I updated the page so hopefully it's a little more clear: Low is 10 m/s. Sometimes programs list the vent mach as a percentage or decimal in relation to the speed of sound. So 10 m/s / speed of sound (~340.29) = ~0.03 I don't like to give definitive answers to something that I haven't personally tested, and especially when one size doesn't fit all. Ie: higher vent mach may not be noticeable in one install, but problematic in another. I will however tell you what I know. WinISD wants it under 0.16 54 m/s, yet I've read from plenty of people that that is way too high, and causes audible port noise. I've also read an article from JBL that suggested 10 m/s for reducing port compression and port noise. My last large install had a vent mach around 20 m/s, and it performed quite well with no audible port noise. So ultimately it's up to you to do the research, and decide for yourself, all the while taking into consideration your install. Enclosure in trunk? Another thing to consider is what number your putting in for power. If your amp is rated for 1500watts at blah blah blah it's very unlikely that the driver will ever consistently see that much power, so you could very well do the calculation with 1000watts, or even more likely: even less.