can someone explain how port velocity works

[akuma4u]

So the 5 i have in red circle is the same as 0.05 Correct?

W is acoustical power not electrical power so that result is invalid, that said those formulas are not as reliable as modeling curves, some 35-37 square inches of port area would be good enough for the 40Hz you want.

i see ok.. there was some confusion that arose because another user told me that sundowns recommendations are very off - purposely- so that the customer can buy the next best sub so the company can make more money. This user told the the 32 sq inches of port they recommend for 2 cubes would make me lose a lot of output and would not make the sub perform well. He told me that 60 sq inches was better to use...

so this caused me a lot of confusion... for my next enclosure i dont know whether to keep it at around 36 or 37 or to go as close to 60 as i can.. he said my port velocity was over 110 and need to be under 70 i believe - this is what got me researching port velocity ..

[JimboJizm]

So if the rule of thumb is 12-16 inches per cubic foot then how do you choose between that range?

[Triticum Agricolam]

i see ok.. there was some confusion that arose because another user told me that sundowns recommendations are very off - purposely- so that the customer can buy the next best sub so the company can make more money. This user told the the 32 sq inches of port they recommend for 2 cubes would make me lose a lot of output and would not make the sub perform well. He told me that 60 sq inches was better to use...

so this caused me a lot of confusion... for my next enclosure i dont know whether to keep it at around 36 or 37 or to go as close to 60 as i can.. he said my port velocity was over 110 and need to be under 70 i believe - this is what got me researching port velocity ..

60 sq in is an excessively large port, it would be nearly impossible to get the tuning down to where you want it to be with that much port area in a 2 cu ft box. As far as Sundown purposefully recommending undersized ports in the hope of upselling on subs, that's ridiculous. I've found Sundown's recommendations to be pretty reasonable assuming RMS input power.

So if the rule of thumb is 12-16 inches per cubic foot then how do you choose between that range?

Forget the rule of thumb. Its based on assumptions that are not always true. Because of this it won't always give you good results.

[fecupe2001]

port area needed is determined by your net volume not the subwoofer

Oh my!

[Krakin]

well ok what i meant by its all about port area is that following the rule will ensure your velocity isn't so high that it makes excess port noise most of the time. How ever could you go over 16 sq inch... i suppose so under certain circumstances but we could talk all day how nothing is really true about audio equipment under all circumstances.

Except the port per cubic foot of internal volume is never correct under any circumstance.

[Jessica]

well ok what i meant by its all about port area is that following the rule will ensure your velocity isn't so high that it makes excess port noise most of the time. How ever could you go over 16 sq inch... i suppose so under certain circumstances but we could talk all day how nothing is really true about audio equipment under all circumstances.

Except the port per cubic foot of internal volume is never correct under any circumstance.

you cant expect layman to workout how winisd works, you cant expect normal people to understand how port velocity works, it isnt practical. If guys can buy prefab boxes and be happy then what is the incentive to go out and buy $300 worth of tools, make 1300 mistakes and rebuild 84 times to have the experience to know what works? The port per cube formula is a very good starting point for most people. It isnt the best formula but it gets people interested and encourages them to try new things.

I say this because it worked for me. my first ported box was 14 per and it smoked my sealed enclosures i had built before.

Not trying to undermine what you said but its one of those better safe than sorry formulas. and it works quite well.

[akuma4u]

Would the following be true: the more excursion the sub has and the more power handling the sub has or more power that will be used then the port area must be bigger in relation to those conditions.

Im sure there is more to it but am i at least in the right direction?

[Jessica]

it is more complex than that. Go back to where i said ihad 4 12's on 3k and now i have 1 10 on 3k. completely different animals. there is no clear cut formula. like krakin said the whole ^2 per ^3 doesnt always work. i wish there was an easy way to explain it but there isnt.

[Triticum Agricolam]

Would the following be true: the more excursion the sub has and the more power handling the sub has or more power that will be used then the port area must be bigger in relation to those conditions.

Im sure there is more to it but am i at least in the right direction?

How much port area you need doesn't have much to do with excursion.

What does affect it is: box volume, input power, tuning frequency, and the parameters of the sub. There really are no GOOD shortcuts, using modeling software is the best thing to do if possible. Things like the "port area per cube" guidelines CAN work, but they are based on assumptions that may or may not be true in each scenario. If someone wants to use a shortcut like that, its important to know what those assumptions are. Since the "port area per cube" rule only factors in box size, its making assumptions about tuning frequency, input power, and sub parameters.

Things that are constant is that assuming everything else is the same, if you increase box volume, input power, and/or tuning frequency = you need more port area.