# Vent Mach and WinISD

Discussion in 'Amps and Cabs [BG]' started by rllefebv, Jun 29, 2002.

1. ### rllefebv

Oct 17, 2000
Newberg, Oregon
I'm having a difficult time sussing this out, and so decided to come to the source...

When toying with cab designs on WinISD and clicking on the 'Vents' tab in the project window, I see an information box containing 'Vent Mach:' information and a red or green number, depending on the number or size of the ports that I choose. Can anyone explain what 'Vent Mach:' is, or what type of numbers that I should be aiming for??

Many thanks,

-robert

2. ### bgavin

May 3, 2001
Orangevale, CA 95662
MACH is the speed of sound in air, 1130 feet per second, and 0.045 MACH is 50.85 feet per second. The spec refers to the air velocity in the vent. I've found that a velocity higher than 0.10 MACH at full power is asking for port noise. The MACH value of 0.045 or less is the "optimum" velocity for noise-free vents.

Link to Robert Bullock Design Program DOSBM.EXE

This directory on my web site has a bunch of drivers worked out for Robert Bullock's DOS Box Model program. It will run under Win9x or NT/2000 and will show you the velocity of air in the vent for various power levels and frequencies.

The vent velocity is highest at the tuning frequency, and varies by the driver displacement and applied power. For example, my Rockford subs displace a large amount of air at full power and require a very large diameter vent. If I run the same driver limited to 50 watts, the vent diameter can be much smaller without getting vent noise.

Another point is where the box is tuned. I tune my subs at 23 Hz, but my lowest usable note is 31 Hz. I never reach the actual tuning frequency, so I can use a much smaller vent diameter. The Bullock program will show you this also.

I aim for 0.10 or less. If I can reach 0.045, that is great, but 0.10 at full power works for me, since I rarely/never hit the tuning frequency at full power. If your tuning is inside your usable range, you must take this into consideration.

The Bullock program is also very useful for seeing how much power can actually be applied to a driver before it exceeds its cone excursion limits. Check out the Carvin PS10 driver for this.

3. ### rllefebv

Oct 17, 2000
Newberg, Oregon
Thank you for the helpful reply Bruce. This is beginning to make sense

-robert

Sep 2, 2005
North NJ
bump

;-)

5. ### bgavin

May 3, 2001
Orangevale, CA 95662

6. ### Filthybass

Jul 25, 2012
Northern California
Help..i run a svt pro 4 thru 8x10 and a gk rb 800 thru 2x15,when svt is off,the gk doesnt buzz,once I turn on the svt,the gk buzzes,the svt never buzzes..thanks

7. ### Filthybass

Jul 25, 2012
Northern California
The 2 heads are housed together in a rack case..bass is running thru a "y" connector..one in to the svt,other into gk

8. ### will33

May 22, 2006
austin,tx
???

Grounding problem.

If the 2 amps are running on different circuits. YOUR BODY CAN BECOME THE PATH TO GROUND.

9. ### Filthybass

Jul 25, 2012
Northern California
It doesnt buzz until I turn on the svt

10. ### 1958Bassman

Oct 20, 2007
Think of a vent as a bottle and you're blowing across the top- if the air passes by quickly, it acts as a whistle and if it's too slow, it doesn't. It also works well at one frequency, just like the bottle. Change the length or diameter- you'll also change the tuning frequency, just like adding/subtracting liquid from the bottle. The diameter has a huge effect on the air speed needed to produce port noise- larger needs high speed and smaller will make noise with less speed.

11. ### nashvillebill

Do you guys realize this was a 10 YEAR OLD thread, bumped once three years ago, and then somebody out of the blue revived it by asking a totally irrelevant question?

JJ is Coo likes this.
12. ### bgavin

May 3, 2001
Orangevale, CA 95662
Indeed.
I let my ancient OFGB web site go years ago, so my link above is not valid.

13. ### 1958Bassman

Oct 20, 2007
He started it!

So tell a moderator and have it deleted or moved.

14. ### KJMO

Feb 6, 2015
I'm going to revive it again, because the question wasn't answered super clearly, and I ran into it when looking for the answer. With one 10cm port, the vent mach number in winisd is red (why did it spit that out if it doesn't work?), so I added another 10cm port and it is now at .04, and green. So I take it that is better? Course, it spit out a port length of 1.5cm, so I don't know what to think about that?

Fwiw, if exactly the question I have has been asked before, even if it was a long time ago, why not continue this thread? It came up when I searched, so it may help someone else.