What makes a High Efficiency Speaker

Sensitivity is more often quoted these days, and for solid state amplifiers it is more appropriate. Sensitivity is dB per 2.86 volts, which is one watt for an 8 ohm speaker and 2 watts for a 4 ohm speaker. The reason it is appropriate is that for 8 ohm speakers it is the same, and for 4 ohm speakers most solid state amplifiers will put out the same voltage loaded with 4 ohms or 8 ohms so the dB numbers can be compared directly. A tube amp will not output the same voltage into 4 or 8 ohms, (the 4 ohm output tap provides half the voltage of the 8 ohm tap) so if you use a tube amp of a given 8 ohm power rating, your maximum SPL will be lower.

Lot's of talk about dB/watt vs dB/volt, but let's not forget that this spec is measured at a particular distance, one meter I think. SPL decreases with distance from the source, and different speakers distribute their sound differently. In particular, sound from a planar or line source speaker system falls off much less than sound from a point source. For example, with my Maggies, I can walk from the far end of my room right up to put my ear to the speaker and hear almost no change in SPL. Maybe I'll do that with my RS meter and post numeric results. Someone else can do a PS speaker. Bottom line is that for two speakers with the same sensitivity or efficiency spec, one planar and one point source, from a normal listening distance the planar will sound louder.

An exception to this would be highly directional Low bandwidth horns...and I am thinking of a Bullhorn, such as used by police and fire departments for crowd control. These must be well over 100 dB.

Here are some SPL measurements.

Planar is a MG1.6. PS is a Dynaudio Gemini (small MTM).

Signal is pink noise generated by Prepro (intended for setting levels).

Distance is feet.

Distance..Planar...PS.....PS
........0........100.....100
........1.........98......90.....108
........2.........96......88.....102
........3.........96......85......99
........4.........95......83......97
........5.........94......81......96
........6.........93......80......94
........7.........92......79......93
........8.........91......77......93
........9.........91......77......92
.......10.........90......77......92
.......11.........90......77......92
.......12.........90......77......90
.......13.........88
.......14.........88
.......15.........87
.......16.........87
.......17.........86

The first PS column shows the SPL leveling out at 77 dB. I think this reflects room effect. For the second PS column I increased the volume so that the SPL at 12 feet was the same as the planar, 90dB. This required the near field SPL to be a lot higher, 108 dB at 1 foot.

I don't think anyone listens at a distance of 3 feet, although that is close to the 1 meter distance used for the spec. At a more reasonable listening distance, between 8 and 12 feet, the Planar SPL falls off by 1 dB over this distance range, while the PS falls off by 3 dB. To produce equal SPL at 12 feet the PS speaker would need to be about 3dB more efficient at 1 meter. I think that these results are generally in agreement with theory.