I'm looking at purchasing a couple HDS212D2's, but I'm attempting to plan out the build as much as possible before actually getting started.  I see that the recommendations for box size per 12" woofer is around 2 cubes, with a 32"sq port (equating to 2"x16" slot size) at 21" long.  This equates to a 40hz tune, which if recommended, so be it.  However!  Looking at the port velocity at this size, I see that the velocity spikes to over 70 at 38hz, and the driver over-extends before hitting 28-30hz.  SQ doesn't recommend a smaller port cross section than 32"sq due to chuffing, but I see that a 16"sq by 27" long slot port will give a 25hz tune and the same air velocity, with the spike at 24hz.  Do you suppose this would be SAFER for the woofer, to even better prevent chuffing by moving the spike lower, into the subsonic filter range, or would it in fact be WORSE for the woofer?  I want to make sure I don't bottom out the driver or chuff the cone at the rated power of 600w, but it seems like whatever I tune it to the velocity spikes into chuffing range.  Or is this not a concern for these drivers?  I'll post some charts below, YELLOW=SQ suggested specs, BLUE=my proposed:

Air velocity at port:

 

SPL:

 

XMAX:

 

edit to correct tune freqs.

Edited March 20, 201510 yr by blacklotus

Would really help if we could read the pictures.

As for chuffing that is all personal preference and what you can deal with. Do realize that you are most likely showing results at full power which of course is also not a "normal" listening level.

SoundQubed's box recommendations are so generic they are almost useless. It's up to you to decide what frequency response you desire, but in my opinion the 27 Hz tuning looks much more preferable to the 40 Hz one. I would then size your port to be whatever it needs to to keep port velocities under 30 m/sec.

SoundQubed's box recommendations are so generic they are almost useless. It's up to you to decide what frequency response you desire, but in my opinion the 27 Hz tuning looks much more preferable to the 40 Hz one. I would then size your port to be whatever it needs to to keep port velocities under 320 m/sec.

20 m/s would be more preferable.

You are already modeling, so just adjust parameters to keep VV low at your chosen tuning and power level.  Also see that bump/ripple in the response of the recommended alignment @ 50Hz?  That is what I call the "Best Buy" bump, meaning when you walk into Best Buy and they have a bunch of subs going with some crap music on the radio you notice the one with this crappy response when it sticks out... But after having it in your car for a while it gets annoying because it has stupid emphasis around 50Hz which wears on you...  Moral of the story, tune flatter and you will be happier in the long run...

SoundQubed's box recommendations are so generic they are almost useless. It's up to you to decide what frequency response you desire, but in my opinion the 27 Hz tuning looks much more preferable to the 40 Hz one. I would then size your port to be whatever it needs to to keep port velocities under 320 m/sec.

20 m/s would be more preferable.

Of course. The problem I suspect the OP will run into is the balancing act between port area, port length, and tuning. With a low tuning and large port area the port could end up being a mile long, which creates its own problems. Just about every design is compromised in some way. Having a slightly smaller port area will only have a negative effect at max output, which will only be a limited amount of time. If the OP raises the tuning to try to keep the port length reasonable that will effect output at all volume levels.

The OP's goals for their system will have to determine just where compromises should be made.

Edited March 21, 201510 yr by Triticum Agricolam

Fantastic discussion!  Thanks are due all around, all great information.  I'm definitely with everyone on getting rid of the "best buy bump", as my musical tastes range from orchestral brown notes to metal double kick drums to electronic nonsense noise.  The head units I'm generally looking at seem to have response ranges going down as far as 10hz, and though I have a subsonic filter (3rd order) on my Kole QX-2400D, I'd like to keep my cones attached as long as possible like most folks here.  So, I understand I'm looking for the impossible, and all boxes are made almost entirely of compromise and tears.  I guess the main thing I'm wondering is that since the suggested box will result in velocities well over 30meters/sec range, the cones of the HDS212's should therefore tolerate the horrendous velocities in the proposed box design as well?  If so, then we have a winning box!  If not... I'll have to increase the port diameter to reduce the velocity at the expense of a little low end response, and that's fine I guess, just not preferred =)

So I'm a little confused by your comment on cone strength, port velocity has no effect on your cone holding together or not.

It is exclusively the speed of air coming out of the port.

From what I understand, and what I've seen with inexperienced box tuners (and those that buy pre-packaged driver/box combos at Best Buy etc), what you run into is a situation where the air resistance or lack of resistance combines with the phase delay of the port's exiting air to create a "vacuum" inside the box during specific points of excursion, increasing the stress on the driver's softparts where at the peak of excursion the driver is "sucked" back into the box with the velocity of air exiting the box.  When the exit velocity is too high, the momentum (mass x velocity) of the mass of air currently exiting the box at the time the driver's cone is extending out creates a high vacuum behind the driver and snaps it back with more force than the driver's cone can handle (like having a sealed box that's too small, but far more intense).  When the exit velocity is too low (port too large), the driver doesn't have enough damping and hits massive over-excursion spikes when the mass of air is accelerating back INTO the box and pushes the driver outwards and there's not enough vacuum behind the cone to prevent over-excursion.  Depending on the tuning, this "spike" in excursion can occur at, above, or below the tuning point of the box and driver depending on how the phase delay responds to the tuning.  Long story short, think of a ported box as an interference engine: air interacting with physical driver just like air/fuel and pistons.  Too much velocity might be perfectly safe for a driver in some cases, or might violently rip the cone from the surround in other cases, since the cone is where the vacuum pressure is being exerted.  Most cases of cone separation you hear about arise from this situation....

 

 

SoundQubed's box recommendations are so generic they are almost useless. It's up to you to decide what frequency response you desire, but in my opinion the 27 Hz tuning looks much more preferable to the 40 Hz one. I would then size your port to be whatever it needs to to keep port velocities under 320 m/sec.

20 m/s would be more preferable.

Of course. The problem I suspect the OP will run into is the balancing act between port area, port length, and tuning. With a low tuning and large port area the port could end up being a mile long, which creates its own problems. Just about every design is compromised in some way. Having a slightly smaller port area will only have a negative effect at max output, which will only be a limited amount of time. If the OP raises the tuning to try to keep the port length reasonable that will effect output at all volume levels.

The OP's goals for their system will have to determine just where compromises should be made.

 

True, but if you just make the enclosure larger that makes up for a better you end you can tune higher (and use a few tricks to lower the tuning without increasing the physical length of the port).

I've built a enclosure recently for my two ten inch JL W6v2, and moddeled it to have a peak air velocity of 21.17m/s around 30Hz at peak power and I noticed while testing that it had port noise at peak power.(not audible from the front seat, but audible from the trunk and that means the port won't behave as predicted, and so the frequency response won't be as predicted)

From what I understand, and what I've seen with inexperienced box tuners (and those that buy pre-packaged driver/box combos at Best Buy etc), what you run into is a situation where the air resistance or lack of resistance combines with the phase delay of the port's exiting air to create a "vacuum" inside the box during specific points of excursion, increasing the stress on the driver's softparts where at the peak of excursion the driver is "sucked" back into the box with the velocity of air exiting the box.  When the exit velocity is too high, the momentum (mass x velocity) of the mass of air currently exiting the box at the time the driver's cone is extending out creates a high vacuum behind the driver and snaps it back with more force than the driver's cone can handle (like having a sealed box that's too small, but far more intense).  When the exit velocity is too low (port too large), the driver doesn't have enough damping and hits massive over-excursion spikes when the mass of air is accelerating back INTO the box and pushes the driver outwards and there's not enough vacuum behind the cone to prevent over-excursion.  Depending on the tuning, this "spike" in excursion can occur at, above, or below the tuning point of the box and driver depending on how the phase delay responds to the tuning.  Long story short, think of a ported box as an interference engine: air interacting with physical driver just like air/fuel and pistons.  Too much velocity might be perfectly safe for a driver in some cases, or might violently rip the cone from the surround in other cases, since the cone is where the vacuum pressure is being exerted.  Most cases of cone separation you hear about arise from this situation....

No, what you are noticing isn't actually happening, the inertia of the air is negligible.