Enclosure for dual SoundQubed 10's HDC3.1

[caraudionoob]

19 minutes ago, Triticum Agricolam said:

Given the amount of power you are going to be running, I'd like to see more port area, like 90 sq in if you can make it work.  I understand space is limited though, so compromises may have to be made.  

I played around with Torres for a bit more and I'm afraid the most I can get it is ~70sq in of port area. 

In order to achieve 90sq in, I would have to increase the port length to almost 7" (6.875" to be exact), as well as increase the dimensions, which I don't have.

That would increase the port area per ft to almost 20in sq (I thought it was desirable to stay between 12-16?) Can you please explain why 90sq in would be preferable given the power that I am running to it? I really am curious and would like to learn as well.

 

[Triticum Agricolam]

Port area is all about keeping air velocity through the port under control.  Air resistance quadruples for every doubling of velocity, so if velocities get too high you can lose a lot of energy pretty quickly, any energy lost to air resistance is energy that would otherwise be sound output.  That loss in output is referred to as "port compression" and it is not your friend.  The other problem with high port velocities is they often result in port noise, which is annoying.

Ideally I like to keep peak port velocities under 22 m/sec, with 32 m/sec as an absolute do not exceed value.  This is often not an easy thing to do.  Port velocity is a function of input power, net volume, tuning frequency, and port area.  The problem with "port area per cubic foot" guidelines is they don't take into account input power or tuning frequency.  Since mostly people tune between 30-34 hz, ignoring tuning frequency isn't a big deal (but it would be if you tuned much above or below that range).  Ignoring input power is a big deal though, in your case you are putting around 1,000 watts per cubic foot into that box.  Its been my experience that 1,000 watts per cube is where it really gets tough to get enough port area.  No matter what you do,  you are going to end up with some port compression.  Getting as much port area as you can is the best way to combat that.  Other things you can do are to do everything you can to help the air moving through the port smoothly and with as little resistance as possible.  Things like rounding over the corners with as large as radius and you can, making sure everything transitions smoothly, adding 45s inside the port, flaring (kerfing) the port mouths, etc.  

[Triticum Agricolam]

For example, here is a box I built a couple weeks ago for a pair of Sundown X-12s.  Its a 4.5 cu ft box with 90 sq in of port area.  These subs are powered by a SCV-4000d, so fairly similar to what you will be running.

Its kind hard to tell, but I flared the port mount as much as I could.  I also kerfed the side of the port where it curves around to the back, I want air flowing as freely as possible.  

[caraudionoob]

11 hours ago, Triticum Agricolam said:

Port area is all about keeping air velocity through the port under control.  Air resistance quadruples for every doubling of velocity, so if velocities get too high you can lose a lot of energy pretty quickly, any energy lost to air resistance is energy that would otherwise be sound output.  That loss in output is referred to as "port compression" and it is not your friend.  The other problem with high port velocities is they often result in port noise, which is annoying.

Ideally I like to keep peak port velocities under 22 m/sec, with 32 m/sec as an absolute do not exceed value.  This is often not an easy thing to do.  Port velocity is a function of input power, net volume, tuning frequency, and port area.  The problem with "port area per cubic foot" guidelines is they don't take into account input power or tuning frequency.  Since mostly people tune between 30-34 hz, ignoring tuning frequency isn't a big deal (but it would be if you tuned much above or below that range).  Ignoring input power is a big deal though, in your case you are putting around 1,000 watts per cubic foot into that box.  Its been my experience that 1,000 watts per cube is where it really gets tough to get enough port area.  No matter what you do,  you are going to end up with some port compression.  Getting as much port area as you can is the best way to combat that.  Other things you can do are to do everything you can to help the air moving through the port smoothly and with as little resistance as possible.  Things like rounding over the corners with as large as radius and you can, making sure everything transitions smoothly, adding 45s inside the port, flaring (kerfing) the port mouths, etc.  

Wow. Thanks for taking the time to explain that; that was extremely helpful. That explains why the box I built (2 12" ZCON 4.5cubes with only 60sq in of port area) was a complete failure - way too much port noise. 

How can you measure peak port velocities? You said you like to keep it under "22 m/sec, with 32 m/sec as an absolute do not exceed value." How would you calculate something like that?  Is there a formula?

What about subs that take massive amounts of power? For example, the Fi TEAM 12" takes 5000w into a 2.5 cube box. That's 2000 watts per cube. If you experience problems with 1,000 watts per cube, wouldn't building a box for that amount of power (2,000w per cube) be impossible due to the overwhelming port velocity?

[Triticum Agricolam]

1 hour ago, caraudionoob said:

Wow. Thanks for taking the time to explain that; that was extremely helpful. That explains why the box I built (2 12" ZCON 4.5cubes with only 60sq in of port area) was a complete failure - way too much port noise. 

How can you measure peak port velocities? You said you like to keep it under "22 m/sec, with 32 m/sec as an absolute do not exceed value." How would you calculate something like that?  Is there a formula?

What about subs that take massive amounts of power? For example, the Fi TEAM 12" takes 5000w into a 2.5 cube box. That's 2000 watts per cube. If you experience problems with 1,000 watts per cube, wouldn't building a box for that amount of power (2,000w per cube) be impossible due to the overwhelming port velocity?

I calculate port velocity using box simulation software, I use WinISD & HornResp, though BassBoxPro and TermPro provide the same functionality.  Port velocity can be measured with a hot wire anemometer, which is something I'd like to do sometime but its kind of an expensive tool so I have not at this point.  

Really high power subs, like the FI Team, are going to have issues with port compression.  It will be impossible to get port velocity low enough to avoid losing much output, that's just the way it is.  However the increase in power those subs support is still going to allow them to get louder than what would be possible on lower wattage.  For every doubling of input power you should see a 3 dB increase in output, in theory.  At lower power, like going from 100 to 200 watts, you are likely to get very close to a 3 dB increase.  As power levels get higher you are going to start getting less return on your power investment.  Going from 2500 to 5000 watts per sub may only gain you 1.5 dB or less even.  The reminder of your 3 dB theoretical gain is being lost to compression.  This is why with these very high power systems putting significant effort into port optimization is going to be time well spent.  

If you have access to an SPL meter you can measure how much output you are losing to compression.  Its pretty easy to do, you just have to take a couple low power and high power measurements at a couple different frequencies and then you can determine what you are losing to compression from the results.  In addition to port compression there is also power compression which is what happens as the voice coil heats up and its resistance increases.  Power compression can be just as bad as port compression and they both occur at the same frequencies.  

[ShadeTreeMechanic]

 Some boxes end up extremely big just because the port takes up so much room. It often gets to the point where the port won't fit inside the box. Then a compromise has to be made, make the box bigger or make an external port or a little of both.