Effects of varying Mid/Tweeter spacing

[fjhuerta]

Hi all,

Ever since my microphone was calibrated, I'm a happy man. My designs now sound like I thought they should. I also noticed I "learned" to get around my calibration issue by designing at 30° off-axis, which basically made my designs flat on-axis with my calibrated microphone. Heh.

Anyway, I was playing around with the Miniliths:

https://www.htguide.com/forum/showthread.php?t=26798&highlight=miniliths

A design which places the Neo3 on top of a Neo8, with a 7 KHz crossover, designed at 115 cms, 0°, for flat response, taking care of avoiding any flares off axis.

I began noticing the speakers were a bit dull on my listening position. After using SynRTA, I noticed there was a null at the tweeter-mid crossover frequency. I was surprised, and re-measured using JustMLS at 1 meter. The speakers were fine. My listening position is 2.30 meters away.

I inverted the tweeter polarity on both speakers, and re-measured with SynRTA. +-5 dB flat in-room at my listening position. And a null at the crossover frequency at 1 meter using JustMLS.

Am I correct in assuming the vertical distance between tweeter and midrange in my design is the culprit? Will the design have nulls or be flat depending on listening position?

I suppose it could be. The Neo8 is a very tall driver, and crossing over at 7 KHz may not be the best of ideas.

[mazurek]

Here is what I did when I was tweaking my latest speakers.

Get in fairly close (<0.5m) because reflections can confuse the story. I measured between the drivers at the exact vertical center point (my drivers have a traditional layout, not sure what layout yours have), and a bit above and below the center point to find the reverse null. In my case the reverse null was about 5 degrees off the normal axis. Then you can pull the measurement mic out along that line to see how the room/etc factors into the null measurement. Then I found out what the time error was associated with a 5 degree tilt and fixed the crossover, and now the reverse null correctly appears on axis (though gating and reflections can still muddy the picture). My best results for measurement reflections are with the speakers on a box in the middle of the room.

[Dennis H]

Yeah, I think it's a path length thing. And, at 7K, the Neo8 is acting like a line source and that complicates things. Simplifying a bit, at 1m on the tweeter axis, the path length to the bottom of the mid is almost half a 7K wavelength longer than to the tweeter. With point sources, I'd say just lower the mic for the 1m measurements to get the phase right. But with the line source behavior of the mid, I think you have to measure from farther away. Take them outside if you need to. What does JustMLS look like with the mic at the seat?

[fjhuerta]

Hi Dennis,

I didn't took an MLS measurement. I'll probably do it this week-end.

I'm thinking about re-doing the speaker, with a more sensible cross-over point, but to be honest, 1m measurements aside, it sounds terrific! I did measure using SynRTA on various points on my listening room, and everywhere I saw a very flat response from 200 Hz - 10 KHz.

I'll probably leave the speaker as is. Working with the Neo drivers is really a pain. The results are definitely worth it, though. As I've said before, these are my reference speakers - I suppose it has a lot to do with their waterfall plots; they are incredibly detailed without being tiresome.