Full range plus supertweeter in the nearfield: cap or real crossover? - diyAudio
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Old 7th November 2004, 09:56 AM   #1
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Default Full range plus supertweeter in the nearfield: cap or real crossover?

I have my CSS WR125S's and I'm starting to play around with them, and I think I would prefer to add a supertweeter. Lots of folks seem to recommend just running full-rangers, umm, full range, and just putting a cap on the tweeter. The CSS drivers don't roll off until 12 or 13k though, and I wonder if there is a problem rolling in a tweeter that high with a good 3 inch center-to-center seperation of the drivers. Am I better off bringing it down to something like 8k/first order with a coil on the full-ranger too? Or is that just tossing out the benefits of a nice wide range driver?

These will be on my desktop about a meter from my head, so I'm also wondering if frequency overlap/lobing may be more of a concern than it would be in the far field; another reason I'm thinking I may be better off with at least a simple crossover.

Any thoughts would be much appreciated.
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Old 7th November 2004, 08:16 PM   #2
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I am also very interested in this question... Same situation exactly. For now I am building the stereo pair with the woofer mounted as high on the baffle as possible, that way I can put a tweeter in its own enclosure of the same width and stack it on top.

I don't really mind missing the 12-20k region, but these will eventually be used for critical listening of various compressed audio formats and I need to be able to hear what's going on 'up there' in terms of artifacting.
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Old 7th November 2004, 09:17 PM   #3
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It's all a matter of dispersion. Aside from the fact that full rangers aren't truly full range, even when they are reasonably flat on axis to 15kHz or whatever they may be down 15dB at 30 degrees off axis. What's accepted as best for imaging is to crossover at the highest frequency where the midbass is no more than -6dB 30 degrees off-axis. As to whether to low pass the midbass it's generally preferred to have ta least a total 3rd order rolloff combined electrical and acoustic; if the driver does so by itself wonderful, but if not having LP components will help prevent deleterious driver interactions. The same applies to how much electrical slope to use on the tweeter HP.
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Old 7th November 2004, 09:46 PM   #4
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Thanks for sharing your knowledge Bill; that certainly makes sense. The WR125S has a nice clean rolloff starting at 12K and is down about 18 dB by 20K, so in that sense it looks like a natural for a supertweeter brought in quite high with nothing required on the midwoofer. But the off-axis is down by about 12 dB there. I wonder though if the off-axis response even needs to be considered for a nearfield monitor.

Also I'm still worried that driver spacing will create an unsolvable problem at these frequencies, at this close a listening distance. One wavelength is 1 1/8", which clearly isn't going to happen. Am I worried about nothing?
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Old 7th November 2004, 10:17 PM   #5
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If you consider how many degrees off axis you ears are shifting one foot left or right from a distance of four feet as opposed to a distance of ten feet you'll understand why off-axis response is even a greater consideration for closefield listening, and one reason why closefields use smaller midbass drivers, for their natural wide dispersion.
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Old 7th November 2004, 10:35 PM   #6
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And nearfield monitors are usually used in small rooms (why would you use them otherwise?). Unless the walls are damped weird off-axis response leads to a weird reverbant field and listening fatigue.
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Old 7th November 2004, 11:43 PM   #7
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I'm in the "let the FR run as far up as it goes" camp.. i've tried many a tweeter on top of my BD-Pipes with far than optimal spacing to the tweeter. It is covering at most the top octave and just gives you some air back.

http://www.t-linespeakers.org/FALL/bd-pipes.html

Click the image to open in full size.

Closer placement would probably be better, but...

Using a horn so that the dispersion of the tweeter is a closer match to the FR as it drops off seems to work quite well.

dave
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Old 8th November 2004, 12:13 AM   #8
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Thanks again guys; I'll probably play it safe and plan on cutting a hole for the tweeter and possibly needing to design a crossover at around 7k, depending on how things sound up higher with just a cap. I'll try the cheap and simple way first and go from there. Dave, your experience makes me hopeful; I guess it will be my experiment to see how such things work out in the near field.

Mainly in the interest of close physical placement I've been considering this little Audax TM025F9 tweeter, which looks decent and reasonably extended although I'd have to pad it down a fair bit. It also happens to be 6 dB down at 12k, 30 degrees off axis, so maybe that will help match the dispersion at the CSS driver's upper range.

Click the image to open in full size.

Anybody used one of these?

Another thing I didn't really stress before is that these will be desktop computer speakers, so I will probably be thrilled with the improvement over my current "multimedia" crap either way as long as I can keep the costs in line.
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Old 8th November 2004, 01:33 AM   #9
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Whoops, I wasn't aware that we had a forum for this. It wasn't here when I signed up, and my bookmark has taken me directly to the general Loudspeakers forum ever since.

Sorry, and thanks to the mod who moved it for me.
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Old 8th November 2004, 04:29 AM   #10
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Quote:
I'm in the "let the FR run as far up as it goes" camp.. i've tried many a tweeter on top of my BD-Pipes with far than optimal spacing to the tweeter. It is covering at most the top octave and just gives you some air back.
Ditto. This design uses only a cap on the super-tweeter too.
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