I have been thinking of doing a line source for a while, and looking a the massive McIntosh XRT2K triggered a new round of thinking and sketching. A unique (to my knowledge at the least) attribute of the XRT2K is the 'dual-baffle', where the mid and tweeter arrays are put in front of the woofer array. This greatly reduces the width of the speaker - very important in the place I want to use it.

Now, I am not interested in cloning this beast (the thought of wiring 220 drivers alone puts me of this...) but I am looking into replicating the principle in a scaled-down speaker. My ideas so far are:

-A woofer cabinet consisting of 9 x 6,5" woofers in a sealed enclosure of about 22(w) x 35(d) x 195(h) (all measures in centimeters). The Peerless SDS164 is a good candidate due to its favourable pricing here in Norway and easy mounting (yes, I am lazy...). 9 of these gives me an SD of about 1300cm2 per side, equal to 2,5 12" woofers. Should be sufficient down to about 60hz with plenty of punch!

-1 BG RD75 in its own chamber suspended in front of the woofer-array. This chamber would be about 15cm wide and deep (and 195cm tall), and would have a curved back (using a cut-off paper-tube - the kind used for sono-tubes etc.) to cater for the output of the woofer line as well as possible. Sufficiently damped and about 70% filled, this should work well with the RD75 down to about 300hz with these dimensions, any lower in the X and I need to increase chamber size.

There is one major issue here: The X-over frequency relative to the radiation pattern and shortest wave-length of the woofer-line output. The XRT2K is crossed at 250hz, and that obviously works well. I _could_ cross this low, but as this would stress the RD75 a bit more than I like and require a somewhat larger chamber for it, I would like to go as far up as possible without messing up the sound of the woofers (500hz would be great, but anything above 300 should be fine for the RD75). So far I have been unable to track down any info on how large an obstacle I can place in front of a woofer at a given frequency. Does anybody have any experience with this, or any sort of guideline that can be applied?

I suppose I could build and measure, but this will cost a bit, and I would prefer not to fail miserably attempting an impossible task...

FYI this will be an active system using a modified Behringer DCX2496 as well as a DEQ2496 for equalizing. That means that I am not that concerned about (moderate) frequency curve deviations. Power response and dispersion pattern issues are the main concern, as I cannot do anything about these electronically. I also have a couple of Peerless XLS-12 in sealed boxes that will take over below the woofer-lines.

Any suggestions for improvements will be greatly appreciated!

At that rate, why use 6.5 in mid woofers? Skip the mids altogether. Go with a group of 5- 10 inchers. You'll be able to go really low. NHT makes a 10 incher with an FS of 23.5. surely you can find something over in Europe like that.

I believe that Jim Griffin recommends woofer arrays not less than 10 inch.

Marlboro

If you want to make a good line array, my suggestion is;

1. Don't use RD75
2. Don't replicate the concept of the XRT2K

Here is a collection of line array pictures found in cyberspace,
some are commercial line arrays, many are DIY, and only a few
are good designs. Study the pictures and send me an email and
I'll give you one opinion on which ones to get inspired by.
http://home.pacbell.net/lordpk/speaker/

/hehe

But what reasons do you have THY, for not using them. Surely if you put the tweeters 6 to 8 inches in front of the woofer coils you will need to put in a time delay, but with the Behringer, I assumed that the poster would do that.

I should think that horizontal woofer to tweeter combing wouldn't matter.

But I'd be interested in your reasons for not choosing this design, it seems viable, though the RD75 is not my choice.

Marlboro

A 350 X-over to the RD75 would be excellent. You don't think that will be any issue in terms of obstructing the output from the woofer-array up to 350?

10" will be to wide I'm afraid, there's some serious WAF restrictions on the width (height and depth are less critical) of the speaker. That's also the reason why I want the mid/tweet in front of the woofers - I would certainly place it besides the woofer-line if possible.

8-inchers may work though. I have been looking av 8 Peerless SLS-8 (830667) per side, also a nicely priced alternative with a healthy 13mm P-P excursion. This would allow for a full-range, skipping the subs altogether. Could be a good idea...

Regarding the choice of tweeter/mid I can't see that I have a choice for this design. Going with an array of smaller units (like the Neo8) will not allow a cross at 350 without serious compromises i SQ. A long and robust ribbon could do the trick, but short of DIYing one (which I don't dear attempt) they seem to be in short supply... Do you have any suggestions?

If you dont want sub/mid and tweeter. try something like the 1 aurasound cougar which is pretty flat from 300hz-20khz. It also will not have comb filtering until much higher in the frequency so it could be a good alternative.

Thanks for your feedback! I am a little bit curious as to exactly why you think replicating the XRT2K concept is a bad idea? I know there are issues connected to obstructing the woofer-line like this, but as it works for McIntosh it should surely be possible to replicate? Or am I missing something?

As for the RD75, I agree that this is not perfect. Sensitivity is a bit low, it rolls of too early towards the top and it costs more than a line of shorter drivers outperforming it in the top octaves. However, if I am to attempt this design I can't really see any alternatives - crossing as low as I have to (below 400) means smaller ribbons and planars are out. And, I am hopeful that the RD75 (even with all its limitations) with the help of some EQ could sound very good given the reports I have read on it.

BTW; Thanks for the link to the images - some interesting designs there! One I have not seen before is http://home.pacbell.net/lordpk/speaker/26.jpg. Do you know what drivers have been used in this one?

Now, wiring 45 drivers is a little bit more work than wiring 1 driver though...;-)