Port area, cone area vs. box volume

[rambronco]

At which point does cone area of the subs take priority over keeping with X sq. In. of port area per cu. ft. of internal box volume? I'm getting ready to build a large 4th order for 2 18's where the ported side box volume would make the area of port larger than I would think necessary, even possibly large enough to start degrading performance? Im looking at approx 25 cu. ft. of ported box which staying with the usual guidelines puts me at approx 375 sq. in. of port, seems excessive for 2 18's. What do you guys think?

[CleanSierra]

Any would a big port(if proper for the subs), degrade performance?

Are you thinking square inches per foot rule of thumb is the way to go about designing a bandpass enclosure?

[alaskanzx5]

Sq" per cuft rule is kind of a bad rule of thumb.

Port area has nothing to do with box volume. It's about the subs specs.

I'm still learning but a sub with higher xmax and cone area needs more port area. So say you have 2 different 10" subs. Birth can work good in a 1.5cuft box. Sub a has a lower xmax and needs 15-20" of port. Sub b in that same box with the same port area has a higher xmax and can create port noise and have a smaller range in freq response bear the port isn't adequate. So sub b needs a larger port of 25-30".

Now from my understanding of 4th order bandpasses you typically want a larger port then what the subs would need for a regular ported box.

[rambronco]

Any would a big port(if proper for the subs), degrade performance?

Are you thinking square inches per foot rule of thumb is the way to go about designing a bandpass enclosure?

Not at all, but to design the box I need to factor in port area for port displacement to adjust box volume and ported box dimensions. My original design had me at about 225 sq. in. of port, which is pretty close to half of the cone area of the subs, and now looking at 375. I guess I can throw some numbers into a formula for "vent mach" and go from there. I had never seen it and was just curious if there is a rule of thumb for largest port area to cone area ratio.

[Joe X]

At which point does cone area of the subs take priority over keeping with X sq. In. of port area per cu. ft. of internal box volume? I'm getting ready to build a large 4th order for 2 18's where the ported side box volume would make the area of port larger than I would think necessary, even possibly large enough to start degrading performance? Im looking at approx 25 cu. ft. of ported box which staying with the usual guidelines puts me at approx 375 sq. in. of port, seems excessive for 2 18's. What do you guys think?

Nice thing about 4th bandpass is high tuning (and large ported chamber) makes port displacement small even with large port area, a 380 sqin port tuned around 45Hz on 25 net would only need to be about 4" deep, that's only about 0.8 cubic feet of total displacement, so basically nothing to care about.

[rambronco]

At which point does cone area of the subs take priority over keeping with X sq. In. of port area per cu. ft. of internal box volume? I'm getting ready to build a large 4th order for 2 18's where the ported side box volume would make the area of port larger than I would think necessary, even possibly large enough to start degrading performance? Im looking at approx 25 cu. ft. of ported box which staying with the usual guidelines puts me at approx 375 sq. in. of port, seems excessive for 2 18's. What do you guys think?

Nice thing about 4th bandpass is high tuning (and large ported chamber) makes port displacement small even with large port area, a 380 sqin port tuned around 45Hz on 25 net would only need to be about 4" deep, that's only about 0.8 cubic feet of total displacement, so basically nothing to care about.

I'm sure some people will disagree with my build but, people are people and I might be a little out of the norm when it comes to tuning. My port will end up coniderably longer than 4" if that is what you came up with for 45hz tuning. I have a very narrow bandwidth at which I like my subs to play, I have my x-over set at 50hz and although I do not have the midbass for the system to intergrate seemlessly, I prefer that sound over subs playing at a higher x-over point. With that being said, I am able to do a large 4:1 ratio 4th and for my tastes it is still completely musical. I realize for the majority of people 45hz on a 4th would be a good tuning frequency, the sealed side providing the low end response would allow you to play down into probably the high 20's with some authority. I am looking at 36-39hz for tuning (still need to throw some more numbers around) which will increase port length.

With all that has been said the original question still has not been answered, is there any relation from cone area to port area that can be given as a general rule as farvas dont go under "this" or dont go over "that"?

[alaskanzx5]

You can determine port area based on come area and xmax. I forget the formula but the formula is also based on pushing the subs to full xmax. A lot of high xmax subs don't typically reach full xmax before reaching thermal power limits though.

[Triticum Agricolam]

Model the box in the simulation software of your choice based on the power you are going to run. Keep port velocity under 17 m/sec. That's my suggestion.

[rambronco]

You can determine port area based on come area and xmax. I forget the formula but the formula is also based on pushing the subs to full xmax. A lot of high xmax subs don't typically reach full xmax before reaching thermal power limits though.

Thank you, that was what I was refering to. I was just hoping that after designing a box there was just a formula you could use to double check the port volume, just as a secondary check.

[rambronco]

Model the box in the simulation software of your choice based on the power you are going to run. Keep port velocity under 17 m/sec. That's my suggestion.

For some reason my vent mach isnt working in Win Isd, I may just try and update or remove & reinstall