First aero port build, question about port area

[Nightshade]

The way to calculate aeroport port area is Pi*r^2 or for example a 6" port would look like this: 3.14*(3*3)=28.26sqin of port area. When using an aeroport for a daily setup that will be primarily used for listening I try to keep the ratio of port area to enclosure volume at approximately 10-12sqin of port area per cuft of.

My daily setup is a single 8" aeroport in a 4.25cuft enclosure that houses a single Sundown Audio X15. The port area is (4*4)*3.14=50.24sqin of port area. Next, to find out the ratio of port area per cubic foot of enclosure size just take 50.24sqin/4.25cuft=11.82sqin of port per cuft.

When calculating port area for an enclosure that is going to be used for SPL competitions only a much higher ratio of port area to enclosure volume is needed. I prefer to use anywhere in the 12-20sqin per cuft range for this type of application. By using this much port area to box volume you are gaining a significant amount of efficiency by reducing box rise. This results in your amplifier seeing a lower reactive load and in turn producing more power since the rise is not as extreme as with a lower ratio of port area to enclosure volume.

Some people go even higher in their port area to box volume, but I have not found this to always increase my overall SPL. There are other variables that need to be accounted for that will contribute to the SPL increase on top of the proper ratio of port area to box volume.

If you have any questions or need any suggestions, I have only been using aeroports for all of my enclosures for the past 8 years. I really believe that they are superior if the application allows for the use of them.

Also to answer your initial question I would personally go with a single 8" port. A single port will be more efficient and louder than multiple ports. Always use a single port whenever possible. With an 8" port you will have 50.24sqin and when you divide that by your 4.4cuft you come up with 11.42sqin per cuft. With the decent amount of power that you are going to be using that ratio of port area will work out very nicely. If you were going to run more power I would shrink the box size and port area. If you plan to run less power then increase the box size and keep the same amount of port area.

[Krakin]

I have no idea how Port area per cubic foot has ever been something that relates to box volume. The area needed to move a certain amount of air doesn't change based on internal volume of an enclose.

[IKnowImLost224]

The way to calculate aeroport port area is Pi*r^2 or for example a 6" port would look like this: 3.14*(3*3)=28.26sqin of port area. When using an aeroport for a daily setup that will be primarily used for listening I try to keep the ratio of port area to enclosure volume at approximately 10-12sqin of port area per cuft of.

My daily setup is a single 8" aeroport in a 4.25cuft enclosure that houses a single Sundown Audio X15. The port area is (4*4)*3.14=50.24sqin of port area. Next, to find out the ratio of port area per cubic foot of enclosure size just take 50.24sqin/4.25cuft=11.82sqin of port per cuft.

When calculating port area for an enclosure that is going to be used for SPL competitions only a much higher ratio of port area to enclosure volume is needed. I prefer to use anywhere in the 12-20sqin per cuft range for this type of application. By using this much port area to box volume you are gaining a significant amount of efficiency by reducing box rise. This results in your amplifier seeing a lower reactive load and in turn producing more power since the rise is not as extreme as with a lower ratio of port area to enclosure volume.

Some people go even higher in their port area to box volume, but I have not found this to always increase my overall SPL. There are other variables that need to be accounted for that will contribute to the SPL increase on top of the proper ratio of port area to box volume.

If you have any questions or need any suggestions, I have only been using aeroports for all of my enclosures for the past 8 years. I really believe that they are superior if the application allows for the use of them.

Also to answer your initial question I would personally go with a single 8" port. A single port will be more efficient and louder than multiple ports. Always use a single port whenever possible. With an 8" port you will have 50.24sqin and when you divide that by your 4.4cuft you come up with 11.42sqin per cuft. With the decent amount of power that you are going to be using that ratio of port area will work out very nicely. If you were going to run more power I would shrink the box size and port area. If you plan to run less power then increase the box size and keep the same amount of port area.

Very informative response, thank you! Since my dimensions won't work, if I was around 3.8-4.1 net cu ft would I still stick with an 8"?

[Krakin]

Very informative response, thank you! Since my dimensions won't work, if I was around 3.8-4.1 net cu ft would I still stick with an 8"?

Cubic feet doesn't correlate to the need of more or less port area.

The minimum port area is needed is calculated using cone area, tuned frequency, and the excursion of the sub.

[Krakin]

A few formulas for vented enclosure design.

Maximum air volume displaced by cone excursion:

maximum air volume displaced by cone excursion.

cone effective radiation area

cone peak linear displacement

Cone effective radiation area:

cone effective radiation area

cone diameter plus one third of surround

Port or vent length:

port or vent length

volume of enclosure or box

tuning frequency

end correction factor

minimum port or vent diameter

minimum port or vent diameter

maximum air volume displaced by cone excursion

tuning frequency

[Nightshade]

Very informative response, thank you! Since my dimensions won't work, if I was around 3.8-4.1 net cu ft would I still stick with an 8"?

Cubic feet doesn't correlate to the need of more or less port area.

The minimum port area is needed is calculated using cone area, tuned frequency, and the excursion of the sub.

Actually it does when trying to reduce box rise and overall SPL output. When the ratio of port area per cuft is higher the efficiency of the enclosure increases as well as tuning. When you increase the efficiency of the enclosure you will get an increase in peak SPL output as well. Using the same enclosure size and reducing the amount of port area will not only lower tuning but will also hurt efficiency in the process.

In my own personal testing to achieve the least amount of box rise as possible in order to make the most power, I am able to add more port area to combat a high rise that may be occurring. For example, take an enclosure that I was working on for a single 15" SPL setup. With the total of 3 cubes and 3 4" aeroports ( 37.7sqin) tuned to 46hz box rise was from .5 ohms nomial to 2.15 ohms reactive. The ratio of port area per cuft of box volume was 12.56sqin per cuft.

Then the 4" ports were replaced with a single 8" port (50.24sqin) and tuned to the same frequency of 46hz. The nominal load of .5 ohms only rose to 1.07 ohms reactive, effectively doubling the amount of power that was being made due to the less rise.

This is only a recent example of this but is a well known procedure in the SPL competition scene to lower box rise and increase the efficiency of the enclosure without changing the tuning of the setup.

[Krakin]

Very informative response, thank you! Since my dimensions won't work, if I was around 3.8-4.1 net cu ft would I still stick with an 8"?

Cubic feet doesn't correlate to the need of more or less port area.

The minimum port area is needed is calculated using cone area, tuned frequency, and the excursion of the sub.

Actually it does when trying to reduce box rise and overall SPL output. When the ratio of port area per cuft is higher the efficiency of the enclosure increases as well as tuning. When you increase the efficiency of the enclosure you will get an increase in peak SPL output as well. Using the same enclosure size and reducing the amount of port area will not only lower tuning but will also hurt efficiency in the process.

In my own personal testing to achieve the least amount of box rise as possible in order to make the most power, I am able to add more port area to combat a high rise that may be occurring. For example, take an enclosure that I was working on for a single 15" SPL setup. With the total of 3 cubes and 3 4" aeroports ( 37.7sqin) tuned to 46hz box rise was from .5 ohms nomial to 2.15 ohms reactive. The ratio of port area per cuft of box volume was 12.56sqin per cuft.

Then the 4" ports were replaced with a single 8" port (50.24sqin) and tuned to the same frequency of 46hz. The nominal load of .5 ohms only rose to 1.07 ohms reactive, effectively doubling the amount of power that was being made due to the less rise.

This is only a recent example of this but is a well known procedure in the SPL competition scene to lower box rise and increase the efficiency of the enclosure without changing the tuning of the setup.

Yes of course lower port area decreases efficiency that is in one of the equations I posted.

But how does the volume matter in your choice of port area?

You never stated how keeping the port area the same and changing your volume changes anything at all.

[Triticum Agricolam]

Lots of good discussion in this thread. Here are my thoughts.

I disagree with Krakin, box volume does effect port area requirement, all else being equal. With a larger box volume the subwoofer is acting on a larger mass of air, more air being moved needs more port area to move though. This is closely related to why larger boxes produce a larger gain in output around tuning. If a larger box produced the same gain as a smaller box did then they would need the same port area, but this is not what happens. Where people go wrong with things like "port area per cube" rules is when they don't consider anything else other than box volume when trying to determine adequate port area, which is what I think Krakin is speaking against.

As far as impedance (or box rise as its commonly called) goes, I largely agree with what Nightshade is saying but I would like to expand on it a bit. Its easy to think that higher impedance is always bad, but this is not the case. Impedance is closely related to how hard the sub has to work. The harder it works the lower impedance is. Just because a sub is working harder doesn't always mean its putting out more output though. Also its important to remember that amps do not put out watts, they put out volts. When you are trying to get as loud as possible efficiency (which is dB/watt) is less important than sensitivity (which is dB/volt).

With a sealed box (or infinite baffle) there is a peak in output at the resonant frequency of the sub. Due to this, sealed subs are VERY efficient when playing content at their resonant frequency, you get a lot of output relative to the amount of watts the system is consuming. This is because the sub naturally wants to move easily at the frequency so very little power is wasted just overcoming the suspension of the sub, thus high impedance. The sensitivity isn't much better or worse at that frequency though, which is why there isn't (or shouldn't be) a peak in output at Fs. With a ported box there is a dip in impedance at the tuning frequency. This is because a ported box is a resonant system and the sub has to work hard to make that system resonate, thus low impedance. Due to the low impedance efficiency is also low at this point since the amp has to put out a lot of watts. Sensitivity is usually very high though, which is why people build ported boxes in the first place, they get you a lot of output per volt (which like I said earlier, is what amps put out). Ideally you want high sensitivity and high efficiency since you get lots of output and its easier on you amp and electrical system, but that's not always easy to do.

Now the trick is determining when high impedance is good and when it is bad. When impedance is raised due to compression its bad. Port compression is what you get when your port is too small and the port velocity is too high. This is why Nightshade saw his impedance go down with larger ports. Too small of a port restricts the amount of work the sub can do, this may seem counter-intuitive, but its just like when you plug the hose on a vacuum cleaner and the motor speeds up, you restricted the amount of work it can do. In addition to port compression there is power compression which is what happens when the voice coil gets hot and its resistance increases, also bad. When higher impedance is bad it decreases sensitivity.

Higher impedance is good when its corresponds to an increase in sensitive. This is what happens when you make a ported box bigger, it will have higher impedance, so your amp will do less work, but you will get greater output due to higher sensitivity. If the impedance is raised enough, you may be able to wire to a lower nominal impedance load and get even higher sensitivity out of your system. Higher impedance isn't always bad.

[Karkov]

Droppin' knowledge as always!

[IKnowImLost224]

Okay, so... After reading all of this and trying to get a decent understanding, if I go smaller with the box it will less efficient correct? Unfortunately I don't really want to build in the trunk so a smaller box is really the only option. This is what I came up with

3.79 cu ft

50.24 in^2 port area

(1) 8"x18" aero tuned at 38.69 Hz

^^ that leaves me 4" away from the rear of the box