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On multiple tweeters

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In Reply to: Other Drivers & tweeters for Straight 8, is it possible ? posted by SBK on January 31, 2000 at 07:05:11:

Doc B. referred to "beaming" above 4kHz with the array of woofers, and both he and Cam mentioned using multiple tweeters. I'd like to comment.
With an extended-range driver used in a line array, there is a significant chance that the beaming problem is really lobing - that is, multiple lobes at different angles, but of nearly equal loudness. For instance, a large array of sources spaced uniformly 6.75" apart (close to the str8-8 spacing) at 4kHz will have far-field peaks at 0 (on-axis), +/- 30 degrees, and +/- 90 degrees. There will be nulls between, at +/- 14 degrees and +/- 49 degrees. At higher frequencies, there will be even more beams, more closely spaced.
The best way around this is to move the drivers more closely together. Tweeters spaced 1.35" apart will work up to 20kHz, or 2.7" apart will work to 10kHz, before becoming excessively lobed. Here I am generalizing from Doc's experience that the Str8-8 can go to 4kHz.
Unfortunately, tweeters that can be spaced that closely are difficult to find. A few are available with Neodymium magnets, but are usually not the best models. Remember, the high-frequency sweet spot will in the limit be only as tall as the array, so if the tweeter array is more than a couple tweeters, you'll probably want to make it at least 3 feet tall, which means some two dozen tweeters per speaker. On the plus side, this gives lots of power handling, so a lower crossover becomes possible, but it's sufficiently unusual that there's not a large body of experience to draw on; lots of eXperimenting will probably be needed to get it really right.
hope this helps!

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Re: On multiple tweeters Doc B 14:57:22 01/31/00 (4)
Is that spacing center to center?
It may be a lobing issue, but the beaming I'm mentioning is really beaming, I think, because the effect is noticable even on a single driver speaker, that is, say if you run the woofers full range to 15 kHz, there is a noticable drop off in HF as you get off axis, to one side.
I think the lobing issue is more likely to create a bit of vague imaging or recession at certain heights, as certain frequencies dip, while the beaming effect is noticed more off to one side.
- Re: On multiple tweeters Paul Joppa 17:50:35 01/31/00 (3)
  Yes, center to center. I should have been more clear, sorry.
  OK, it's beaming then.
  Lobing is probably more of a problem with tweeters, which remain wide dispersion at high frequencies. Incidentally, this problem resolves itself if the array is tall enough and you sit close enough - the difference in speaker-to-ear delay times spreads the apparent directivity. You can also do this with a gently curved baffle if you want to sit further away.
  The real problem with severe lobing is that small head movements give large changes in the first-arrival spectral shape. You want the principle lobe to cover at least +/- 15 degrees generally, so that there is a sweet spot bigger than your head.
  - Re: On multiple tweeters Doc B. 08:42:23 02/01/00 (2)
    Bigger than my head? You're asking a lot.
    In fact we are enjoying sitting close to the straight 8s. I measured last night and the current listening position is at 8 feet from the middle of driver array, with about 6.5 feet between them. I did a lot of neck craning and shifting around in my seat, and while there's a definite sweet spot, there doesn't seem to be any lobing effect - it doesn't really seem to change the character of the speaks as you move.
    - Re: On multiple tweeters Paul Joppa 10:03:10 02/01/00 (1)
      Ummm - not sure if this was clear, but that's my point exactly. The Str8-8s would lobe above 4kHz if they were not crossed over to a single-point tweeter. An array of tweeters, spaced 6" apart like the Str8-8, would have some problems, too; IMHO they would have to be spaced much closer to work out well.
      - Re: On multiple tweeters Doc B. 10:57:19 02/01/00 (0)
        
        In Reply to: Re: On multiple tweeters posted by Paul Joppa on February 01, 2000 at 10:03:10:
        
        Sorry, I was trying to support your point, bud. You may recall that I initially ran the 4 driver FIM model 4 midwoofs all the way up to 15 kHz, and crossed the tweeter in with a 1 mfd cap. When we improved the tweeter damping and could then move to a lower crossover point, as used in the Straight 8, things got a lot more coherent.

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