Ported Enclosure questions by the boatload

[harlemn8ive]

determining what type of port to use in which applications, why some vents are so intricate,the level of reliability in these programs everyone uses... basically real knowledge... not just plug in some numbers and voila

[Triticum Agricolam]

For figuring out how large your port needs to be here is a decent calculator: http://www.carstereo.com/help/Articles.cfm?id=31

That calculator is going to have you error on the large side, but that's better than having a port that is too small.

As far as which port types of port to use when, here is a couple things to keep in mind. A properly sized and tuned port is going to have the same performance regardless if it is a round port, slot port, octo port, etc. A well flared round port (aeroport) is going to allow you to use least amount of port area before you start having negative consequences. They will also use up the least amount of internal box volume. The disadvantage of round ports is you are limited in size by the materials you have available (PVC pipe & sonotube sizes) especially if you are buying pre-made aeroports. This is the big advantage of slot ports, you can make them any size you want and they are made out of the same material as the rest of the enclosure.

The reason some ports are so intricate (I assume you are referring to labyrinth style ports) is that's what you have to do to fit the port inside the enclosure. As you increase port area, lower port tuning, or decrease enclosure volume your port needs to get longer. So if you are building a small box, that is low tuned, with a lot of port area you are going to have a really long port and you are going to have to get creative to fit that whole thing in there. If you ports get too long (much over 40" in length) you can start to have issues with port resonance, basically your port start acting like a pipe in a pipe organ. How much effect this has really depends on your application and with cars its usually not big deal.

When it comes to the reliability of the modeling programs, it really depends on the situation. If you are using round ports programs such as WinISD are very accurate at determining what your port length should be. If you are going to use slot ports, this is where it becomes not so easy. The problem with slot ports is end correction. With any port the airspace just outside the port moves with the air inside the port and effectively makes the port perform like it is a little longer than it actually is. To factor this in you use a variable called end correction. WinISD allows you to pick between three end correction values: both ends flared, both end free, one flared & one free. None of these are appropriate for slot ports. I don't know of any programs that allow you to use proper end correction for slot ports. I use a spreadsheet that I've entered the formula by hand to do it. Even then its not as accurate as I would like.

The good news is if your tuning is off by a Hz or two, you are never going to hear the difference so I wouldn't worry about it too much. If you are a perfectionist, like me, then you use the formulas to get you as close a possible and then test to see where you are at and adjust the design from there to get your tuning spot on. Its not quick or easy, but it works.

[harlemn8ive]

Triticum Agricolam Thanks alot you have shined some needed light on the subject