Box for Eminence Sigma Pro 18A-2

[Mark H]

I picked up one of these for cheap off Ebay (with the 120oz magnet) and have been running some numbers in WINLSD. If I run the numbers for the Eminence designed "High Powered" 7 cuft tuned to 38hz it says I'll exceed the 6mm Xmax by 2mm @ 400 watts. Is it true that Eminence is conservative on their Xmax numbers? How can this speaker be rated for 650w RMS if it reaches Xmax at 250w input unless you tune it @ 50hz?

[Music is life]

I picked up one of these for cheap off Ebay (with the 120oz magnet) and have been running some numbers in WINLSD. If I run the numbers for the Eminence designed "High Powered" 7 cuft tuned to 38hz it says I'll exceed the 6mm Xmax by 2mm @ 400 watts. Is it true that Eminence is conservative on their Xmax numbers? How can this speaker be rated for 650w RMS if it reaches Xmax at 250w input unless you tune it @ 50hz?

Hi Mark
The thermal rating and the exertion rating are two separate indicators of performance which must be taken into account for the intended application.
http://www.eminence.com/pdf/cab-sigmapro-18-2.pdf

Eminence says that you are ok to 400 watts at 7cu ft with high pass filter set at 35hz. That is not too bad really. Not bad at all. The extra 250 watts is almost negligible in volume. Maybe 1.5 db if you are lucky, with power compression already setting in.

This speaker here just as an example:
http://www.eminence.com/proaudio_spe...5&SUB_CAT_ID=3

is one of the few in which its exertion limits come close to matching its thermal limits in the larger box sizes.

This speaker here as another example:
http://www.eminence.com/proaudio_spe...5&SUB_CAT_ID=2

is good for about 50 watts in a 5.5 cu ft box. So maybe a sealed midbass would be a better application for this driver.

"Watts" does tell us only a small portion about performance and application.

Good luck with your project!

[Paul O]

With WinISD Pro you can also model filters with your box simulation and it will show you how these affect response and driver excursion. The apparent reduced power handling you see here is something all speakers are affected by, pro audio drivers are designed to get loud but give up some extension in exchange, so resist the urge to tune them real low like a home audio sub. Eminence provides generous over excursion capability on most of their drivers, this one in particular has an Xlim of 18mm, that's how far the cone can travel before one of these things happens..

spider crashing on top plate
Voice coil bottoming on back plate
Voice coil coming out of gap above core
or the physical limitation of cone.

[billfitzmaurice]

How can this speaker be rated for 650w RMS if it reaches Xmax at 250w input unless you tune it @ 50hz?

Because the rating is thermal. All driver manufacturers only publish thermal ratings. Determining the displacement limited power is up to the cabinet designer, in this case, you.

[dotzs]

Sorry but software is only software ,i have had drivers that sounded crappy in the approved cabinets and then just popped them in something that should never work and wow hey do what i never thought they could do.
The practical solution is to say " I have this much space,i have to carry it myself and need it to go from this frequency to that frequency .
Its a good strong driver as a designer its your job to make it do what it can do without building a mammoth cabinet or a cheesey plastic resonating price point sacrifice.The emmo software is good but they make so many speakers its hard to believe they could come up with the ultimate cabinet for everything they make.its come a long way from when you bought a raw driver then attempted to build some imperial folded horn cabinet that only a master carpenter could build -no matter how spiffy the driver is you can weld the voice coil shut if you push it too hard.As a boring aside i used to have a few Ampeg b 15 N cabinets that did nothing till i used a different amp with some power and a good pre amp = it brought them to life and they were a total compromise cabinet with ports that were guess work before the Thiel/Small port research -this has saved so much time and trial & error .

[Mark H]

Thanks for the tips on the box. Looking at other commercially available pro subs if I can get it flat to 40hz then that should be plenty low enough.

One question I have is if there is any pro/cons of using a slot type of vent vs four vents placed in the corners assuming the area and tuning is the same. What I would like to try is putting four triangle shaped ports in the corners so I wouldn't have to use 4" pvc pipes.

[Paul O]

Ports can be done any way you like as long as the total port volume meets the tuning requirements, just don't make the individuals too small or you'll get port noise at high excursion.

[wg_ski]

Also be aware that box modeling programs only take into account the more common end effects of standard PVC ports. Putting triangle vents in the corners will change the tuning compared to either a duct or a vent hole in the middle of the baffle so the models will be a little off. You may want to build a test baffle and tune it manually until its right, then cut the final one. The impedance curve will tell you where it's tuned.

[billfitzmaurice]

Putting triangle vents in the corners will change the tuning compared to either a duct or a vent hole in the middle of the baffle

Not enough to make any difference; I know, as I just tested both. Shelf style vents, or any with a very high length to width ratio, do alter the tuning as they result in higher airflow resistance than vents with more symmetrical shapes.

[Mark H]

Ripped from the help files on WinISD:


Port terminology: Physical/acoustical length, end correction
If you have read articles on port design on the Internet, you have probably stumbled on various terms involving ports. This document is to explain some of those terms.


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Effective radius/diameter
If your port has some other shaped cross-section than circular, then we'll need to calculate effective radius or diameter. Effective radius is radius of the circle, which has same cross-section area than non-circular object. For ports, only inside area contributes to port area, not it's walls. For calculating effective port radius/diameter, use following formulas:
effective radius = sqrt(S/pi) and
effective diameter = sqrt(4·S/pi),
where sqrt means square root.

For example, you have square port which has 100 square inches of cross-section area, what is effective diameter and radius?

Now, just plugging the values to above formula we get:
radius = sqrt(100/pi) = 5.642 inches
diameter = sqrt(4*100/pi) = 11.284 inches.

Of course, diameter could have been easily obtained from radius by just multiplying radius by two.


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Acoustical length
First, we need to discuss what is the idea behind port, and how it works. Basically, port forms a mass-spring resonance circuit with enclosure volume compliance. Helmholtz was first to discover such a resonator, hence it was named after him. Mass in port resonates with compliance of box air. By tuning the enclosure, we chose mass in port, so that resonates at desired frequency. As our port will have some area, we can calculate, how long this "tube" should be to have desired mass. This length is so called "acoustical length" of our port.


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Physical length
Unfortunately, effect of the port doesn't stop at the end of our port. Moving mass extends somewhat beyond of the port. Researchers of acoustics say that radiation impedance has reactive component, mass. So if we make our port length equal to calculated acoustical length, we'll find out that tuning frequency is lower than what was expected. We can make good approximation to this extra mass by adding this radiation load mass from desired port mass and then calculating required port length.


By default, WinISD assumes that you are going to mount your port above way. And therefore gives you physical length of your port assuming mounting like in above figures. See also FAQ 3.1.


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End correction
End correction factor is analytically or empirically determined factor, how much port extends beyond its physical ends. For free end, end correction is 0.30665 times port effective diameter. For flanged end, more analytical expression is available, 4/(3·pi) ~= 0.42441 times port effective diameter. Flanged end is calculated assuming that tube terminates to infinite baffle. Which is not exactly true, but.. Following table summarizes various port configuration types and their total end correction factors (refer to above picture to various end types):

Summary of end correction values Port configuration end correction 1 end correction 2 Total end correction
Two free ends 0.30665 0.30665 0.6133
One flanged and one free end 0.42441 0.30665 0.7311
Two flanged ends 0.42441 0.42441 0.8488

Physical port length is obtained by subtracting port effective diameter multiplied by suitable end correction value from port's acoustical length.


If I were to use 4" PVC pipes like the Eminence designed Ext Bas Shelf box http://www.parts-express.com/pdf/290-427c.pdf isn't the rule of thumb that the ports need to be a minimum of of the port diameter from an inside cabinet wall? With an 18" driver the baffle would have to be pretty large with front mounted 4" pvc ports.


I really want to keep it small so it's easier to move and not use more than one
4x8 sheet of plywood. That's why I'm shooting for the corner ports.

thanks

[billfitzmaurice]

I really want to keep it small so it's easier to move and not use more than one
4x8 sheet of plywood. That's why I'm shooting for the corner ports.

thanks

As I replied above, I recently measured some prototypes using 4" PVC ducts center baffle mounted and 5" leg triangle ducts at the cab corner, so both have 12.5" sq.in cross-sections. With ducts of the same length fb was within 2 Hz.

[Mark H]

Bill,

If the baffle face is inset into the cabinet say an inch from the front how do you determine what length the port should be if two of the three sides are longer?

[billfitzmaurice]

Bill,

If the baffle face is inset into the cabinet say an inch from the front how do you determine what length the port should be if two of the three sides are longer?

That's exactly how mine was and it made no perceptable difference.