In the past I have designed and built several mobile audio systems including dozens and dozens of speaker enclosures and consider myself to be fairly knowledgeable on the topic. Depending on the driver, I usually got really good results from tuning to 30-35hz. However, since I have been perusing though the conversations on this site the past couple of weeks I have notice a lot of people tuning their sub enclosures to frequencies below 20 hz. Now, if recall correctly the frequency to which an enclosure is tuned is the frequency that is produced most efficiently with this efficiency trailing off at about 6-12 db per octave. If this is indeed the case, why would you tune your sub to be most efficient at producing a frequency that you can't even hear? What am I missing? Does it have something to do with the different acoustical environment? Have the laws of physics changed?