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Old 12th November 2004, 03:06 PM   #21
Sherman is offline Sherman  United States
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Quote:
Originally posted by johninCR
For planning ahead purposes, leave at least 13" of space at the bottom of your array...

Yesterday I "finished" a new set of line arrays using the PE 49 cent drivers. They were inspired by posting in another thread forgot the poster's name, sorry. (ceramic?) I put them in square plastic sleeves from Home Depot about 42 inches tall and 4 1/4 inches square. They have 9 drivers per side, no tweeters just yet.

Unlike my other arrays where I designed the boxes to give at least some bass these have virtually nothing below 100Hz. In order to get 9 drivers in the sleeve the first driver starts only about 4 inches up from the bottom.

After reading your suggestion I'm thinking I might mount the posts with the arrays on boxes with a 6" or even an 8" woofer. That would raise the highest driver well above ear level, raise the lowest one a decent distance off the floor and allow me to get some decent bass response. I had been considering a separate sub but this might be a much better solution.

Thanks for that suggestion!
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Old 12th November 2004, 04:37 PM   #22
cjd is offline cjd  United States
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Two comments...

1. Why open baffle? Because these drivers prefer it. They have no bass no matter how you look at them - they're really mids. If you put them in a too-small box you end up building a strong "hump" on the bottom end and you can hear the box. Too big and you still hear the box some.

2. Onkyo tweeters - if you wire them for 8ohms, you'll need TWICE AS MANY tweeters as NSBs. Because they're a 1:1 match as far as efficiency. And wiring a pair in series gets you zero efficiency change, but more power handling with the higher impedance. Anything less and you'll have to pad the NSBs, which IMHO sounds like crap.

My vote is to keep the crossover no higher than 4000hz. An 8uF cap on the tweeters if you keep them at 4ohms (Onkyo, PE P/N 269-702) and about a 1mH (to 1.1mH) inductor/10uF cap (series/parallel) on the NSB (at 8ohms). The open baffle at the bottom of the NSB Project page on the PE Project Showcase is mine.

The NSBs get honky and harsh if you play them too high, and most of these tweeters sound great with what is still a "high" crossover, which is why my choice is a bit lower than some - it rolls off the harsher response and breakup of the NSB.

C
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Old 13th November 2004, 02:21 AM   #23
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I'm sure we have Mr. David_Larkins confused by this point...

We currently have votes and lucid arguments for:
1. No cross but cap the tweets
2. Crossing at 4K
3. Crossing at 5K
4. Crossing at 10K

Surely somebody must be right. But that's the fun of it. It turns out that nobody is ever right.

JohninCR... How are you crossing your arrays and how are you driving those big round things on the bottom?

David_Larkins... Have you built those speakers yet and kicked those boxes to the curb?
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Old 13th November 2004, 02:54 AM   #24
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That array is just an early version long sold. Those are shivas at the bottom sealed in about 1.5 ft with a 1st order low pass at 100 if I remember correctly. I was still new to all this and I had an Lpad on the woofer because the speaker was a little The 6 wide rangers have a 1st order high pass at 100 to protect them since they are dipole. Those aren't NSB's, but I imagine quite similar, nice and reasonably flat up to just over 10khz where they fall off and no nasty peaks along the way. The tweaked bullet tweeter has just the supplied cap and an Lpad.

That pair sounded really good despite the 5.5" center spacing on the line drivers. The limiting factors were the short line length and lack of high max SPL of only having 6 of those little drivers. Plus they were a little bulky for my evolving tastes.
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Old 13th November 2004, 05:22 PM   #25
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Nothing built yet. I think I'm gonna go with a single Dayton ribbon in the middle of an 8 or 10 driver NSB array - open baffle. Don't know about the crossover yet...

With the OB design, I can decide on all of that at the very last minute, and if I want to make a change, it should be no big deal - at least to get to it.

Thanks again for all your help.
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Old 13th November 2004, 05:39 PM   #26
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Quote:
Originally posted by David_Larkins

With the OB design, I can decide on all of that at the very last minute, and if I want to make a change, it should be no big deal - at least to get to it.
That's one of the great things about OB. You can make changes literally in seconds.

Regarding a single tweeter in the center, you need efficiency in excess of the array and a Lpad to adjust it's level. This is necessary because the tweeter's SPL will decrease by 6db per doubling of distance vs 3db for the line array, so an adjustable tweeter is important.

To me this is still a better answer than a tweeter array (except a vertical array of ribbon tweeters) because, for example, at 15khz you need center-to-center spacing of less than 1 inch to avoid comb filtering attentuation of the high end. The wavelength of a 15khz wave is .9" . Not to mention all the extra holes to cut and wires to connect.
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Old 13th November 2004, 06:13 PM   #27
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Quote:
Originally posted by johninCR


That's one of the great things about OB. You can make changes literally in seconds.

Regarding a single tweeter in the center, you need efficiency in excess of the array and a Lpad to adjust it's level. This is necessary because the tweeter's SPL will decrease by 6db per doubling of distance vs 3db for the line array, so an adjustable tweeter is important.

To me this is still a better answer than a tweeter array (except a vertical array of ribbon tweeters) because, for example, at 15khz you need center-to-center spacing of less than 1 inch to avoid comb filtering attentuation of the high end. The wavelength of a 15khz wave is .9" . Not to mention all the extra holes to cut and wires to connect.
Regarding the 3db and 6db falloff for line arrays- I recently read this paper- http://www.audiodiycentral.com/resource/pdf/nflawp.pdf and learned that the line arrays transition from 3db falloff to 6 db falloff (point source behavior) at a distance determined by line length and frequency.

As it turns out, a one meter line source transitions to 6 db falloff at less than 2 meters below 2 or 3 khz. What this means is- the famous bottlehead Straight 8 does not act as a nearfield line array! If you're sitting 15 feet from the speakers, you're listening in the far field, and once you've matched the tweeter level to work with the far field level, it will fall off at the same rate as the "line"

On the other hand, if you've got the NSBs stacked up to the ceiling (at least close enough to interact with the ceiling as a boundary) then it's possible to get that nearfield line array effect, with 3 db within a useful range.

That's not to say that it's not useful to have an array that you listen to in the far field- it still controls vertical dispersion, and has the advantage of greater sensitivity, power handling, and lowered distortion because there are a lot of speakers to split up the work.

So JohninCR- I totally agree that in this case, a single tweeter is better than a line array- with a caveat.

If the crossover frequency is too high, the line will be in transition from nearfield to farfield at the listening position. I'd say to cross that tweeter over around 4khz, maybe as low as 3khz.

Also, does anyone have any ideas for a woofer to pair with the NSBs? I've got a serious urge to build something, but right now it's got to be cheap (almost throw-away cheap), and listenable. I'm considering a super-cheap Straight-8 sort of thing, but with the NSBs, I'll need something for bass up to 150-200hz. Any ideas?
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Old 13th November 2004, 06:47 PM   #28
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Joe,

There are 2 parts to the nearfield/farfield transition distance. It is a function of both line length and frequency. The higher the frequency the farther the distance to the transition to farfield with the same line length.

Dr. Griffin's paper regarding line arrays is really a must read for anyone building an array. See http://www.audiodiycentral.com/resource/pdf/nflawp.pdf

I've discussed my real world results vs the theory with Dr. Griffin because I've found the nearfield to extend much farther than predicted. His answer was that the transition from nearfield to farfield is a gradual change not a finite one. The bottom line is that as long as you have a nice tall array you are going to be in the nearfield in a typical size room. Short arrays can be a problem because their tonality will change in relatively short distances as you are in the nearfield for certain frequencies and the farfield for the lower frequencies, so just build tall arrays and you'll be ok. The other advantage of tall arrays is that they still sound great and balanced when you are standing up, perfect for parties and get togethers because the room is filled more evenly with sound than ANY point source speaker is capable.

For example, I built an array pair for a bar consisting only of a 6ft array of 4in drivers. They already had adequate bass from some cheap speakers and just needed good sound. At 25 ft the arrays still sounded great with no noticeable change in tonality and there was still a fairly distinct line between being in the plain of the array and above it when standing on a table.
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Old 13th November 2004, 06:56 PM   #29
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Here's another of Dr. Griffin's papers that is great and includes information about power tapering. That's where you allocate more power to the drivers in the middle of the array and less to those at the ends. This improves imaging because with an equal power allocation you can audibly hear the drivers at the ends as their waves reach your ears later than those in the middle, which distorts the image.

here's the link:
http://www.prism.gatech.edu/%7Egte929u/LinusWP.pdf
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Old 15th November 2004, 11:35 AM   #30
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I meant to ask about this before, but it slipped my mind...

The NSB's are rated for 10 watts max each... If I use 8 or 10 of these in an array, am I going to be ok driving them with my Adcom GFA-555 which is 200 wpc into 8 ohms? or am I going to have to be VERY careful?
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