Some speaker driver measurements...

As to what is the true Xmax of a driver, it depends on what your design will allow. Many times, the Xmax on a spec sheet is at a higher distortion level than what you would like to tolerate.
True. Some are more focused on SPL than SQL. Some state that you can't really hear issues in the transient domain bellow 50Hz and that it is pointless to address or try to fix the cabinet such that maximum transient (low group delay) is displayed sub 50Hz or even 100Hz.
I do not agree to such views - the speed of a woofer determines the overall clarity of the loudspeaker. A slow bass/sub bass makes the whole loudspeaker sound bloated and sluggish.
I'd rather have a loudspeaker that is fast, aka have stellar performance, compared to heaps of SPL output, because listening to elevated levels (90-95dB or more) for a prolonged time is very unusual.
 
Regarding multiple woofers vs single. Typically multi, like 2 woofers, will perform better since in part, you have the same SPL output as one large but half the excursion with the 2 drivers.

Less excursion retain more motor control, the total distortion will therefor be lower with 2 (or more) drivers compared to 1 large.
It's actually pretty interesting... this is the expectation and seems to be quite intuitive, but it is not always the case.

Let's look for example at the SB15NBAC30.

We see that H3, H4 and H5 barely change when going from 2.83 V to 5.6 V input. So, if we use the higher level as a baseline, we see that half the voltage and half the excursion are not going to deliver half the distortion (no matter how you calculate the "half"). Of course, many drivers have distortion that scales up at least 1:1 with input and excursion.

Strangely, you see more of this behaviour with tweeters than with woofers.
 
Right, I see what you are getting at. The individual distortion does not change, so in that regard I agree.
What I was getting at is that typically, when you demand less excursion for a given SPL, the cone will travel less and do the same job due to a companion. Less motion equal less material stress.
So while the individual driver's distortion remain the same, the system (loudspeaker) will express less distortion compared to using a single woofer.

Some evidence for this statement can be found in looking at distortion in low signal vs high signal input. Very few drivers maintain the same harmonics profile, but again the exception, so there are drivers that indicate the same harmonic distortion as the voltage increase.

I wouldn't go so far as to say that this is a rule, but more as an indicator or potential benefits of using multi vs single woofers. Every rule has exceptions and so does this one.

:)
 
The SB26CDC was measured on this page :

SB ACOUSTICS SB26CDC-C000-4 (Tweeter 1", 4 Ohm, 200 Wmax)

Very good measurements, just like the SB26ADC. Personally I don't think there will be much difference, if any, between the aluminum and the "ceramic" (anodized aluminum) versions.

However, one driver that does not seem to have been reviewed or measured anywhere is the small SB12CAC (for which there is no aluminum version for some reason). Since the BAC/CAC family measures so well, I would be curious to see what this small driver can do in the higher midrange.

4” SB12CACS25-4 / Ceramic – Sbacoustics
 
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the nac tweeter has a biger bump but beyond 20 k...
looks like the cac tweeter has a little fantasy perhaps towards 6K hz that can be ringy ? Don't know if one can hear it though cause it's very little bumpy but ears are sensible also here. I like the dip and the power response towards 2K to 4/5 K hz :)...


max spl ?
 
The SB26CDC was measured on this page :

SB ACOUSTICS SB26CDC-C000-4 (Tweeter 1", 4 Ohm, 200 Wmax)

Very good measurements, just like the SB26ADC. Personally I don't think there will be much difference, if any, between the aluminum and the "ceramic" (anodized aluminum) versions.

However, one driver that does not seem to have been reviewed or measured anywhere is the small SB12CAC (for which there is no aluminum version for some reason). Since the BAC/CAC family measures so well, I would be curious to see what this small driver can do in the higher midrange.

4” SB12CACS25-4 / Ceramic – Sbacoustics

Thanks for this info, I have these guys ( 17CAC and 26CDC) on going 2way flooring. I just want to know how good they are. :)
 
I suppose at this price point you can afford to route into the tweeter faceplate when creating the rebate for the driver that sits below it, bringing them closer together.

Like this...

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Even if it is a pain to do.