# WinISD question on port size.



## Hogger (Nov 25, 2009)

I just started playing around with WinISD, and when I adjust the port size there is a box titled "vent mach" that also adjusts. Sometimes the numbers are red, and sometimes green.
What is this number for?

Side note: Good program, but the whole metric thing stinks.


----------



## jeremy7 (Feb 7, 2008)

I've never used winisd, but I've used similar programs. To my understanding, "vent mach" is the air speed in the port (vent), related to the speed of sound (mach). So in other words, hypothetically, if the number in that box is [.10], then that would mean that the speed of the air in the port is .10 or 10% the speed of sound.
As far as I know, when air speed in the port increases, the port noise increases, and the color represents the audibility of this. In other words when the number is red, the port noise overcomes the accepted audible threshold (you get port noise). 
When I use my modeling program, and the "vent mach" number is red I make the port wider (slowing the airspeed) until the number turns green.

hope this helps
jer


----------



## Hogger (Nov 25, 2009)

O.K. that's about what I was thinking, thanks.


----------

