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Jimmy79
01-25-2012, 05:30 PM
Whats the difference in using 1 - 7.5" aero port vs using 2 - 17" aero ports? in a 4 cuft box either way it will be tuned around 30Hz (using 4" aero ports), I know people always say use 2 but why? Do you get a gain in spl or a gain in sq by using 2? if so can someone explain why/how. Thanks

SMS
01-25-2012, 08:35 PM
It has to do with the square inches of port area. your 4cuft box needs two 4" aero ports to move air effeicently.

Jimmy79
01-25-2012, 10:27 PM
My question wasn't really whether or not I should use 1 port or 2, Everybody always says use 2. My question is what benefits someone would get by using 2 vs 1.

tommyk90
01-25-2012, 10:39 PM
My question wasn't really whether or not I should use 1 port or 2, Everybody always says use 2. My question is what benefits someone would get by using 2 vs 1.

Usually more output and less port noise. Your subs won't have to work as hard to produce more output (i.e. be louder).

SMS
01-25-2012, 10:48 PM
the port needs enough area to move air in and out to work correctly. using one 4" port for a 15 or two 12s would result in air moving much to fast creating port noise and taking potential output from the subs.
an educated box designer could explain this much better than i can.

Moble Enclosurs
01-25-2012, 11:43 PM
Basically, the port cross-sectional area deals with cutoff frequencies as well as length. Though in a dual 4" aero setup, the cutoff is extremely high, you are allowing a lower range of frequencies to be more efficient in that area. By using only one, you are allowing less low end efficiency due to the area being smaller. There are two major formulas for this, but they are only for figuring cutoff, not controlling it. By using two, you decrease the cutoff, allowing more potential in the lower region due to the fact that any given response band is limited to a certain range. So, you want them as large as possible to increase efficiency closer to the sub frequency range of music.

In horn design, this is common to understand the effects of low frequency response efficiency and how the cross-sectional area couples it to the environment. Any other design can benefit from this concept.

The flaw of this that many know of, is to keep tuning low, the length is commonly increased to account for efficiency on the port end when in phase with the rest of the system. So, it has been known that a quarter or an 8th of the cutoff cross-sectional area can be utilized effectively to keep efficiency higher than a single conventional port alone. The also, well known effect of this is increased resonance nodes which usually creates a peaky response.

If that does not make sense, I'll explain more, and even give the formulas if necessary.

Jimmy79
01-26-2012, 02:53 AM
Thanks for the answer :thumbsup: now I know.

TaylorFade
01-26-2012, 03:18 AM
The flaw of this that many know of, is to keep tuning low, the length is commonly increased to account for efficiency on the port end when in phase with the rest of the system. So, it has been known that a quarter or an 8th of the cutoff cross-sectional area can be utilized effectively to keep efficiency higher than a single conventional port alone. The also, well known effect of this is increased resonance nodes which usually creates a peaky response.

If that does not make sense, I'll explain more, and even give the formulas if necessary.

You lost me here. Care to expound with a dumbed down version?

Moble Enclosurs
01-26-2012, 04:01 AM
you're welcome man! And taylorfade, no problem! Basically, the larger the area of height and width, the higher the efficiency of lower frequencies due to coupling. Think of coupling as a "close enough to 1:1 as possible" in this case, even though coupling is more complex. And think of the first 1 as being the area of the port and the second 1 in the ratio as the area of your vehicle. Length not included yet. So, the closer to the area of the vehicle that the port opening is, the better it will sound as long as all other factors are correct.
The average vehicle is 3ft tall and 5ft wide.
Since it is unlikely you will make the port a 3ft x 5ft opening, the 1/8 of this can be utilized for "close enough" efficiency for the frequency range that the example 3x5ft area will allow.
So, take for instance you want the efficiency to be highest around 30-50Hz. Then a cross sectional of this should be for a full wavelength cutoff of the average of 30-50hz(which is 40Hz) is going to be 1130/40...giving us a wavelength of 28.25ft. This equates to a cross sectional area greater than 3x5ft because the cutoff of 3x5ft is around 140hz. The actual cross section needed for a 40hz efficiency is around 9 ft squared but 1/8 cross section needed for a 40hz efficiency is about 3.25ft squared. So, this is more usable in a vehicle....though still very large. You can get about 30% the efficiency higher than the average 5% by using a 1/16th cross sectional area of the cutoff wavelength.
i know the math here was not simplified but the rest is said about as simple as I can.
Do you see how making the port opening smaller equates to less efficiency, without me getting into the specifics too much?
The more the opening allows for lower frequencies to travel, the higher the efficiency.
Interestingly enough.....in conventional design, we all know that a smaller port opening, if all else is constant, will result in lower tuning, right? Well, this is where people do get confused. The reason is, I said all else is constant. So making the opening larger will lead to higher tuning and overall higher efficiency at higher frequencies....the exact opposite of what we want. But when other variables such as length and compression volume and others are changed, the efficiency changes for where in the frequency spectrum it is by pure physics. So, yes, you do have to accommodate port length in this concept, but by using 1/8th scales, we can get pretty compact. And 1/8 scales are for great efficiency, around the 30-40% range as a smaller design with smaller ports will be around a 2-10% range, less than 1/10th the efficiency....which is not as great.
I really hope that was understandable.
The cutoff basically allows higher efficiency much like the tuning of a ported box. But, unlike tuning, it is for a broader range of frequencies.

Moble Enclosurs
01-26-2012, 04:10 AM
Think of this......you are in a room. Sitting in the center at the front of the room. Now, on the other side is a sub box. The port faces you as well as the driver on the same baffle. Now, if you were to extend that port closer to you and connect it to the walls and floor and curving at an angle relevant to coupling, then you will here much more coming from the port because now it is part of the room and the room is an extension of the port. So minimal acoustical losses are present from before. You are essentially now sitting in the port because the port is coupled with the room by extending itself into the rooms boundaries.
So efficiency is higher because you did not have to turn anything up but the sound increased due to coupling. And since the port is larger, the boundaries act much like resonances in a room where the larger the room, the lower the resonances that are excited, or heard.
Maybe that will help a bit also.

Moble Enclosurs
01-26-2012, 04:23 AM
Keep in mind also, that this concept is very connected to driver specifications. Do you want to know why tlines are so popular? Its not because of added efficiency, or wavelength phasing or any crap like that. It is because for this efficiency port idea to work the best, the compression chamber has to be also coupled to the driver, and the best way to make sure compression is there and not lost, we can make it non existent and rely on wavelength to couple without a compression, so we do not have to couple the driver specs much anymore.

So, the best way to do it with a compression chamber, I.e. Ported boxes, is to use a small compression and a long and large port...as long as all of the frequency parameters work out, like electrical and acoustical coupling, phase response, output efficiency, driver control, etc. It is a work of art to get every one of those factors correct, and I can nearly guarantee that not many people knew that was why small compression chambers work well for output.
Everyone thought because of high tuning....noooo...its because a smaller compression allows for less acoustical losses when adding a port that I s properly coupled to the environment. It just so happens that tuning is high because of the port characteristics. And efficiency is high because of crossectional area when tuning is high.
You want a good spl box, go with a huge port and a small compression chamber. Watch what happens. :D