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View Full Version : Big ports vs small ports

DBBOOM
11-27-2008, 04:43 PM
Increasing the area of the port drastically increases the length needed to get a low tuning frequecy. If you are tuning high, such as for SPL a huge port doesnt affect that as much. But when tuning for sound and music, usually lower tuning, a bigger port makes it harder to incorporate the extra length required into an enclosure design.

11-27-2008, 04:55 PM
Yes. I understand that.

I'm trying to figure if it's true that more port area = more SPL, how, and how much is too much.

Bigger ports are more efficient, too a point.

Its hard to say how much is too much, but yes more port area = more SPL, until you get to 'too much'

'Too much' is very application dependent. Also, the more port area you have, the less effective your box will be when you aren't near the tuning frequency, and it'll act more like free air

Most I've ever used so far is cone area = port area, and it did exactly what I expected it to, so that's clearly not 'too much' at least in my application. I intend to use cone area x2 for port area on a test box for a 10" sub in a couple days to find out if that is justifiably 'too much'

Eventually you basically will get to a point where the sub cannot pressurize the port and will act like it's in IB or free air

If sub is excurting 2 inches peak to peak, and port area = cone area, that means the air in the port is moving 2 inches peak to peak (at tuning frequency). If we halve the port area, it must now travel 4 inches peak to peak, and increase port noise as well as cause more of the pressure to be lost through friction. Think of the port almost acts like a hose in this manner.

The bigger the port, the less pressure is lost due to friction, because the speed of air movement is significantly decreased. The less pressure is lost to port friction, the more output there is.

Edit: sry this post wasn't more organized..kinda just typed as I thought of how to explain it

Pioneer~Saturn
11-27-2008, 08:49 PM
Bigger ports are more efficient, too a point.

Its hard to say how much is too much, but yes more port area = more SPL, until you get to 'too much'

'Too much' is very application dependent. Also, the more port area you have, the less effective your box will be when you aren't near the tuning frequency, and it'll act more like free air

Most I've ever used so far is cone area = port area, and it did exactly what I expected it to, so that's clearly not 'too much' at least in my application. I intend to use cone area x2 for port area on a test box for a 10" sub in a couple days to find out if that is justifiably 'too much'

Eventually you basically will get to a point where the sub cannot pressurize the port and will act like it's in IB or free air

If sub is excurting 2 inches peak to peak, and port area = cone area, that means the air in the port is moving 2 inches peak to peak (at tuning frequency). If we halve the port area, it must now travel 4 inches peak to peak, and increase port noise as well as cause more of the pressure to be lost through friction. Think of the port almost acts like a hose in this manner.

The bigger the port, the less pressure is lost due to friction, because the speed of air movement is significantly decreased. The less pressure is lost to port friction, the more output there is.

Edit: sry this post wasn't more organized..kinda just typed as I thought of how to explain it

x2

Bigger port=louder (due to the air moving slower because of the larger opening...rashaddd already explained this though) unless you get to a point where the port is just too fackin big...

boomzilla
11-27-2008, 09:35 PM
Bigger ports are more efficient, too a point.

Its hard to say how much is too much, but yes more port area = more SPL, until you get to 'too much'

'Too much' is very application dependent. Also, the more port area you have, the less effective your box will be when you aren't near the tuning frequency, and it'll act more like free air

Most I've ever used so far is cone area = port area, and it did exactly what I expected it to, so that's clearly not 'too much' at least in my application. I intend to use cone area x2 for port area on a test box for a 10" sub in a couple days to find out if that is justifiably 'too much'

Eventually you basically will get to a point where the sub cannot pressurize the port and will act like it's in IB or free air

If sub is excurting 2 inches peak to peak, and port area = cone area, that means the air in the port is moving 2 inches peak to peak (at tuning frequency). If we halve the port area, it must now travel 4 inches peak to peak, and increase port noise as well as cause more of the pressure to be lost through friction. Think of the port almost acts like a hose in this manner.

The bigger the port, the less pressure is lost due to friction, because the speed of air movement is significantly decreased. The less pressure is lost to port friction, the more output there is.

Edit: sry this post wasn't more organized..kinda just typed as I thought of how to explain it

Good explanation, you made me learn something new today!
:veryhapp:

kmanian
11-27-2008, 09:43 PM
so there is a ratio that equals the point that you no tonger have ample pressur in the port for it to be effective? correct?

cone area to port area
1:1 ok
1:1.5 ?
1:2 ?

To the OP thanks for bringing this up, I need some schooling!

Max_Power
11-27-2008, 10:35 PM
not to threadjack.... well I guess I am but..

I plan to use a 15" woofer, so pi(sq r)= 3.14(7.5*7.5)= 176.625in^2

I was thinking of using (1) 8" port, 3.14(4*4)= 50.24in^2

I would need 3!!!! ****!

would only 1 8" be enough? It will be in a 5^2ft box.... if that matters.

splferrell
11-27-2008, 10:40 PM
1:1 ok
1:1.5 ok
1:2 ?

the bigger the port the bigger the drop off. make sure you have a good subsonic filter

11-27-2008, 10:50 PM
so there is a ratio that equals the point that you no tonger have ample pressur in the port for it to be effective? correct?

cone area to port area
1:1 ok
1:1.5 ?
1:2 ?

To the OP thanks for bringing this up, I need some schooling!

There's no golden rule. Even cone area alone isn't enough if there was a golden rule...its about how much air the sub can displace, meaning power and xmax come into play as well. I'm sure its possible to calculate, but you're better off just trying for yourself, and remember it'll be different with every different box and sub you use.

Also, remember, bigger port is only good close to tuning and the box's peak. As said before, you will experience MAJOR differences in dampening roll off below tuning by doubling port area for example. You could easily go from not even needing a ssf with a 40hz tuning, to needing a ssf set at 35hz with the same tuning, same driver, same sized box, just by increasing port area enough (as an example)

splferrell
11-27-2008, 11:20 PM
its about how much air the sub can displace

yes

this mite help

The Vas is a number that represents the amount of air that has the same compliance (springiness) as the speaker. Since the speaker will need to compress the air, how compliant it is will have a major effect on enclosure size. Typically, the larger this number is, the larger the enclosure will need to be for a proper response.

alvitae
11-27-2008, 11:43 PM
This info is good stuff. I want to learn more on ports as well.

kmanian
11-28-2008, 12:08 AM
not to threadjack.... well I guess I am but..

I plan to use a 15" woofer, so pi(sq r)= 3.14(7.5*7.5)= 176.625in^2

I was thinking of using (1) 8" port, 3.14(4*4)= 50.24in^2

I would need 3!!!! ****!

would only 1 8" be enough? It will be in a 5^2ft box.... if that matters.

1 8" port is fine

splferrell
11-28-2008, 12:44 AM
Aren't you the guy that wanted to run 9 15's ported with 15 cubes?
ya, just case I'm not making a 9 15 box right I'm automadicly an idiot:p:

njaykaplan
11-28-2008, 01:17 PM
Very interesting read, I hope we can get some of the other very knowledgeable people in the forum on here.. This will definitely help me with my next build. Could you give an example of how vas affects the box size? I'm currently using a bravox subwoofer, a vas of 44.5liter or 1.57 cu ft. They recommend rediculously small, but i'm using 2.5 cubes now with a pair of 4 inch aeros, and unreal sound for a single 12... I'm also feeding it double or triple its RMS, with a box LARGER than recommended and the same size port as recommended. Sorry to threadjack, but this thread touches on a topic i know very little about, and would like to learn a lot more before i enter the competition scene :).

azimuth
11-28-2008, 01:25 PM
alot more needs to be considered, other than vas. cms, xmax. i don't fully understand either, but stiffness (cms) and xmax, along with many others i don't get yet have to be considered.

njaykaplan
11-28-2008, 03:36 PM
Why does this thread have to die, when theres so many uneducational posts sitting around up top.. I cant find anywhere else to learn about how the sub specs reflect upon the box building..

lincolntc690
11-28-2008, 03:54 PM
http://www.ajdesigner.com/speaker/ajvented.php

nice little box site w/ explanations