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View Full Version : Is xmax directly related to port area?

amartin_72
01-27-2008, 10:10 PM
I have come to the conclusion from all my building expirience that when I have a low xmax sub they seem to like alot less port area than the high xmax ones. So hence would my proposed theory of using xmax in mm * .75 for inches of port area per cubic foot be a good starting point for my future builds? Thanks jw what some guys who really know the effect and requirments of port area are.

Ex: An FI SSD 15. 21mm xmax, 4ft @ 34hz = 63 in port area
ehqs12 9mm xmax, 2.5ft @ 33hz = 17in port area

I dont really know about this it doesnt seem to work out too well, but then what about a speaker really determines how much port area you should give it?

bonesninja
01-27-2008, 10:12 PM
short answer = yes

amartin_72
01-27-2008, 10:18 PM
So is there a general rule of thumb that we can use or what?

JimJ
01-27-2008, 10:19 PM
short answer = yes

How so? :)

mjf
01-27-2008, 10:21 PM
atleast compare two subs of the same size........................

bonesninja
01-27-2008, 10:21 PM
my guess is that it has something to do with the larger amount or air displaced with the larger excursion.

MyFartsStink
01-27-2008, 10:27 PM
Not to thread jack, but I think I might have this problem. I got an mt 15, and the port is less than 5" deep port area is 12". Is this kind of like what he is saying? Would this cause a problem?

bonesninja
01-27-2008, 10:28 PM
12" of port for an MT? you might as well have a sealed box that leaks real bad

amartin_72
01-27-2008, 10:29 PM
haha yeah i was looking at your avatar and hoping that was done in MS paint or photoshopped

Tehgregzor
01-27-2008, 10:30 PM
MyFartsStink....you could SERIOUSLY increase the output of your MT by constructing a new box. How much space does your box have?

MyFartsStink
01-27-2008, 10:32 PM
12" of port for an MT? you might as well have a sealed box that leaks real bad

I did the math to tune it to 34 hz. Checked online calc, it said its tuned to 33. So i dont know. Is it easy to knock that other port out and put a new one in?

MyFartsStink
01-27-2008, 10:33 PM
MyFartsStink....you could SERIOUSLY increase the output of your MT by constructing a new box. How much space does your box have?

4 cu ft. And I was instructed to do the porting this way, I was not sure about it, but I want to know how it sounds first.

MyFartsStink
01-27-2008, 10:33 PM
haha yeah i was looking at your avatar and hoping that was done in MS paint or photoshopped

vinal wrapped

amartin_72
01-27-2008, 10:33 PM
wow this is an emergency man that looks like a decent box you built there except for the major problem with the port, sorry to say but you need a new one.

bonesninja
01-27-2008, 10:34 PM
try like 80-100 of port and see what sounds like...and no, its not that easy to just change the port on an already constructed box

MyFartsStink
01-27-2008, 10:36 PM
try like 80-100 of port and see what sounds like...and no, its not that easy to just change the port on an already constructed box

:( Time to sell it to a hood rat for 100

amartin_72
01-27-2008, 10:36 PM
Does anyone know a rule for port area then or what?

MyFartsStink
01-27-2008, 10:37 PM
Does anyone know a rule for port area then or what?

Ive read somewhere that more port length gives the sub a flatter response

Edit: lol never mind you said area

amartin_72
01-27-2008, 10:42 PM
Yes a longer port yields lower tuning resulting in flatter response, but the ultimate unanswered question is when you have a sub, and know the paramaters, what the FU*ck do you do for port area?

bonesninja
01-27-2008, 10:42 PM
Does anyone know a rule for port area then or what?

here's a decent port calculator....it sometimes looks a bit on the high side but i've never had an issue with the calculations when the box was finished

http://www.carstereo.com/help/Articles.cfm?id=31

01-27-2008, 10:42 PM
No, Xmax is not directly related to port area. It is a bit more complicated than that.

As far as rules are concerned, the 12-16 square inches of port for every cubic foot of volume works well for most. Obviously things change a bit with your application but this is maybe the best general rule.

A longer port may mean a lower Fb but it doesn't mean flatter response.

amartin_72
01-27-2008, 10:47 PM
devildriver thanks for your help it seems like your one that at least has an idea whats going on. how can you say that 12 to 16, which is a big variance, can still be the rule, thats no rule. What determines wether we choose 12 14 or 16? I know given the same sub cu ft and tuning changing from 12 to 16 will have a dramatic effect on the output but how can we be guided before we design the box?

MyFartsStink
01-27-2008, 10:47 PM
No, Xmax is not directly related to port area. It is a bit more complicated than that.

As far as rules are concerned, the 12-16 square inches of port for every cubic foot of volume works well for most. Obviously things change a bit with your application but this is maybe the best general rule.

A longer port may mean a lower Fb but it doesn't mean flatter response.

Sorry im still noobish for making boxes. Whats fb? frequency?

amartin_72
01-27-2008, 10:48 PM
Im always guessing between 12 to 16 but how can i KNOW?

bonesninja
01-27-2008, 10:49 PM
Sorry im still noobish for making boxes. Whats fb? frequency?

i believe its the resonant freq of the enclosure

MyFartsStink
01-27-2008, 10:51 PM
i believe its the resonant freq of the enclosure

hmm Is there any way to determine the resonant frequency? Is it just the same as the tuning?

01-27-2008, 10:52 PM
devildriver thanks for your help it seems like your one that at least has an idea whats going on. how can you say that 12 to 16, which is a big variance, can still be the rule, thats no rule. What determines wether we choose 12 14 or 16? I know given the same sub cu ft and tuning changing from 12 to 16 will have a dramatic effect on the output but how can we be guided before we design the box?

From 12-16 square inches is not a big difference unless you're using a high displacement driver with an enclosure that results in a port velocity that would be higher than normal...thus requiring the use of more port area.

Sorry im still noobish for making boxes. Whats fb? frequency?

Fb is what many call a "tuning" frequency.

amartin_72
01-27-2008, 10:52 PM
Wow that link is alot of help! Thank you I think I will use that for a good starting point on my boxes.

amartin_72
01-27-2008, 10:54 PM
So say if your going for SPL or sound quality would anyone safely say that more PA is going to yeild more spl but less clarity? Also would more port area increase the boominess of a ported box?

MyFartsStink
01-27-2008, 10:55 PM
From 12-16 square inches is not a big difference unless you're using a high displacement driver with an enclosure that results in a port velocity that would be higher than normal...thus requiring the use of more port area.

Fb is what many call a "tuning" frequency.

Ok but I dont see how people have port lengths that are about 5 ft when someone can have the same tuning at a shorter length. I read that it is for a flatter responce but like I said Im still a noob at boxes and want to learn.

helotaxi
01-27-2008, 11:14 PM
More port area means lower port velocity which means lower potential for port noise. Port noise indicates a loss in SPL if that's what you're after and it sounds annoying if you're after SQ. There is no difference in how it actually sounds and you either have enough port area or you don't. There are various ranges of not enough from the point where the port is so small that the box behaves like a leaky sealed box to sound correct but having a lot of port noise at high volume. If you are running lower power, you can have less port area without problems. You can have too much port area as well. At that point the port is not able to load the sub and the sub behaves as if it were in free air and there is next to no output. There are trick to allow you to run slightly smaller ports without noise, such as flaring the ends, as well.

MyFartsStink
01-27-2008, 11:21 PM
More port area means lower port velocity which means lower potential for port noise. Port noise indicates a loss in SPL if that's what you're after and it sounds annoying if you're after SQ. There is no difference in how it actually sounds and you either have enough port area or you don't. There are various ranges of not enough from the point where the port is so small that the box behaves like a leaky sealed box to sound correct but having a lot of port noise at high volume. If you are running lower power, you can have less port area without problems. You can have too much port area as well. At that point the port is not able to load the sub and the sub behaves as if it were in free air and there is next to no output. There are trick to allow you to run slightly smaller ports without noise, such as flaring the ends, as well.

please explain what flaring the ends mean.

BrianChia
01-27-2008, 11:23 PM
please explain what flaring the ends mean.

http://www.subwoofer-builder.com/port-flares/6_inch_port.jpeg

IDSkoT
01-27-2008, 11:29 PM
Flaring is when it's rounded off at the end of the port. Like you can see on that Aeroport, the end is flared out at a nice soft angle so the wind doesn't "whistle" off the edges. Someone explained it to me like a whistle.

MyFartsStink
01-27-2008, 11:32 PM
Flaring is when it's rounded off at the end of the port. Like you can see on that Aeroport, the end is flared out at a nice soft angle so the wind doesn't "whistle" off the edges. Someone explained it to me like a whistle.

Now would this at all take away from the port length though? would I have to add port length to it?

helotaxi
01-28-2008, 12:01 AM
The flare actually difuses the air coming out the port which reduces the velocity at the port's ends. There are equations out there to account for the flare when figuring port length. Devil Driver has them and should also have a link to an Excel spreadsheet that I made to calculate it.