DIY 2-15 port question/confusion

I suggest designing the box to accommodate the longer length, then measure and trim if necessary:

"Measure twice, cut thrice."

It might not make a huge difference. I think port frequency (IIRC) goes as the square root of length, so you might only be off by a few Hz. I've noticed differences in the published port length equations.

 

There's also a way to test it. The port tuning frequency is the frequency where the cone grinds to a halt. Lay the speaker on its back, and put a grain of rice on one of the cones. Use a program like "Visual Analyzer" to generate frequencies, and run the frequency up and down until the rice stops bouncing around. (Use uncooked rice. )

Be very careful with your amp and ears while doing any kind of testing like this. When you're getting things set up, start with some music from your PC to get levels set, then turn the amp down to a very low level, so you don't suddenly generate a very loud sound.

I'm pretty sure TB'er bgavin first introduced me to the rice method.

 

I'd search up the "dancing salt" or "dancing rice kernels" method and use that to determine your actual tuning frequency. That'll help you dial in your slot/shelf ported cab as well. Some of the modeling programs get a little off when using different types of ports where cabinet walls function to lengthen the port a bit.

 

Perhaps the discrepancy is because the longer port length calculated by the Precision Ports calculator is for a flared port, while the Eminence estimate is for a straight port. Both would be correct. Given your application, flared ports might be a good idea (helps prevent chuffing).

Also, given the external cab dimensions and wall thickness, the internal volume is more like 5.5-6 cf (with the drivers, ports and 'some' bracing subtracted).

If you enter 6 cf, 4 ports, 4 inches diameter and 48 Hz Fb into this calculator (http://www.psp-inc.com/tools.html), you will get 6.87 inches for the straight port, and 7.87 inches for the flared port.

Should be a great cab!

 

What did you find wrong with slot porting?

Have you seen the johnk_10 Starliner thread? He used 3015's and did another with Faitals.

 

Looking at the pic, it looks like the port shelf is very short in height compared to it's width. Look this up before just taking my word on it, but I remember reading somewhere that when that width/height ratio gets over something like 8 to 1, you can get into an area where the port can loose it's effectiveness or start to wander in tuning under higher power or something. Or it could be just that when it's under a couple inches in height, there is too much friction along the walls in comparison to the open area.

I think it's this port calculator http://www.bcae1.com/spboxnew2.htm that won't accept a shelf port if the ratio is greater than 8 to 1.

 

Sweet. Legend CA154 or 4ohm Faital or other? Ported half or sealed half?

 

Indeed.
Been there, done that.
Prototype(s) are mandatory, at all levels of skill and knowledge.

May I suggest you do two 1x15 instead?
Easier to haul, easier to get closer to the ideal box volume.
The current 3015LF requires a much larger box than the older versions (mine).
Check the current PDF for T/S parameters, which will be obvious when you run the numbers.

The vent area is sized for no chuffing (5% MACH) at the highest usable power level within your frequency range.
If your driver exceeds Xmax at 50 Hz at 100 watts, it is pointless to size a huge vent are for operating at 450 watts.
A larger vent area is not "louder" than a smaller vent.
The larger vent creates less chuffing noise as a result of slower air velocity.

WinISD and others can calculate excursion and show you where Xmax is exceeded for a given box volume, tuning and power level.

 

The CA154 is a quite nice driver, and far less expensive than neos.