I have been catching on to an interesting phenomena with ports that may solve my design issues but I'm not quite sure it's legit.
It seems strange that two ports half the length and diameter of one port would work just as well, and even have the exact same vent mach, but I believe it's true. This checks out in WinSID where I have been running different parameters. What is even stranger is that four ports at a quarter of the length and diameter, or ever five ports at a fifth of the length and diameter, all produce the same vent mach, however the latter configurations take up a small fraction the the space in the box!?!
In my case my enclosure required a 2” port 18” in length, but I am going to replace it with five square ports, side by side, like a partitioned slot, that are 5/8” square by 6-1/2 inches long, at the bottom of the box. This is great! It takes up hardly any space in the box, easy to build, and, in fact, with these figures I have an even lower vent mach than with the original port! Moreover I don't have to do any pluming inside of the box.
Is this crazy?
It seems strange that two ports half the length and diameter of one port would work just as well, and even have the exact same vent mach, but I believe it's true. This checks out in WinSID where I have been running different parameters. What is even stranger is that four ports at a quarter of the length and diameter, or ever five ports at a fifth of the length and diameter, all produce the same vent mach, however the latter configurations take up a small fraction the the space in the box!?!
In my case my enclosure required a 2” port 18” in length, but I am going to replace it with five square ports, side by side, like a partitioned slot, that are 5/8” square by 6-1/2 inches long, at the bottom of the box. This is great! It takes up hardly any space in the box, easy to build, and, in fact, with these figures I have an even lower vent mach than with the original port! Moreover I don't have to do any pluming inside of the box.
Is this crazy?
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Nope not crazy at all.
Check this out:
http://archives.telex.com/archives/EV/Builders%20Plans/TL806%20Builders%20Plans.pdf
Check this out:
http://archives.telex.com/archives/EV/Builders%20Plans/TL806%20Builders%20Plans.pdf
The higher efficiency pro/home woofers have modestly robust spiders and surrounds. Putting just one cabinet port with these light construction woofers can create a pressure variation across the cabinet volume which can rock and tilt the cone to where the voice coil can start to rub against the side of the pole pieces.
Many old school pro woofers use two or even four ports(each corner) to help keep equal pressure across the entire cone area. Each port should be identical and placed symmetrically around the cabinet. As mentioned, small diameter ports create audible huffing noise.
Super-duper subwoofers with thick rubber surrounds, double spiders and very heavy cones are robust enough to live with just one large port, which because of the huge subwoofer airflow requires special large radius entry/exit venting and is costly to replicate.
Many old school pro woofers use two or even four ports(each corner) to help keep equal pressure across the entire cone area. Each port should be identical and placed symmetrically around the cabinet. As mentioned, small diameter ports create audible huffing noise.
Super-duper subwoofers with thick rubber surrounds, double spiders and very heavy cones are robust enough to live with just one large port, which because of the huge subwoofer airflow requires special large radius entry/exit venting and is costly to replicate.
What you are building is a shelf port. It will work fine, but if you find it is not tuned to the desired frequency it can be a bitch to cut or lengthen.
Ports are a compromise no matter what you do. You have to juggle port velocity, length, pipe organ effects, and linearity. You can't have it all, so you just have to determine which compromises you want to make least and order things in that priority.
I really don't know the extent your software models all the attributes of ports. I used BB Pro and some spreadsheets to get mine tuned. However, testing revealed some anomalies that I had not expected. It wasn't until I got LEAP and began more modeling that I realized what was happening and later why.
Larger diameter ports tend to have longer pipes. By reducing the cross sectional area you shorten the length of the port, but vent velocity goes up and so does noise and linearity issues. Larger openings tend to reduce port speed, but you also get pipe organ effects higher up in frequency. Subs don't care about this because they cut off the upper bass.
Good point, Loren. I suppose it's worth considering if computer modeling was involved in the design of the TL806, they used a mainframe.
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