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Re: 4Pi crossover study [message #77257 is a reply to message #77254] Tue, 23 July 2013 01:21 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

We can definitely see the response change corresponding to C1 value changes. So that's good, to be expected. But for whatever reason, the system takes a nose dive at 8kHz or so. It will be interesting to see if all tweeters show this trend (indicating a likely measurement anomaly) or not (indicating a tweeter problem). Thanks for keeping us posted.

Re: 4Pi crossover study [message #77271 is a reply to message #77257] Tue, 23 July 2013 21:16 Go to previous messageGo to next message
Joe Sever is currently offline  Joe Sever
Messages: 11
Registered: May 2009
Chancellor
I just remembered that my daughter had a little Samson USB Mic, so I thought I'd give that a go just for grins.

It's too late to do any serious measurements or super accurate mic placement (and I forgot to save the couple measurements I did do), but with the crossover in place (no C1), more or less on axis, roughly 18" from the CD diaphragm, the freq resp from 3K-20KHz is actually tipped up!

More measurements tomorrow. In the meantime, I'll resist the urge to throw my USB mic pre in the lake.
Re: 4Pi crossover study [message #77273 is a reply to message #77271] Tue, 23 July 2013 22:08 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

Very good. Now you can have another run at it.

Re: 4Pi crossover study [message #77283 is a reply to message #77257] Wed, 24 July 2013 20:09 Go to previous messageGo to next message
Joe Sever is currently offline  Joe Sever
Messages: 11
Registered: May 2009
Chancellor
Not out of the woods yet.

Here's 20-20K on the standalone USB mic. The downward trend from ~7K-20K is clearly there:

index.php?t=getfile&id=1222&private=0

So I'm starting to feel more confident of - but more discouraged by - the previous measurements.

Once I scare up some way to mount the other CD I'll see how it measures.
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Re: 4Pi crossover study [message #77284 is a reply to message #77283] Wed, 24 July 2013 21:10 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

Interesting. The response from the SEOS12 is as I would expect between 1kHz and 6kHz, but above that, it has more rolloff than it should have just from mass. There's 20dB drop from 6kHz and 10kHz, more than 18dB/octave. This would only happen if voice-coil inductance and the front chamber conspired to rolloff at the same point, and they're usually staggered, with mass starting around 4kHz, inductance around 12-16kHz and front chamber just above that.

Could be that the phase plug is blocked with debris, or maybe the diaphragm is misaligned. You might open the compression driver and check it out. Sometimes just taking it apart and putting it back together fixes that kind of thing.

Re: 4Pi crossover study [message #77296 is a reply to message #77284] Fri, 26 July 2013 15:46 Go to previous messageGo to next message
Joe Sever is currently offline  Joe Sever
Messages: 11
Registered: May 2009
Chancellor
Tried the other CD I had on hand, and the results floored me:

index.php?t=getfile&id=1223&private=0

While I have some work to do on the cabinets (bracing, damping of standing waves, etc.) and perhaps more traps in the room, I think we're done here. That said, I'm always open to suggestions...

Thanks, Wayne, for your guidance and support.
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Re: 4Pi crossover study [message #77302 is a reply to message #77296] Sat, 27 July 2013 10:01 Go to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

Yes, that looks good. The other driver must be messed up.

Can you send it back for a warranty replacement? If not, you might want to take it apart and see if there is debris in the phase plug. Make sure it is clear and free of dust. Sometimes just taking it apart and putting it back together recenters the voice coil and fixes problems like these too.

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