Center Cabinets - Sealed or Ported?

One curious phenomenon I noticed while designing my center channel speaker was the almost universal use of ported cabinets.  This included designs from:

• Legacy
• ATC
• Troels Gravesen (DIY) 
• Wilson 

The reason I scratch my head here is that usually we cross centers around 80 Hz, and a ported cabinet is just so much bigger than it's sealed counterpart and you lose dynamic range trying to get lower than you otherwise would.  If I couldn't make my own center channel I'd probably buy something from Monitor Audio like the c250 which follows all of the principles I've been writing about, including using a sealed cabinet, and 3-way crossovers with optimum crossover points:

My goal for the center, besides dispersion and low distortion was high dynamic range.  Deep bass was not important at all.  Lets look at a simulation from WinISD for two configurations: 

• Sealed, f3 = 80 Hz
• Ported, f3 = 50 Hz

In these two simulations we assume the same two 7" drivers, but the ported cabinet requires about 2-3x as much volume.  Yes, it's huge.  For this simulation we assume the limiting factor is cone excursion at the lower -3 dB boundary:

That is, we ask, how loud can we play a sealed cabinet vs. a ported cabinet if we limit the -3 dB point for each? Given those excursion limits you see above, we draw the SPL curves.  Note these do not include the effects of the high pass filters, they are cabinet only:

 

 

Notice that the difference in -3 dB points is only about 25 Hz!! So, we make a very large cabinet to gain 25 Hz.... but going deeper has a cost, excursion.  

At 80 Hz the limits of the driver excursion is around 111 dB, but if I attempt the same excursion limits at 50 Hz (green) I'm limited to about 105 dB.  That's a significant difference in output.  One thing you could argue is: "Well, I can set my high pass filter to 80 Hz with the ATC and have more dynamic range" and you'd be right, but you'd still have a cabinet 3 times bigger than it has to be. 

The center I designed is still large, 24" x 12" x 12" (approximately).  All the other designs are much larger than this.