Hello all, I don't mean to be disrespectful to those individuals who have spent years in the industry, but I don't understand the logic behind port noise. With the decision to build my tempest sonosub, I downloaded team linear's WinISD. This nifty little program was telling me that it wasn't safe to have a vent mach of over .08. I've heard from some on this board that this is because you get compression of the air, which causes unwanted noise. However, in the field of aerodynamics, any air flow velocity under mach .3 is still safely considered in the INCOMPRESSIBLE mach regime. Even beyond this up to .8 mach is considered subsonic, with some regions approaching mach 1 possibly because of localized accelerations of flow over a body. In addition, when taking into consideration the effects of elevation and temperature and humidity, this still doesn't bring a mach number of even .1 remotely close to compressible. Now I'm really not looking to rock the boat with this one, and I will say that many elements of speaker design are beyond my knowledge and experience, but this matter has piqued my interest. I apologize for the long post, and thank you for any thoughts you may have.