# Port length effects how?



## JAX

In laymans terms how does changing the port length effect the sound? 

I should know this bit I have been so used to prefabbed boxes. I got one built with a 3 inch port . Box itself is bigger than what was listed in the manual but my port length is the same. 


It is an adjustable port so I can change it. I just don't know what changing it does. 

Longer port=? Shorter =? 

That kind of answer. 

And since I mentioned it. How does size effect the design If its bigger than the recomended?


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## dragonrage

Longer port = lower tuning. Shorter port = higher tuning. What tuning to use depends on several factors (speaker, box, what kind of response curve you're going for, etc.) and it significantly changes the sound.


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## voltij

Box design basics:

VARYING BOX VOLUME
- Bigger boxes require shorter port (same port area) to hit same tuning
- Bigger boxes with same port length and area will lower tuning

VARYING TUNING
- To lower tuning, make port longer (with same area) but note this will decrease box volume (more displacement due to port)
- To lower tuning, make port area smaller (with same length) but note this will increase box volume and may lead to chuffing/port noise if you make port area smaller than recommended
- To lower tuning, make box bigger (with same port area and length)

In your situation (variable port length) if the port extends into the box, if you want to know what length yields what tuning make sure that you count the volume displaced by the port as you make it longer.

AKA if the shortest port makes the box 4 cubes at 38 hz, a longer port would be more like 3.5 cubes at 33 hz. (rough guesses)


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## cubdenno

don't forget that increasing length of an interior port decreases available box volume which affects the tuning freq.


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## sqshoestring

If you have a given box and can change port length only, longer is lower and short is higher. Note that lower you tune the less output you get; the port makes a peak in response that will be larger the higher (and more 'SPL') you tune it. Also you should run a subsonic filter to take out under tuning frequency if you tune high and run a lot of power because the sub will xmax under tuning when its unloaded. You can use that to figure out what the tuning is by playing tones. The sub will play above tuning, the port will play at tuning and sub plays/moves less, below tuning the xmax goes up on sub and the port starts huffing air not much sound.


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## JAX

What about bigger port? If I put in bigger port can it then be shorter? I found a piece of some piping at work that looks just like a port with a flange but its like 5 inches wide.


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## JAX

Well I guess I need to find out displacement of sub and port and work from there. I was just wanting to verify what I thought was the relation of port and length. 


My box is bigger than manifest recommended but my port is same length only cause it's currently stuck at that length


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## voltij

Generally when I'm making my boxes, I design them on paper or on the computer, THEN i measure the effective volume and port area and port length. Then calculate tuning, and if it's not right, I go back to square one and start all the way over.

If you try to change just one parameter of a designed box you're likely to affect others significantly.


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## mikey7182

The easiest way to take all the guesswork out of designing a ported enclosure from scratch is to determine what you want your net volume and tuning frequency to be and work backwards. Say you want an enclosure that's 4cf net tuned to 38hz and you're going to use a single 6" Precision Port. Figure out how long the vent needs to be in 4cf to achieve a 38hz tune, then calculate how much volume that length of 6" port will displace. Let's say it displaces .63cf. Add to that the displacement of the woofer and any bracing (let's say another .4cf), and you now know how large your enclosure needs to be (5.03cf gross volume).


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## dragonrage

JAX said:


> What about bigger port? If I put in bigger port can it then be shorter? I found a piece of some piping at work that looks just like a port with a flange but its like 5 inches wide.


A bigger port needs to also be longer to have the same tuning.


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## JAX

mikey7182 said:


> The easiest way to take all the guesswork out of designing a ported enclosure from scratch is to determine what you want your net volume and tuning frequency to be and work backwards. Say you want an enclosure that's 4cf net tuned to 38hz and you're going to use a single 6" Precision Port. Figure out how long the vent needs to be in 4cf to achieve a 38hz tune, then calculate how much volume that length of 6" port will displace. Let's say it displaces .63cf. Add to that the displacement of the woofer and any bracing (let's say another .4cf), and you now know how large your enclosure needs to be (5.03cf gross volume).



thats the problem. I dont know what I want it to be or whatever..ha ha...I always build sealed or buy sealed. Only had 1 ported box in my life and I dont have it any more. 

jbl box was calling for 1.25 cube and a 8.5 inch x 3 port. my box is bigger than that but not sure by how much...in the neighborhood of 1.70 but I need to remove the sub and the port area. 

I am going to install the sub in a minute while on lunch and see how it works as it is.

it has to be better than the way it was in a small .70 cube box sealed.


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