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Discussion Starter · #1 ·
I am building my first subwoofer box.

Here is my current box plan:
Wood Rectangle Audio equipment Triangle Font


It is 2 cu ft, tuned to 35hz. The port is 14.5" x 2.69".

I will be using a TXX-BD2-12. It is 750w rms / 1500w max.


In WinISD, the port velocity, at 750w, peaks around 22.18 m/s (72.8 ft/s):
Rectangle Slope Font Parallel Pattern


How much port velocity is too much? and at what wattage?

I've seen people check port velocity at 75% of the RMS rating. and I've seen people check port velocity at the woofer's max rating.

How do you go about port velocity?


This box will be in the back of hatchback with limited space. I would like this box to be as small as possible.

Could I get away with 30 m/s? 40 m/s?

This site has been a great help! Thanks!
 

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everything under 20-30m/s is good and more than accetable.
 

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Generally the larger the port diameter the high the velocity can be before niose becomes a problem.

If the woofer is in your trunk you're not going to hear it so under 34 m/s is fine. If it is in the same area as your ears I like to stay below 25 m/s.
 

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Generalizations:
20 m/s is SQ limit in your space
40 m/s is SPL or trunk sub limit
60 m/s is where port compression can start to affect the box tune and you will definitely have port noise.

The tuning point and shape DO have an affect on these numbers as well. If you have a box tuned really low (like below 25Hz) then the rising portion of the waveform is slow enough that the air crashing into itself on the reversing wave doesn't create as much turbulence as it does at higher frequencies.

Aeroports let you fudge those numbers about 10-15% (the larger the diameter the greater the factor) and slot ports give you about 5% depending on the width/height ratio of the port (too wide and short or too square less benefit.)

Edit: WinISD port velocity is only at the wattage on the input tab so if you input the maximum wattage for the mechanical excursion above the box tuning point then you get the maximum calculated port velocity. This is worst case scenario. So you use WinISD to create the best port for your worst case scenario and have that be something in the 40s then know that you RARELY listen to the sub at maximum volume at the tuning frequency with a sine wave. So then you back down to 50-75% wattage and see what the results are and if it fits your needs.

One other consideration given your OP - the smaller the box the longer the port needs to be to achieve the same tuning frequency.

Sent from my SM-G965U using Tapatalk
 

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Discussion Starter · #6 ·
Thanks for the help! I'll shoot for around 30 m/s. Now Im realizing I should use a wedge shaped box instead of a square one. I gonna try to draw it up in sketchup. I'll post a picture when I figure it out.
 
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