Awkward box shape + ported?

[helotaxi 5,000+ posts]

Ok so i found the air velocity graph in WINISD. I'm getting max 34ft/s with a 2" diameter port that only needs to be 1.29" long. That's way under 110ft/s. This can't be right. It's almost like just poking a hole in the box, and I'm never seen anything like that. I'm starting to get really confused.
If I change it to "two free ends" it only increases the length to 1.75". Which if this is right, would mean there would be absolutely no reason to ever go with a huge slot port because a tiny piece of pvc would work just fine. And that can't be true. Can it?

Change the power in the signal tab to the neighborhood of what you will be sending to the subs and watch the port velocity go through the roof. You're gonna want much more port area than what you are considering. Run too small a port and you're gonna get a nasty whistle from the port. With the port way too small it will cease to function as a tuned port and the will end up as nothing more than a leak in a tuned box.

 

[Rawr-DQ 10+ year member]

Also, you want a flat response? that box wont give it to you, you'll have all sorts of peaks and valleys in response all along the frequence chart, a sphere will give you the flattest response.

But really, when you need a flat response it takes all kinds of tuning and adjustments with an EQ/ect. to get a flat response, and is mainly done with the desire of an all out SQ system...that and a Home audio system.

 

[helotaxi 5,000+ posts]

Also, you want a flat response? that box wont give it to you, you'll have all sorts of peaks and valleys in response all along the frequence chart, a sphere will give you the flattest response

That really only holds true for a full range speaker. A sphere won't hold a standing wave. Standing waves aren't an issue in the sub bass range in a small room.

 

[Rawr-DQ 10+ year member]

such as a car =p

 

[Trendkill 10+ year member]

Yeah I wasn't expecting perfect, just as close to an in-car-sealed-box version of "flat" with a ported box instead, if it's possible, which I've read it is. Thanks for the tip about the signal tab. That was definitely the missing factor. Now a few more questions:

1. Why is it that when I change the end correction, only the length of the port increases (and only slightly) but the air velocity doesn't budge? I thought the flared ends were supposed to reduce air velocity.

2. How useful is the "1st port resonance" stat

3. Is the "110ft/s" guideline only for audible frequencies? Or would subsonic frequencies produce audible port noise when pushed through the port too fast?

Thanks.

 

[helotaxi 5,000+ posts]

Flanged ports only mean that the port is flush with a baffle of some sort. Free ends are the other option in WinISD which means that the port ends are in free space. WinISD doesn't do flared port calcs.

such as a car =p

Exactly. Standing waves aren't an issue in the sub bass region in a car, and a spherical enclosure is pointless for sub bass. The response will never be perfectly flat, but the response curve in the bass region is usually MUCH smoother than in the upper freqs.