Finally got around to that test I mentioned earlier.
So I built a single test speaker with 5 individually sealed Dayton DS135-8's (5"). Not normally a choice for a sealed enclosure but fairly conventional except for a high Qms,low volume requirement and low price. These are nominal 8 ohm rated drivers. They were arranged vertically with 9 inches center to center and a 9 inch wide baffle. That approximates a typical MTM setup but without the T. I kept the volume and the microphone fixed. This is an Omnimic USB microphone. I tested with both "pseudo-noise" and short sine sweeps and measured max SPL. The amp used was a bridged Plinius SA-100.
For a starting point, I measured a single driver at 78 dB C weighted. (+- 0.1 dB for either test)
Two in parallel (4 ohms) measured +4 dB higher than a single. A touch lower on the short sine sweep.
Two in series (16 ohms) measured -1.5 dB lower than a single.
Four in parallel/series (8 ohms) measured +1.7 dB higher than a single. In earlier tests with this setup, I did notice a comb filtering effect and a dip around 1 KHz at the same mic location.
Won't draw any conclusions but I encourage anybody else to try a similar experiment. Results do go against standard convention. With closer spacing, or other factors, results may vary.
This test speaker was for more than just this. It also proved that I would need five of these drivers in a parallel/series 2.5-way setup to achieve some BSC and a fairly flat in-room response.
