Hi all. I have been working for about three weeks on my Tempest Sonosub. The sonotube is 18" and 54" tall (I discounted here the MDF's for the driver and the port). My calculations are 225L tuned to 20.50 using a 6" port diameter with a length of 19.7". According to UNIBOX this would give an F3 of 21Hz, which is apparently the case. At the time I ordered the Tempest I ordered a calibrated mic also, but the guy who told me about it resulted to be a fraud. So I don't have the correct tool. Anyway, using my SPL Meter I get this graph at my listening position. Frequency Response from 10 to 100Hz at listening position This the raw data using the SPL Meter as a mic, without calibration. I know it is tunned to 21Hz, but I don't like the fast roll-off after that precise point. Perhaps the SPL Meter has something to do about it, or perhaps my room (which resonate between 40Hz and 50Hz by the way). Anyway guys, what do you think about it? Should I tune it lower? I want to get around 16Hz at decent SPL's if possible.