Possible standing wave question : )

[NEO Dan]

I am about to get started with a mid-bass line array and I am wondering if putting the ports at the ends of the enclosure would relieve the enclosure of any standing waves?

[planet10]

Is it an OB or closed? If closed opening 1 end turns it into a quarter-wave transmission line, if both ends then a half-wave transmission line.

dave

[NEO Dan]

Oh. I dunno if that's good or not, the internal length is 81.25", it is shown without the back panel, so it's "closed" I guess. The ports are 3.5" x 12.5 x .75" hwl tuning is @ 70hz.

I've no prior experience with TL's, so this sounds like the proverbial can of worms to me. Any recommendations?

[planet10]

I didn't see any ports in the box... the holes in the top and bottom, are going to completely screw up your porting.

Even without the holes on the ends, the ports & the high aspect ration are going to make the box a mass-loaded transmission line (ML-TL) not a bass reflex. Your BR model is only valid for cabinet dimensions that don't stray far from a cube.

dave

[godfrey]

Originally Posted by planet10

I didn't see any ports in the box... the holes in the top and bottom, are going to completely screw up your porting.

I think the holes at the top and bottom are (supposed to be) the ports.

Agreed it will behave more like a transmission line than a bass reflex, but as it's driven more or less uniformly along it's length, the standing waves shouldn't be too bad - certainly much better than with a single driver part-way down.

Could be good but not easy to model. I seem to remember that this sort of thing is easier to model i.t.o. transient response rather than frequency response (with home-brew spreadsheets anyway). I'm not sure what fancy software is available nowadays, but if HornResp can cope with multiple drivers, it might be ideal for this. (Horn-modeling software tends to be good for transmission lines and reflex boxes too).

Alternatively, to make it behave like a reflex box, you could have separate ports next to each driver and pretend it's a lot of little BR boxes stacked on top of each other.

btw, Is the large rectangular cut-out 1/2 way down the front for a tweeter? (presumably it's not another port)

Cheers - Godfrey

[NEO Dan]

The holes are the ports .75" being the thickness of the MDF, the internal volume is ~100l.
*******

This is getting very interesting... I thought you couldn't really effectively drive a TL with an array of drivers like this?

I took a look around and I see your SD12 TL is similar in internal height and a bit wider than my proposed test mule.

I should have my drivers tomorrow but I've averaged a couple sets of measurements of this driver I've found on the web. The driver is the 299-250 closeout at PE.

The intended use is as an HT main to be HPF'd @ 80hz with the Econowave JBL horn and a BMS like driver, the JBL 2408h. If I could have an extended response for use without the HPF that would be great provided it does not compromise the sound for extension.

What would you do if you had an array of drivers that summed as a single driver with these parameters?


I HAD a starting point like this:

with and without baffle step

External dimensions 15" wide x 90" tall 1.25" roundovers

[planet10]

No this won't be a very effective TL in the traditional sense. But loysa gotchas when it comes to trying to figure out what it is gonna do...

1/ it is a half-wave TL (and as such a nominal ~80 Hz line)
2/ substantially driven thruout its entire length
3/ terminus is a significant portion of the cross-section (and short) so not much mass loading
4/ cross-section is small for a TL

If Martin's Sheets actually do multiple drivers now, it could be modeled there, but not in anything simplistic.

Worst case, you can stuff it until it is aperiodic.

Also note that a single vertical brace (driver placement isn't optimal for it to double as a driver brace) would be more effective than the sillion little stubs you have)

dave

[godfrey]

Originally Posted by NEO Dan

What would you do if you had an array of drivers ...

Maybe some sort of rear horn loading to fill in the bottom end?
Perhaps something like the pic below. It would be better (but horribly difficult) to do the horn flare horizontally.

Or a sealed box somewhere between 80L to 240L and separate subs.

I suspect reflex loading tuned high won't sound too good due to lousy transient response.

[Don Hills]

The ports on the ends will be worse than useless.

The dividers / braces will prevent any overall resonant behaviour of the enclosure.

Either seal it, or put multiple ports along one of the long panels - at least one port per two drivers.

This sort of enclosure is a good candidate for modeling in Akabak.

[planet10]

Originally Posted by NEO Dan

What would you do if you had an array of drivers that summed as a single driver with these parameters?

Can i use 10 drivers? 2 on the front, MTM, and 4 on each side push-push, XOed at about the baffle step, for a 3-way system.

A line array XOed to a single tweeter has serious issues with changing radiation patterns.

dave