Read This ...and Learn...

[60ndown 10+ year member]

Quote:

Originally Posted by 60ndown

'In other words, a sealed setup will push the sub to its excursion limits much easier than a vented system"

i believe you, but it makes no sense to me?....in a sealed box driver moves forward-creates 'vacume' behind itself...forward motion is harder.....so surely with no box ...or a vented box the driver is able to reach its 'x max' more easily?? please help a struggleing not so new but vey interested in how and why car audion enthusiast...........

First off, forward motion of the cone in a sealed system is no harder than the rearward motion, its exactly the same. When the cone moves out, it creates a vacuum as you said, thereby restricting cone motion. But as it moves in, it compresses the air molecules, giving the same cusion effect with equal force. The amount of force it takes the push a specific sub to its excursion limit will be determined exclusively by the enclosure size. The bigger the box, the more air to compress (or create vacuum), there by giving less resistance.

Vented boxes are an entirely different animal. Many (if not most) people think a vented box simply allows the cone to move freely as the air from the back of the cone has an exit. But this is not true, the 'exit' (port) is designed in such a way that the amount of air the subwoofer moves cannot fully move in and out of the box without restriction. Simply put, the sub is capable of pumping more air faster than a properly designed port can accomodate in through-put. Follow me? Now another thing to consider is in vented systems its all frequency dependant. The closer your signal frequency moves toward the enclosure's tuning frequency (lower and lower), the more the cone is trying to move, and thus the more air the speaker is trying to push out the port, creating more and more back-pressure. This creates a situation where even as output is going up (reaching closer and closer to the boxes tuning), actual cone excursion is going down and down. Many people do not understand this phenomenon because they do not consider that a vented enclosure's efficiency is based on frequency. Closer and closer you move towards tuning gives more and more output even as cone motion is decreasing, BUT enclosure efficiency is rising, which explains the rise in over all output.

This also explains why it is easier to push a sub to its excursion limits in a sealed system rather than a vented one. A vented box actually has more restriction on cone motion over a given frequency spectrum than does its sealed counterpart (assuming a properly designed box). And considering that the lower the frequency the note is, the more air that's needed to be displaced to reproduce that note at a given decibel level. In a sealed box, the lower you go, the more excursion rises. Vented systems do just the opposite. If you try to reach maximum excursion at low freqs, at or near tuning, you will end up pumping so much wattage thru the sub that it will fail thermally before it reaches mechanical limits (overheat). And if you try to do it far from tuning where cone restriction is the least, by the time you get far enough away from tuning in the vented box so as excursion restriction from the port is gone, you've already gone well above the frequency range the sub is meant to play (or that your crossover will allow).

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[dakrazyman 10+ year member]

good explaination mang

 

[Fitz 10+ year member]

audioholic posted all the techie stuff

 

[squeak9798 5,000+ posts]

in a sealed box driver moves forward-creates 'vacume' behind itself...forward motion is harder.....so surely with no box ...or a vented box the driver is able to reach its 'x max' more easily??

http://forum.elitecaraudio.com/showthread.php?threadid=107877

For those too lazy to read (though I sincerely recommend you read it), here is the skinny:

And let's use a specific example :

12 inch sub, piston area = pi*(6 inches)^2

1 cubic foot [sealed] box

Xmax, one way = 1 inch

............

1. Cone at rest:

Outside air pressure = 14.7 lbs/inch^2

Inside air pressure = 14.7 ibs/inch^2

Net air pressure acting on cone = 0

2. Full rearward excursion:

Outside air pressure = 14.7 lbs/inch^2

Inside air pressure = 15.730 lbs/inch^2

Net air pressure acting on cone = -1.03 lbs/inch^2

Net air force acting on cone = -116.49 lbs

3. Full forward excursion:

Outside air pressure = 14.7 lbs/inch^2

Inside air pressure = 13.797 lbs/inch^2

Net air pressure acting on cone = 0.903 lbs/inch^2

Net air force acting on cone = 102.13 lbs

Note that there is actually more force on the cone while excurting rearward than there is while excurting forward.

Pressures are assymetric in boxes. And ported boxes are even worse - you can get twice the internal pressures at resonance, as you do in a sealed box.

Ever play a test tone in a box and notice the driver creeps forward? This is the box "pumping up" - it's easier for the driver to push forward than pull back (as we see here, based upon the stiffness of air in the box), and over multiple cycles the driver doesn't recenter. The center slowly creeps forward, leading to more nonlinearites.

Really suggest you read Dan Wiggins entire post on page 4 of that thread. Good info there........

Also, because I'm tired and slap happy....your username is 60ndown, and you currently have 60 posts. Ironic. Haha

 

[stones 10+ year member]

"Ever play a test tone in a box and notice the driver creeps forward? This is the box "pumping up" - it's easier for the driver to push forward than pull back"

I always wondered why they would creep like that.

 

[audioholic 5,000+ posts]

http://forum.elitecaraudio.com/showthread.php?threadid=107877
For those too lazy to read (though I sincerely recommend you read it), here is the skinny:

Note that there is actually more force on the cone while excurting rearward than there is while excurting forward.

Really suggest you read Dan Wiggins entire post on page 4 of that thread. Good info there........

Also, because I'm tired and slap happy....your username is 60ndown, and you currently have 60 posts. Ironic. Haha

I believe the phenomenon he is referring to with the enclosure 'pumping up' is simply the air pressures equaling out. Rapid pressure changes like what occur in speaker enclosures do not happen instantaneously, there is a lag. Simple physics tells us the air cusion should be the same front and back on a speaker's cone, as the number of air molecules inside the box have not changed, and the force that it requires to expand the molecules (decrease the air pressure) should be the same as the force it takes to compress them a proportionate amount. If this is not true, Id like to have someone explain why not. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

 

[60ndown 10+ year member]

ok enuf b.s. lets talk about tits!

 

[squeak9798 5,000+ posts]

I believe the phenomenon he is referring to with the enclosure 'pumping up' is simply the air pressures equaling out. Rapid pressure changes like what occur in speaker enclosures do not happen instantaneously, there is a lag. Simple physics tells us the air cusion should be the same front and back on a speaker's cone, as the number of air molecules inside the box have not changed, and the force that it requires to expand the molecules (decrease the air pressure) should be the same as the force it takes to compress them a proportionate amount. If this is not true, Id like to have someone explain why not. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

I guess I'm not following your logic. That same "simple physics" was just used to mathmatically prove the forces were not equal.

 

[audioholic 5,000+ posts]

I guess I'm not following your logic. That same "simple physics" was just used to mathmatically prove the forces were not equal.

I see numbers that simply say the pressure was different. I see no explanation as to why. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

Im not arguing you are wrong. But IF my conclusion is incorrect (pressure is the same), I would like to know why, not simply that Im wrong. The reasoning Im using is that if force exerted on the cone is the same in either direction, and the number of air molecules within the enclosure does not change, expanding or compressing this same number of molecules (in the same sized space) *should* be the same. Logic tells us that creating a high or low pressure area of the same magnitude, given the same space, should require the same amount of force. If not, why not?

 

[audioholic 5,000+ posts]

After reading the ECA thread a bit more carefully, I understand where werewolf was going with that. The key equation difference is indeed box volume, and his equation does explain my error. Enclosure volume does indeed change. When the cone moves in, enclosure volume drops. When the cone excurts out, enclosure volume rises. The number of air molecules does not change (in a sealed system at least), but the volume size does, even if slightly.

Thanks for the info squeak, I should have thought of that myself. //content.invisioncic.com/y282845/emoticons/uhoh.gif.c07307dd22ee7e63e22fc8e9c614d1fd.gif

 

[JimJ 5,000+ posts]

Ideal gas law...if volume decreases with N (being the # of mol in the closed system) held constant, pressure must increase...

Now I'll go back to trying to forget a year's worth of chem...

 

[d cha p 10+ year member]

Now I'll go back to trying to forget a year's worth of chem...

lol

 

[audioholic 5,000+ posts]

Ideal gas law...if volume decreases with N (being the # of mol in the closed system) held constant, pressure must increase...

Which is just a scientific way of stating that when the cone moves in, it decreases box volume while the number of molecules remains constant, thus creating a high pressure zone within the box. In a sealed system.

Ported boxes work the same, exscept as I said above the pressure isn't created due to the air molecules having no place to escape (they obviously do), but due to the fact that they cannot all escape fast enough (thus another way to create this high and low pressure situation within the enclosure).

No real breakthrough in logic there. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif