Tweeter- dual impedance peak?

[tktran]

I was checking some tweeters through my DATS and noticed that one of my tweeters has a dual impedance peak.

The other one doesn't.

What could be the source of this problem?

[JonMarsh]

Are they the same tweeter type and part number? I’ve seen dual peak on some models of tweeters, which comes down to having acoustic resistance in series with two different chambers of different sizes and effective compliance.

If they’re the same part number, then I’d say one has an assembly defect. Is the main peak at the same frequency for both?

[tktran]

Hi John,

Yes it’s a T25CF002 Millenium.

The main peak at 600Hz exists for both my units, but the 2nd peak that you see around 1.7-1.8 ish isn’t.

And certainly doesn’t appear on the spec sheet.

This is an old driver and sometime ago I did replace a voice coil on one of my dead units, but I can’t be certain if this the repaired unit. Or whether this is the second one is dying…

I’ve tried unassembling and reassembling the voice coil/faceplate and haven’t managed to fix it, nor can I see what I’ve done incorrectly.

[Scareurpasenger]

I saw this in a rs28 that need some diaphragm/coil alignment. It was a bit temperamental to get resolved and required sweeping 5 times before I corrected the issue.

[Reet]

I'd suggest that the chamber has a leak or they forgot to put the damping material in.

[Zvu]

I've seen similar behaviour when ferofluid starts thickening in time.

If there is FF in those tweeters, check the production date and if it's older than 10 years it could be that.

Also, open both tweeters and replace the magnets to see if the same problem persists with same dome. That will help isolate the problem - which dome or which magnet.

[tktran]

Just circling back on this in case of future reference.

Turns out the voice coil/dome assembly wasn't installed correctly. This was MY fault (as I stated, I had repaired a dead tweeter), but didn't check with an impedance sweep after doing so.

The remedy was to unscrew the voice coil/dome assembly, and rotate it 90 or 180 degrees and reinsert it. Then, whilst applying gentle pressure, and test with an impedance sweep BEFORE tightening all the screws.

One screw that is too tight or too loose, out of 4 screws, may have caused some kind of non-symmetrical rocking mode, or something like that, to give the 2nd impedance blip...

All fixed, and back to Normal now.

[JonMarsh]

Thanks for the follow up TK and explaining the solution- could be a big help to someone else some day!