Effect Ohms Have on db and Speaker Sensitivity

I'm not sure if this is why you could be confused, but some driver manufacturers do offer 2 versions of the same woofer: an 8 and 4 Ohm.

The 4 Ohm allows twice the current to flow in the coil, which means 2x the power for the same V will be dissipated. The advantage in this particular case, everything else being equal, is the 4 Ohm version gains 3 dB of sensitivity.

It is only in this type of situation, where everything except the voice coil impedance is the same, that we can generalize and say that 4 Ohm versions are more sensitive. We cannot take this and compare drivers in general even within the same manufacturer. The magnetic strength, gap, weight, cone diameter, suspension, all come into play.

First, be aware no spec gets fudged more than sensitivity! :) Most speakers are rated 3-5 dB higher than they actually are.

dB at 1W / 1m is "efficiency" and varies by load impedance, which as stated above, is 2.83V at 8 Ohms. Think of it as "power efficiency." With modern SS amps this is a meaningless measure really. You want sensitivity.

dB at 2.83V / 1m regardless of load (and therefore regardlesss of power) is "sensitivity." Again, think of this as "voltage sensitivity" to try to remember why they are not the same.  This measurement makes more sense, since most speakers impedance varies WIDELY at different Hz. Measuring between say 6 to 20 Ohms in the same speaker is quite typical. I don't really care about how much power is being dissipated at 2,450 Hz as much as how loud it will get with a 40 Watt amplifier.

Also, the exact Hz at which the dB are measured varies. I use 1 kHz, but some manufacturers may try to eyeball it.