If I understand Nrenter correctly, this means 0.7 multiplied by the frequency at which your speaker rolls off -3db in the bass region. A common speaker measurement, and what everyone is generally referring to when discussing how low in the bass a speaker can go. So for example, my speaker's -3dB point is 70Hz, meaning that according to Nrenter's rule of thumb, I should crossover at about 50Hz. Which interestingly is where I have the crossover set. I hadn't heard this rule of thumb, but using an SPL meter to calibrate the subs and mains, 50Hz was the point where I could get the flattest bass response. Above that and I was getting some humps between 80-100Hz.
