i think when u're looking for efficient subs look at the sensitivity, for a good sub it should be around 86db and up. i think thats part if im correct.
sensitivity is a common misconception of how efficient a sub will be. look for a woofer that has a loose suspension, and lower rms recomendation, and go ported. aaron, gotta agree with ya, cerwin are quite efficient.
yeah, sensitivity specs are more accurate for comparing mids and highs, but i still use them to compare subs atleast as a starting point.
in addition to a loose suspension, low-mass cones & coils, high energy magnets, and larger diameter contribute to a sub being more efficient.
lower rms recomendations only mean the speaker is more efficient if its coil & cone weighs less compared to its motor strength. some higher power subs use better designs and more powerful motors to more easily push their heavier coils & cones.
as mentioned in previous post, underhung coil designs tend to be very efficient because of the low mass of the coil. they can take lots of power, but only need a few watts to get loud. IMO, this is a very desireable combination.in addition to PPI and crystal, audiomobile MASS subs used this design. it's too bad they aren't making subs any more - i've always wanted to have a set.