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J-Lane

Slotted port area vs Circular port area

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I have only designed basic rectangular enclosures with a standard slotted style port. However a friend of mine wants me to design an enclosure that is more of a wedge shape. Due to the size of the enclosure and wanting to keep the tuning around 30-32 Hz I'm not able to really use a slotted port. When I increase the port area to the recommended 14-16 sq. in. of port area/1cu. ft. the port needs to be super long to achieve the required tuning frequency. Since I'm only designing it for him and not building it, I thought it may be easier to have him just build it with a circular port. I know I have seen some people say that with a circular port the recommended 14-16 sq. in. of port area/1cu. ft. is not as critical. What is the minimum recommended port area/1cu. ft. with a circular port?

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Generally speaking, I've found you can get away with up to 20% less port area with a round port compared to a slot port, but this is under ideal conditions. 

"Port area per cube" guidelines are flawed because they don't take input power or tuning frequency into consideration and those two things have a BIG impact on how much port area you need. If you give me some more info about what sub your friend wants to run, how much power, and what box specs you are shooting for I can give you my recommendation. 

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So basically these are his maximum dimensions he sent me...

5945c468e4c02_maximumboxdimensions.thumb.jpg.ea76c783166d24201dd3e7811500dc27.jpg

I decided a flared port is probably the best option. He has two JL 10W0v3-4 subs and JL recommends an internal volume of 1.125 cu. ft. I'm not sure why but I always like to give each sub it's own chamber. So using bass box pro I came up with this (this is just for one sub)....

screenshot.thumb.jpg.cc09edfbda8596d839cbd005531800b1.jpg

It has a flared 3" port 11" long giving it a tuning frequency of approximately 30 Hz (the same as what JL has their recommended design tuned to). However my box is slightly larger at 1.68 cu. ft. If I try to make the enclosure smaller I'm probably going to have issues getting the port to fit due to the flared ends and length. I'm wondering if I just put both subs in the same chamber and do one larger port in the middle. What do you think would yield better results, individual chambers or sharing the same enclosure with one large flared port (and what diameter port)?

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The box being too big isnt always an issue, you could run less power to the subwoofers and still achieve a nice result. I would personally if it was a box going in something I owned, shrink the box to 1.25 or so, and have it be tuned around 34-35 or even a bit higher (which shrinking the box will make the tuning rise). You can tune higher and still have great low end output, plus I doubt he will be playing in to the 20's with those subs, as most songs dont even get that low without editting. 

Edited by ChevyBoy95

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my experience with a large enclosure, even with proper port area is that cone control below tuning is shit. and output above tuning falls off quick. in other words, peaky.  lower volume with more port works well, but having a 45" long port is not practical. I would say, shoot for the middle on net volume and keep port area as large as you can go with the space you have. If you have port noise at tuning then you do. unless you are chasing numbers or competing most people wont even notice.

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Using one common chamber with a single larger port will be the most space efficient.  How much power is your friend going to be running?

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On 6/18/2017 at 0:48 AM, Triticum Agricolam said:

Using one common chamber with a single larger port will be the most space efficient.  How much power is your friend going to be running?

He said he'll be running around 150 watts per sub. I was thinking about doing a single chamber instead. This box is going in a trunk so I guess port noise isn't going to really be an issue. I had a decent design that I came up with where each sub has it's own chamber but after drawing it up in sketch-up I found out that with flared 4" ports I couldn't fit the sub and the port due to the cutout for the flared port. Since I guess port noise isn't an issue because it's in the trunk, would a 2" flared port be too small? Would there be performance issues? What is the smallest port you would recommend if both 10's shared one chamber?

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18 minutes ago, J-Lane said:

He said he'll be running around 150 watts per sub. I was thinking about doing a single chamber instead. This box is going in a trunk so I guess port noise isn't going to really be an issue. I had a decent design that I came up with where each sub has it's own chamber but after drawing it up in sketch-up I found out that with flared 4" ports I couldn't fit the sub and the port due to the cutout for the flared port. Since I guess port noise isn't an issue because it's in the trunk, would a 2" flared port be too small? Would there be performance issues? What is the smallest port you would recommend if both 10's shared one chamber?

Those subs are pretty sub-optimal for ported box usage, the output rolls off at a fairly high frequency. This does have one advantage though, the peak port velocity frequency is quite a bit below the tuning frequency.  

So lets assume you go with a common chamber box, 2.5 cu ft net tuned to 30 hz running on 300 watts.  Since the port peak velocity frequency is so low you could PROBABLY get away with as little as two 3" ports.  I wouldn't go smaller than that though because while you may not hear the port noise from the trunk, if you have port noise you have port compression, which means you are throwing output potential away. 

Now if it were me, based on the dimensions you posted, you should have enough room to run a side firing slot port along the top of the box above the subs,  You should be able to get at least 20 sq in of port area, which should make all your port problems go away.  

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