port location on linearrays

[justinc]

I need to place the port on the front of the line arrays I am going to build because they will have a curved back. Does the port need to be inline with the woofer line. or can it be inline with the tweeter line/parallel to the woofer line without causing any problems. Also will it be more beneficial to place one at the top and one at the bottom to spread it out some instead of having just one. thanks

 

[VDOSC]

Based on what criteria?

FYI http://www.l-acoustics.fr/

worlds leader in line-array design. Everyone just started to copy after their long lasting success on line arrays. ProSound News was always full of events with them and still...

might be a good source of idea...

The technology behind the idea is pretty heavy. Check out their own basic criterias on WST.

A real line array just works awesome.

 

[RJ]

What drivers are you using for the mid-bass???
I've used 5 1/4" drivers with an fs of 55 hz and was able to get an F3 of 40 hz. in room with a sealed enclosure.
Room gain took over from there.
I've tryed ported and passive radiators on mine before settling on a closed enclosure. There was just too much midrange noise coming from the ports and the passive radiators.

 

[justinc]

I was going to use 12 dayton rs125 and 8 pt2B's In about a 2.8cft enclosure tuned to about 50hz getting and F3 of 52... in the same size enslosure sealed it looks like I would get an F3 of 80hz and qts around .5

 

[johninCR]

I would put at least 3 or 4 ports. You want the port output to have array dispersion to the extent possible. Also with such a long narrow cab I'd question whether 1 or 2 ports would behave the same as with a more traditionally shaped box.

 

[justinc]

well I coule put two at the top of the tweeter line and two at the bottom would be the most reasonable locations for them. Is it important if they are in an array really since most of what you hear coming from them will be less than 100hz anyways? In a previous line array I built a square port into the back and this worked out fine. But like I said these will have a rounded back so the port will have to go in the front.

 

[johninCR]

The difference is a 6db reduction in spl per doubling of distance vs 3db if you can get them to act like an array. For bass you need one as close to the bottom and the top as possible because bass needs as long an array length as possible. Spacing isn't important since the wavelengths are so long. I don't know if just 2 will get array dispersion or not. 4 should though. 1 top and bottom and 1 at each end of the tweeter array.

 

[justinc]

would putting them inline with the tweeter array parallel to the woofer array cause cancellation problems similar to if if you put two woofers next to each other in an array? thanks

 

[johninCR]

would putting them inline with the tweeter array parallel to the woofer array cause cancellation problems similar to if if you put two woofers next to each other in an array? thanks

No, the ports will be a source of lower frequencies, but even if they were playing the same frequencies, comb filtering only comes into play when the wavelengths are in direct relation to the source spacing and you're talking about wavelengths in excess of 6 meters.

 

[cmckasty]

just a question

Have you thought of a narrow slot loading the length of the array. It may be a challenge to build but if your building rounded backs, your probably up for a challenge. If the length of the slot is possible it may behave more like a line source. I am a beginning builder so pardon me if this is an ignorant question.

either way good luck, Chris M

 

[GM]

Greets!

You're right on track! Many moons ago when I was experimenting with LAs I learned that a ~aperiodic slot vent worked the best since drivers nearest a standard vent or vents would unload quicker. In a way this was good since it creates a broader/flatter BW vent output, which unfortunately is also its downside, mass quantities of group delay (GD).

If the cab is tuned low enough and at frequency between the first fundamentals or harmonics, it's a great way to do it and is used in some better prosound cab designs, but tuned higher up it's better to match the vent to the app, which means to keep the vent length reasonable/GD low requires that it be ~aperiodic.

What I wound up with was a slot down the length of both sides of the LA, and when critically damped you get more gain than sealed, though less than vented, but the GD of a ~0.6 Qtc sealed alignment. With the rounded back though, sounds like it would be a natural for a full length slot.

GM

 

[justinc]

thanks for the help. Great idea about the slot port but im limited to 10 inch with for the baffle and the 3/4 inch roundovers are going to come directly up to the edge of the woofer and tweeter line. the tweeter line will be about 12 inches or so shorter than the woofer line which is another reason i would like to place the ports above and below the tweeter line making the two about the same length.

 

[dhenryp]

I'm building a line array with 14 mid/woofs and I'm actually building seven seperate compartments, each with it's own port. I was concerned about pipe resonances if I built it with one long narrow box. I'm sending my ports out the back to reduce the possibility of hearing any port noise.

 

[johninCR]

I don't really understand why either of you are going to all the trouble of porting the arrays, since you're both still going to need a sub for the bottom anyway. I find it's more difficult to integrate a sub if you have ported bass above it making the extra construction complexity counterproductive.

 

[pinkmouse]

I'm with John on this, just go sealed.

 

[justinc]

The F3 sealed will be around 90hz. I would like to cross over to the sub around 60hz though, which is why I wanted to go ported. Do you think room gain will be sufficient enough to bring it down near that? Also sealed because it will be easier to build, or do you feel that the sound quality will be much better. thanks

 

[johninCR]

If you have a sub with variable XO points slopes, then integrating with ported array may not be too difficult. If you are using a typical 12db/oct fixed subwoofer XO, then matching the more gentle rolloff of a sealed array is likely to be much easier and it may even end up that the 60hz XO point works.

Also, if you go sealed and leave the back panel for last, then you can try it as my favorite, an OB line array.

If you do go ported, then you do have the option of adding some resistance in the ports like GM suggested. That will give you more extension than sealed and improves the group delay over ported, which is likely to help with sub integration. Plus you can always plug the ports later.

Personally, I like to take the easy route. Plus I think the 60-90hz range is very important for bass SQ, so I'd avoid causing group delay in that range if at all possible. GM is one of the very experienced people here who gives really sound advice, so if you do go ported, plan on going aperiodic for best results.