# Kind of a silly question regarding a leaky ported sub box



## voltij (Feb 15, 2011)

So let's say I take an awesomely constructed ported sub box that is ~6.5 cubes tuned to 28 Hz with ~110 sq in of port area and drill two 3/8" holes into it, purely for science.

What effects would be experienced?


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## Oliver (Jun 25, 2007)

minimal resulting air hiss, as the air would take the path of least resistance which would be the port itself. [ 110 sq in of port area ]


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## minbari (Mar 3, 2011)

probably nothing would happen. with a port volume that big, you would have to have a pretty big hole to change the porting characteristics.


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## Fricasseekid (Apr 19, 2011)

voltij said:


> So let's say I take an awesomely constructed ported sub box that is ~6.5 cubes tuned to 28 Hz with ~110 sq in of port area and drill two 3/8" holes into it, purely for science.
> 
> What effects would be experienced?


It would bring your tuning frequency up to 28.3 hz. Just kidding...

I've actually heard the a good seal can be more critical on a port box than a sealed on, ie. sealed boxes are more forgiving? 

Subscribed! I wanna see what people have to say.


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## minbari (Mar 3, 2011)

no they are not! sealed boxes need to be absolutely sealed! if you have a leaky sealed box the woofer can pump out the air through the leak and actually pull the woofer to the bottom of the suspension. once it bottoms out, its done.


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## OldSchoolRF (Aug 29, 2011)

Agreed. Sealed boxes definitely need to be completely sealed. The small holes wouldn't make to much of a difference on your ported box though.


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## XtremeRevolution (Dec 3, 2010)

minbari said:


> no they are not! sealed boxes need to be absolutely sealed! if you have a leaky sealed box the woofer can pump out the air through the leak and actually pull the woofer to the bottom of the suspension. once it bottoms out, its done.





OldSchoolRF said:


> Agreed. Sealed boxes definitely need to be completely sealed. The small holes wouldn't make to much of a difference on your ported box though.


Sigh. Do you have any idea how much airspace is displaced by a 10" or 12" subwoofer at 15mm of peak to peak excursion? That's 1.18" of total excursion. For a 12" sub, you're looking at .37 cubic feet in a split second. 

Think faucet in your bathroom sink. You open the valve just a bit, and how much water comes out? A few drips. The pressure at the pipes is the same. You open it a bit more, and the water flow increases. Same concept. 

A 3/8" hole or two will not be enough to make a difference. Don't believe me? Build a test box, put a sub in it, get a radio shack SPL meter, and measure them. Also, give them the exact same amount of power, and check the excursion. Those holes would be way too small to make a difference. 

Sealed boxes do not need to be completely sealed. In fact, it is good to have a very small amount of air leaking. If you push the cone down, it should go down very slowly with a bit of pressure, and should come back up slowly as well. 

This is in order to account for changes in atmospheric pressure and air expansion due to temperature. If you build the sub at sea level and drive up to colorado where the atmospheric pressure is much less, do you want a perfect seal so the sub's surround rips as the air in the box expands? Probably not. 

A couple of 3/8' holes will not cause a sealed box to over-extend and reach the limits of its excursion. We're talking a LOT of airspace displaced in a matter of milliseconds. You're not going to be pushing that much air through a couple of tiny holes. 

To answer the original question, the correct answer that has been noted is that you will get a hissing noise and an inconsequential loss of output.


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## Brian10962001 (Jul 11, 2009)

minbari said:


> no they are not! sealed boxes need to be absolutely sealed! if you have a leaky sealed box the woofer can pump out the air through the leak and actually pull the woofer to the bottom of the suspension. once it bottoms out, its done.


LOL DO WHAT?!


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## minbari (Mar 3, 2011)

I have seen it happen more than once. laugh if you like.


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## Brian10962001 (Jul 11, 2009)

I'm not trying to be rude here, but no, you haven't.


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## minbari (Mar 3, 2011)

Brian10962001 said:


> I'm not trying to be rude here, but no, you haven't.


so, I must have been blind?

guy plays bass heavy track in a sealed box, woofer starts to bottom out. turns off music and the woofer slowely moved back to center.


what did i see then?


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## Fricasseekid (Apr 19, 2011)

minbari said:


> I have seen it happen more than once. laugh if you like.


How would you have evidence to prove that that is the exact circumstances which cause said subwoofer's demise? 

Did you have a gauge that read negative "average" air pressure Inside the bix just prior to the subs bottoming out?


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## minbari (Mar 3, 2011)

lol, I have eyes. he turned off the music and the sub slowely moved back to the center resting point. so there was a vacuum in the box. turn the music back on and it plays for a few seconds before bottoming out again. then same thing again.

I dont need a gauge in the box to figure this out anymore than I need to test the airpressure of a flat tire.


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## voltij (Feb 15, 2011)

The way you describe it makes it seem like there was something acting like a "check valve" (allows flow one way but not the other) somewhere in the box.

I could actually very easily see MDF do this in certain circumstances. Say that there's a split piece of MDF and it's got a wood flap sticking out into some gap between pieces at an angle. When air pushes from the inside of the box to the outside, the air pushes the flap out of the way. But when air is pushing from the outside to the inside, the flap closes and stops the air from moving into the box.

Repeat 35 times per second for however long a song's bass notes are and it might add up to a negative air pressure inside the box. How likely it is to happen, though?


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## Brian10962001 (Jul 11, 2009)

Exactly, there was something else going on there. If that were true than EVERY sealed subwoofer enclosure would suffer the same fate over time. That and anyone who built a ported enclosure with too small of a port; doomed. I believe what you saw but as was stated, you had something acting as a one way valve (probably a panel with a loose line of caulk or something).


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## XtremeRevolution (Dec 3, 2010)

minbari said:


> so, I must have been blind?
> 
> guy plays bass heavy track in a sealed box, woofer starts to bottom out. turns off music and the woofer slowely moved back to center.
> 
> ...





minbari said:


> lol, I have eyes. he turned off the music and the sub slowely moved back to the center resting point. so there was a vacuum in the box. turn the music back on and it plays for a few seconds before bottoming out again. then same thing again.
> 
> I dont need a gauge in the box to figure this out anymore than I need to test the airpressure of a flat tire.


What you saw was a poorly built sealed box, or the sub was not suited for a sealed box. 15mm of excursion each way at peak power in 30ms or less for the entire cycle will not be enough to push that much air out of the box if the sub slowly comes back to its resting point once pushed down. 

I will bet you anything that the sub was not suitable for the sealed box. How do I know? See how many subs I've modeled, and try to imagine how many I've built. Just because you saw a sub bottom out in a sealed box and there was a very small leak that allowed the sub to come back to its resting point once pushed down doesn't mean that that insignificant leak caused it to bottom out. Many subs are actually designed with very small leaks so that when you change atmospheric pressure (go skiing in Colorado for example), you don't rip the surround in the box because it has a 100% perfect seal and the pressure inside the box becomes greater than the pressure outside of the box. Many sub surrounds are specifically designed that way. 

That's like saying I'm driving a Lexus, and I get into a crash. Clearly I got into a crash because I was driving a Lexus. 

Sealed boxes don't always control excursion as well as you think they do.


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