohm voice coils in parallel with each other, and then wire the two subs in series with each other. While this configuration will also produce the desired combined load of 2 ohms, series connection between two or more subs will produce an unwanted "back EMF" effect.


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## Grinder (Dec 18, 2016)

Grinder said:


> While the only T/S specs I found for the 11S15L72 are Vas, Fs, Qts and Xmax (at: Kicker 11S15L72 Solo-Baric L7 Series 15" subwoofer with dual 2-ohm voice coils at Crutchfield ), WinISD was able to produce some useful results.
> 
> Two 11S15L72 in 7.4 ft^3 (net volume, with external port), tuned at 35 Hz, 1,500 watts RMS: Two 6" tube vents (56.55 in^2 vent area) x 12.35" long = 21.3 m/s. Peak output 125.67 dB at 49 Hz (+4.87 dB peak), 30.7 Hz F3 (-3 dB roll-off).
> 
> ...


TLDR: Your proposed 18.5” W x 3” H x 22.51” L external port, at 55.5 in^2 and 21.5 m/s, should produce minimal port compression, and zero port noise.

Also, I forgot to mention that the limited T/S specs were unfortunately not sufficient to allow WinISD to produce excursion results. This is important to predict conditions at which the subs might reach or exceed mechanical limits (overexcursion), for example when the cones unload below tuning frequency.

According to this link ( Kicker ZX1500.1 Class D 1500 Watt Amplifier 2 Ohm Stable - 08ZX1500.1-RS ) your amp has a 25 Hz subsonic filter. While I have no idea if that is true, or how that filter might be implemented, it seems unlikely that a 25 Hz filter would be adequate to prevent overexcursion as the subs unload below your 35 Hz tuning. If you have DSP, you could simply set up a high-pass at somewhere around 30 Hz that would do the trick. If your amp has a 25 Hz subsonic filter, and you have no DSP, that would be another reason to consider going with a lower tuning frequency, say 28 - 30 Hz.


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## _Cynic_ (May 8, 2021)

Would it be possible to do a port that isn’t square... like 3” on one side by 6” on the other by 18.5” wide for a port area of 83.25in^2 it would be optimal for my box with 7.6 ft^3 tuned @ 30 hz but I have no clue on how long to make the exterior port because the tuning calculators only do squared ports. Is there a factor to multiply by the area to get the port tuned at 30 hz? Sorry I’d have it done and pictures up of the finished box but life’s been crazy lately. Hoping to put it together this weekend


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## Grinder (Dec 18, 2016)

_Cynic_ said:


> Would it be possible to do a port that isn’t square... like 3” on one side by 6” on the other by 18.5” wide for a port area of 83.25in^2 it would be optimal for my box with 7.6 ft^3 tuned @ 30 hz but I have no clue on how long to make the exterior port because the tuning calculators only do squared ports. Is there a factor to multiply by the area to get the port tuned at 30 hz? Sorry I’d have it done and pictures up of the finished box but life’s been crazy lately. Hoping to put it together this weekend


I don't see why not. You can use a port length calculator just as you would for a square or rectangular port, by simply entering two dimensions that equal the area you want (83.25in^2 in this case), and then construct your 3" x 6" x 18.5" port at the calculated length (which is entirely based on port area and net cabinet volume).


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## _Cynic_ (May 8, 2021)

Grinder said:


> I don't see why not. You can use a port length calculator just as you would for a square or rectangular port, by simply entering two dimensions that equal the area you want (83.25in^2 in this case), and then construct your 3" x 6" x 18.5" port at the calculated length (which is entirely based on port area and net cabinet volume).


Hell yeah so i put in the calculator 
15” x 3” at 7.6 ft3 box at 30 hz and it’s 12.1929” long port and 
6.1846” x6.1846” which makes the total area of both 83.25”^2 and its 9.965” long so together they make 22.1579” long port for my janky rectangular port that leaves me with optimal area for port noise and compression and it’ll fit under my backseat. Thanks you’ve been a great help


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## Grinder (Dec 18, 2016)

It's late and I'm tired, so maybe it's me... I'm having trouble making sense of all those dimensions.

However, simply in terms of 83.25 in^2 (6" x 13.875", for example) port area for a 7.6 ft^3 box at 30 Hz, I'm getting a length of 18.33".





Port Length Calculator


Port length Calculator for sub woofer box design.



www.mobileinformationlabs.com





EDIT: Just noticed when I change "slotted port" from yes to no, the length increases to 25.27". Not at all sure what to make of that.

I'll check WinISD and see what it says.


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## Grinder (Dec 18, 2016)

In WinISD, at 83.25 in^2 (6" x 13.875", for example) port area for a 7.6 ft^3 box at 30 Hz, I get a length of 25.13".


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## _Cynic_ (May 8, 2021)

Thanks for your time grinder you rock
Lol I must be tired too I got the same thing with those dimensions... I thought you meant putting in two different areas that equal 83.25”squared.... it’s been a long week being down a guy in the brush and next week I get a promotion and lose the rigging slinger so I don’t expect it to get better haha have a great night thanks for your help


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## _Cynic_ (May 8, 2021)

Figured I’d post some pictures. Definitely added a **** ton of pocket screws like over 100... went to steal some out of my old box and a bunch were broke in half from box excursion... might just go buy some wooden dowels instead of using mdf. 
Thanks for the help grinder


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