These four tweeter models share the same basic design but differ as follows:TW030WA05: 4 ohm, no ferro fluidTW030WA06: 4 ohm, with ferro fluidTW030WA07: 8 ohm, no ferro fluidTW030WA08: 8 ohm, with ferro fluid.Real high-end tweeter design with extremely low resonance frequency and distortion.Thanks to the extensive internal venting between three internal cavities no air inside is trapped. This feature extends the usable frequency range downwards to an extend very rarely found on tweeters.Adding the high power handling this tweeter is the natural candidate for high-end 2-way speaker systems.