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[Mark Wilkinson]

Ok, my 2 cents...probably worth less...but at least offering a slightly different take...and some more time for popcorn 

I don't think bridged vs dual-channel is even worth discussing RMS wise...both setups should have the same RMS limiting, around 60v...or only around 500w into 8 ohms.... (which is quite often the same for many drivers),
 
both setups deliver this.....

I think bridged vs dual-channel is about head room....
And IMO, 2.4 dB as offered by the amp in bridge mode that is being discussed is huge...HUGE!!!!!!

OP, if you hear clearly different, detrimental,  results from bridged vs dual, something is wrong....
Or your drivers really dip down in impedance...
What does WinISD say about impedance?  It appears you like the correlation between its modeling and measured results, yes?
Unless impedance is dipping below 4 ohms ( for drivers in parallel) I don't think you should be hearing the differences you speak of, or seeing the amp heating up differently.
Really, really,      you simply have to know the impedance curve......

[Patrick Cognitore]

...The spec. sheet for the XTi 6000 claims 6Kw into 4 ohms.

Doesn't the spec sheet specify the power at 1khz, 20ms burst? If so, you may not be seeing anything close to that wattage, whether bridged or in stereo.

Do you have one sub cab and one amp, or are you looking at powering multiples?

[Art Welter]

Even with a 30Hz roll-off and high-pass filter applied, If I were to place input gain at unity and put any sort of transient like a kick drum through the system, the cones would jump across the room. 

Jeff,
A 6 dB increase in power is required to double excursion.
If you have a proper HP (a few Hz below Fb) applied, any transients should be only hitting maximum excursion around 1/3 octave above Fb.

Not to doubt your observations, but one "gotcha" that has got me is that the setting of the inputs for bridge mono operation is a critical step- some amps require both inputs set the same (best "wide open" to avoid a mismatch in the +/- waveform), others disconnect the second (right) input so "user errors" are eliminated.

After RTFM I realized a series of  "bridged mono" tests done on an amp with channel 2 down had actually been only the + portion of the signal- so the "unimpressive" bridged mono output results were actually the same as a single channel doing both +/-. Hard to notice distortion with pink noise, but when I ran some music, doh...

At any rate, just like your previous "illumination" in finding your box Fb, you should verify that both SPL and excursion increases by 4.5/6 dB between one side driving both speakers and two sides bridged mono, if the cones look like they would "jump across the room", something may be wrong in the set up.

Cheers,
Art

 

[Bob Leonard]

Popcorn for the show, anyone?

I don't usually eat popcorn, but I like the caramel covered stuff with a gin tonic or three.

[Jeff Schoonover1]

I think bridged vs dual-channel is about head room....
And IMO, 2.4 dB as offered by the amp in bridge mode that is being discussed is huge...HUGE!!!!!!

I thought less than 3dB would be a barely or not at all audible change?  Electrical dB's or acoustic dB's?

What does WinISD say about impedance?

Impedance at about 35Hz, is about 4 ohms.

Unless impedance is dipping below 4 ohms ( for drivers in parallel) I don't think you should be hearing the differences you speak of, or seeing the amp heating up differently.

I don't remember who mentioned it, but a good point is that when channels are bridged, each sees half of the total impedance.  IOW, 2 ohms.

[Jeff Schoonover1]

If you have a proper HP (a few Hz below Fb) applied, any transients should be only hitting maximum excursion around 1/3 octave above Fb.

I ended up with ~32Hz Fb. And yes, my observation is that excursion increases above the Fb to about 40Hz, after which it begins to subside again.  (Outdoors, my box has a really nice flat response ).  In bridged mode, I'm only able to set the attenuation (input level knob) at about 1/2 to 2/3 max., with EDM or testing sweep before that little voice starts saying, "Man, you gonna blow that speaker right out the frame..."

setting of the inputs for bridge mono operation is a critical step

I will read again, but I'm pretty sure the XTi cancels out the channel 2 attenuation knob. 

At any rate, just like your previous "illumination" in finding your box Fb, you should verify that both SPL and excursion increases by 4.5/6 dB between one side driving both speakers

I haven't done this, I've only run bridged into parallel, or single channel per speaker.

if the cones look like they would "jump across the room", something may be wrong in the set up.

WinISD shows that with 6KW, maximum excursion at ~40Hz is nearly double the 'red line.'  With 2.4K Max excursion is only exceeded by a mm or so. 
On the SPL graph, It shows that 6KW is a gain of ~4dB over 2.4K

[Jeff Schoonover1]

My next thought regarding over-excursion is that since some people (me) are bad at judging loudness by memory, it could be that my perception of "loud" while testing with dual channels was different than many days later testing bridged.  So, maybe I wasn't comparing apples to apples.  Admittedly I didn't measure distances of microphones for testing, I eyeballed it, and moved the mic stand to where the sound wasn't overloading the mic.  The response curves didn't change, but SPL, of course, varied with distance.
But I also eyeballed the cones in both modes.  "Cranking it up" and watching the cones move made my head's warning bells go off if I started moving close to maximum output in either mode.

[Jeff Schoonover1]

Doesn't the spec sheet specify the power at 1khz, 20ms burst? If so, you may not be seeing anything close to that wattage, whether bridged or in stereo.

Yes it does, and I agree.  Perhaps Moreso with bridged than with stereo.

Do you have one sub cab and one amp, or are you looking at powering multiples?

one sub which contains two 18" drivers, and one amp, a Crown XTi-6000

[Mark Wilkinson]

I thought less than 3dB would be a barely or not at all audible change?  Electrical dB's or acoustic dB's?Impedance at about 35Hz, is about 4 ohms.I don't remember who mentioned it, but a good point is that when channels are bridged, each sees half of the total impedance.  IOW, 2 ohms.

Electrical or acoustical should be the same thing within linear range of operation. 
IMO, everyone should decide for them self whether 3dB is audible. 
I think it's very audible...even when just going for crest factor headroom.
Lord knows it costs enough, every time we want to double power.

I'd look at the entire impedance curve within the passband...if it dips below 4 ohms anywhere it represents indigestion for the amp.
For the reason you state...that each channel sees half the impedance..and we're trying to stay above a channel seeing 2 ohms....
Do look higher up than just 35Hz, often impedance drops low up around the sub's low pass freq.....

BTW, where do you see the 20ms burst spec for the xti 6000 or 6002?  I've seen it for the itech HD's but not the xti's......
I was thinking, hoping, the xti's still had older, less obvious marketing specs...
The dang itech HD's spec out like I-Nukes     lol

[Jeff Schoonover1]

where do you see the 20ms burst spec for the xti 6000 or 6002?  I've seen it for the itech HD's but not the xti's

Break out your magnifying glass, LOL
https://www.crownaudio.com/en/products/xti-4002

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