what is Ql ? ported box building questin

[jaygeorge1979]

what is Ql ?

i am trying to use winisd to design a ported box for the tangband 6.5" neo subwoofer

there is already a design on partsexpress's support page, but i cant figure out how the designer came up with this design cuz my port length calculations say it should be longer?

the only thing i havent messed with is the Ql, which i didnt know i could change till i held the mouse over it and it said "Click to change. Do not change this if you do not know what it is." well, i want to know. I may want to change it to see how it affects everything...

someone please help

[kelticwizard]

I believe Ql is the number assigned to the leakage of the box. Usually it is a standard number, and changing it changes things very little. The best thing to do is to leave it in the default setting.

If you change the port lengths in WinISD, you will find that even fairly large changes in port length do not change the tuning frequency much,and certainly not the response to any huge extent. If Parts Express says a port should be 5 inches long and WinISD says 6 1/2 inches long, I wouldn't worry much. Make it either one-it won't change anything drastically.

[jaygeorge1979]

interesting...i was unaware of this

i am using the beta version of winisd, and it willl not allow me to alter the port lenght. It only allows me to change the number of ports, the shape, and the diameter...then it tells me the necessary port lengths and vent mach...

what is vent mach anyways?

[kelticwizard]

Mach: the speed of sound. Varies with altitude, about 760 mph, (or Metric equivalent), at sea level.

There is a lot of air coming through the port when you near the box tuning frequency. In order to squeeze all the necessary air out of the port in time, the speed, (velocity) of the air must increase, just as you must increase the velocity of water through a hose in order to empty a large pool in a certain amount of time.

The larger the diameter of the port, the slower the air speed through the port must be in order to allow a certain amount of air to leave the box. When the air speed through the port gets too great, the port makes noises which are undesirable. On the other hand, if you make the diameter of the port larger, you must increase the length of the port to tune the box to the same frequency. So you try to balance, if possible.

So vent Mach is simply the percentage of Mach speed, (760 mph) that the air in the port must travel using that size port in that size box. Vent Mach goes down as you increase the diameter of the port.

[kelticwizard]

Originally posted by jaygeorge1979
i am using the beta version of winisd, and it willl not allow me to alter the port lenght. It only allows me to change the number of ports, the shape, and the diameter...then it tells me the necessary port lengths and vent mach...

You can do it in a roundabout way. Since lengthening the port results in a lower box tuning frequency, (Fb), simply go to the part of your WinISD display that says "Box" and write a lower tuning frequency in the box there. Then go to the "Vents" section to see how long a length is required to tune the box to the lower frequency you just wrote in.

I think you will find that there is very little difference in the box tuning frequency when you change the box tuning so that Parts Express' port length is used. Assuming, of course, that WinISD is using the box volume that Paarts Express recommends.

[jaygeorge1979]

veery interesting...thanks for the posts!

i do have another mach question tho...how can i know when the mach value given will result in undesired air noise? cuz the box i was designing returned a mach value of .11. is this acceptable? how can you tell?

thanks again

[kelticwizard]

In WinISD, the acceptable velocity values are written in green. The unacceptable ones are written in red.

[jaygeorge1979]

thanks....i like the way you think

i think that is all i need to know for now