What's with 4 ohm speakers?

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

I may be wrong but FWIU halving the impedance will not increase efficiency by 3db. If you look at the spec's more closely you will notice that Morel spec's the 8 ohm at 89 db at 1W/1M which is 2.83 volts into 8 ohms. If you look at the 4 ohm spec sheet they stayed with a 2.83v input which is actually 2 watts into 4 ohms. So the doubling of the wattage is where the 3db comes from.

It seems to be fairly common practice among driver manufacturers to spec a driver that is available in both 4 and 8 ohm versions in this manner. Can't say as I blame them since they believe that the majority of there drivers will be used on a "constant voltage paradigm" type of amp. Thus the amp is going to try to keep the voltage output the same regardless of impedance.

Jjrenman, you are absolutely correct. People commonly assume that Efficiency and Sensitivity are the same but that is only true into exactly 8 ohms.

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.

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.

Actually this sounds pretty predictable to me. The Plinius can double power when impedance is halved, so it would make 3 db more output. Depending on where the mic is placed, you will find out about beaming and line source effect as well.

If you ran the same tests with a tube amp you would get different readings- likely more output with the drivers in series rather than parallel.