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On multiple tweeters

12.13.226.15

Posted by Paul Joppa on January 31, 2000 at 14:01:09

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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