# The Real Port Formula



## jjearl (Jun 18, 2013)

I am troubled by all the different port formula equations surfacing today all spitting out different answers so I came here to ask you. Why are they all different. Does the qtc change with them or whats going on! Before I spend hundreds building boxes, I want to make sure I'm doing it right. Here are all the ones I've found. 

Ported Vented Box Equations Formulas Design Calculator Lv Port Vent Length

Subwoofer Enclosure Calculators, Fraction to Decimal, Parallel, Series, Port Length and Volume Calculators

JL Audio » header » Support » Tutorials » Tutorial: Enclosure Ports

Car Audio - PORT DESIGN FORMULAs for SUB WOOFER ENCLOSUREs

and after downloading the loudspeaker design cookbook 7th edition on page 81 it says the port formula is lv=((1.463 x 10^(7) x R^(2)) / (fb^(2) x vb)) - 1.463R

Lv= Port Length inches
Fb= Tuning frequency hz
Vb= Box Volume inches cubed
R= Port Radius inches.

From my math almost all of them are different.. WTF?


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## minbari (Mar 3, 2011)

how much different are they? 

Some vent calcs take into account the interface between the port and the air its going into (flared? straight? slot?) not sure it makes enough difference to matter. if you tune it for 28.2hz instead of 28, will you hear it?


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## jjearl (Jun 18, 2013)

I'm talking basic round ports non flared. They can be anywhere between 3-13 inches off and from what I know 1 inch = 5hz difference correct me if I'm wrong.


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## cubdenno (Nov 10, 2007)

Also to note, make sure the calculator takes into account the total woofer displacement capability and the power that you are going to use.

Port requirements are different if you are using a 10 inch woofer with 5mm of xmax and 200watts ad a 10 inch sub with 20mm of xmax and 1500 watts. even in the same volume enclosure and tuning desire

The former may only require a 3" ID port while the latter may require a 5"ID port to keep air speed in the manageable area.


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## minbari (Mar 3, 2011)

jjearl said:


> I'm talking basic round ports non flared. They can be anywhere between 3-13 inches off and from what I know *1 inch = 5hz* difference correct me if I'm wrong.


I cant beleive they are all off that much. 1" = whatever the volume of the enclosure and diameter of the port determines. there is no way to make a statment like that.


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## jjearl (Jun 18, 2013)

Yeah Vent air velocity is a whole different formula and its really good to take into account. I'm just looking for a port formula that tells me length for my given tuning frequency and box volume and port diamiter. A qtc would also be nice but i doubt it exists in a port formula built in anyways.


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## minbari (Mar 3, 2011)

Now that I have some time to explore this, this is what I got:
WinISD:

2cuft box
(1) 4" port
tuned to 28hz
.614 end correction.

19.05" long


> Ported Vented Box Equations Formulas Design Calculator Lv Port Vent Length


.614 end correction
19.11" long
WinISD says this is 27.96hz


> Subwoofer Enclosure Calculators, Fraction to Decimal, Parallel, Series, Port Length and Volume Calculators


no end correction
18.67" long
Winisd says this is 28.25 hz


> JL Audio » header » Support » Tutorials » Tutorial: Enclosure Ports


End correction for JL was .823
18.27" long
Winisd says this is 30.38hz

this one is off the most soo far, but they have the largest end correction too.


> Car Audio - PORT DESIGN FORMULAs for SUB WOOFER ENCLOSUREs


no end correction
18.67" long
Winisd says it is 28.25hz


> and after downloading the loudspeaker design cookbook 7th edition on page 81 it says the port formula is lv=((1.463 x 10^(7) x R^(2)) / (fb^(2) x vb)) - 1.463R
> 
> Lv= Port Length inches
> Fb= Tuning frequency hz
> ...


so really the only difference is that some of them use end correction and some dont, which was my guess from the start.


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## jjearl (Jun 18, 2013)

Oh.. My math must have been off then (or I messed up my units), thanks I really appreciate your help! Do you figure the one with the from AJ is the most spot on? Because its also my favorite one to work with.


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## minbari (Mar 3, 2011)

jjearl said:


> Oh.. My math must have been off then (or I messed up my units), thanks I really appreciate your help! Do you figure the one with the .614 is the most spot on? Because its also my favorite one to work with.


it might be, its the default end correction for winISD, which is what I usually use to model with.

TBO, if you are within 1 hz of your target, I bet you cant tell the difference. you can always decide to err on the side of too long and cut-to-tune.


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## thehatedguy (May 4, 2007)

1 hertz...I'd say more than that. Especially if you are using generic published specs and not measuring the speakers you own.


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## jjearl (Jun 18, 2013)

thehatedguy said:


> 1 hertz...I'd say more than that. Especially if you are using generic published specs and not measuring the speakers you own.


Generic published specs, are you talking about thiele smalls?


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## minbari (Mar 3, 2011)

thehatedguy said:


> 1 hertz...I'd say more than that. Especially if you are using generic published specs and not measuring the speakers you own.


you are probably right. in the past I have always made my ports a little too long and trimmed to tune. not hard if you use round ports.


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## Oscar (Jun 20, 2010)

thehatedguy said:


> 1 hertz...I'd say more than that. Especially if you are using generic published specs and not measuring the speakers you own.


I don't understand why you say this. Port tuning is not dependent on T/S parameters. F3 _would_ be impacted by not knowing T/S parameters (or anything having to do with any part of the frequency response), but not enclosure tuning. Enclosure volume, cross sectional area, vent length, end correction is all that is needed. Unless you're trying to get at something else?


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## thehatedguy (May 4, 2007)

This



cajunner said:


> seemed like he was saying don't be surprised if your box ends up more than 1 hz off of the box calculator's tuning result using the factory provided T-S parameters.


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## cubdenno (Nov 10, 2007)

The box calculator should not take into account the T/S specs unless it is a modeling program. For just a box calculator, all it is concerned about is cubic volume, port length and port area for X tune.


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## Oscar (Jun 20, 2010)

cajunner said:


> seemed like he was saying don't be surprised if your box ends up more than 1 hz off of the box calculator's tuning result using the factory provided T-S parameters.


Again, T/S parameters do not determine Fb, enclosure tuning via the Helmholtz resonator. F3 is impacted (because it is part of the response curve) by incorrect T/S parameters. Fpeak is impacted (because it is part of the response curve) by incorrect T/S parameters. In fact, the entire response curve is impacted by varying T/S parameters. No speaker can shift Fb in any way shape or form (port compression and port Q losses aside). It [Fb] is strictly determined by the enclosure and port characteristics. Looks like some people need to hit the books...find me one T/S parameter in the equation(s) that calculate port length (hint: you won't)  

Don't believe me? Use your favorite modeling program and set up an alignment---any alignment where you can easily see the excursion minimum that corresponds to Fb. Now lock the Vent dimensions and don't change the Vb. Now shift one, or even all the T/S parameters (except Sd, which is strictly a physical parameter----play with Qes, Qms, Qts, Fs, Re, Le, Vas as much as you want). Now go back and look at the Driver Excursion graph---it will have shifted at many places, but the dip that corresponds to Fb will still be at the same frequency on the X-axis.


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## minbari (Mar 3, 2011)

^^^ yup, once a box is tuned, you can put an 8" or 15" in there, the box tuning wont change

sent from my phone using digital farts


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## Oscar (Jun 20, 2010)

minbari said:


> ^^^ yup, once a box is tuned, you can put an 8" or 15" in there, the box tuning wont change
> 
> sent from my phone using digital farts


Thanks for confirming me. I knew I wasn't crazy. I think a lot of people now a days that didn't start with books and equations, think that box tuning frequency is dependent on _everything_ that is input into modeling software/programs including speaker parameters, when it's just not the case.


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## Brian Steele (Jun 1, 2007)

You are worrying too much about this. The correct approach to building a vented box is calculate->build->measure->adjust. You'll likely have to adjust the vent length a bit anyway to get it right. So, design and build your box taking this into consideration.


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