Tuned tube ports for 828C cabinets?

[Albert Allen]

I talked to Roland a little about using tuned port tubes in my 828C cabinets and he though some of you could provide me with a direction to take (namely GM)

I have done the standard mods to the 828's to block off the horn flair and I have added extra particle board sheeting to the inside of the large cabinet surfaces as well as a 1 inch MDF rear panel and some inner cross bracing.
I would now like to tackle the job of using tuned tube ports to get the most bass out of my 515 8-G bass drivers and I don't have any idea what diameter to use for the ports or how many and how long they should be...any ideas or thoughts on this ?

Thanks

[Fitts]

Here are some formulaie for caluclating the port.

Fb=.42 X Fs X (Qts)^-.9
and
Lv=(1.463 X 10^7 X R^2)/(Fb^2)X Vb

Fs and Qts are Theile Small paramaters for your woofer.
Vb is the volume of your woofer enclosure in square inches.
Lv is the length of the port in inches
R is the radius of your round port in inches.

If I remember correctly, the back of the port tube should not be too close to any interior surface. If the calculation results in a tube that is too long to fit within the cabinet, use a larger radius tube.
The ^ mark indicates to raise the number to the power that follows.

Good Luck,

[Albert Allen]

Thanks very much Dave! I wondered if anyone was going to pick this one up.

[GM]

If you plug some numbers into the Lv formula you'll see that increasing the cross sectional area increases, not decreases, the vent length.

This is a very condensed formula that results in way too long a vent. Not that this is necessarily a bad thing since you normally still need to tune it by ear at the listening position, just a FYI.

To get really close on the first go-round requires multiple stage ML-TL simming due to the step tapered design that I don't have time for right now, neither the specs for the particular drivers used nor the desired roll off response. Last, but not least, knowing whether you drive them with tubes/SS and wire gauge/length is required to help figure which Fb is the best compromise to start with.

You choose the diameter/qty. of vents based on vent mach, which these formulas don't address. You're really better off using one of the on-line box calculators once you've figured the net Vb of your cabs and the driver's effective Qts if there's any significant added series resistance in the signal path to them.

For instance, your cab should be around 10ft^3 net and a max flat alignment with published specs for this driver, an SS amp and sufficiently low resistance wire runs, only needs ~3.13ft^3 tuned to ~56Hz. At 10ft^3, about the highest it can be tuned without excessive peaking at Fb is the driver's 37Hz Fs.

To get a low vent mach requires at least four 4" dia. vents, but this is a hassle, especially if you have to pull them to change their length so a single 8" heavy duty cardboard tube 4" long looks like a winner for this scenario, so this is where I would start. If it sounds too boomy then make it longer, if too lean, try it shorter.

WRT placement, as Dave alluded to, it needs to be at least one diameter away from any boundary, particularly the surface parallel to the internal end, usually the rear of the cab. You can put it closer to the bottom or side wall, but it will make the vent act as if it were longer due to mirror imaging like happens when you put a speaker near a wall or corner.

A good seal everywhere is required to get max performance from a vented cab, so make sure there's no leaks. Plumber's putty or similar works well for sealing the tube to the mounting board during testing.

HTH,

GM

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All that said, my investigations with WinISD and UniBox indicate that the optimum port for the 828 with the flares closed off, 529-G driver, and .25 ohm series resistance is approximately 3" high X 28" long. Cut a f-i-l-l-e-r piece for the vent opening, seal it with weather strip to adjacent surfaces, and I think you'll be pleased. You're not going to 20 Hz no materr what you do, but it should be a solid 40 (as opposed to 50/60 with the vent "per plan")

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529???? Where did THAT come from??? I meant 515-G.

[Albert Allen]

Thanks guys! I am now on the hunt for a section of 8" dia. cardboard tube. I will keep everyone informed of the results.

[GM]

>All that said, my investigations with WinISD and UniBox indicate that the optimum port for the 828 with the flares closed off, 529-G driver, and .25 ohm series resistance is approximately 3" high X 28" long. Cut a f-i-l-l-e-r piece for the vent opening, seal it with weather strip to adjacent surfaces, and I think you'll be pleased.
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Hmm, Lspcad pro says it will have a huge ringing peak, so unless the grill is very resistive to sufficiently damp it, it wouldn't be my choice and wouldn't recommend it as a first approximation. As I learned long ago though, one man's ringing is another's 'punch', so by all means, try it also. Really, if the published specs are reasonably accurate, then for best tonal balance/max BW in 10ft^3 without completely killing efficiency is ~3ohms series resistance tuned to ~39Hz. For a bit more extension, and my choice for a typical room curve, is upping it to 4ohms total with the proper size speaker wiring and tuning to ~30Hz. This one yields excellent transient response and will cover all but 16Hz stopped pipes and some movie special effects. Of course in any alignment the HF horn has to shelved to match, with a bypass cap to lift the extreme HF up.

Regardless of the final config/tuning though, using an 8" pipe is my recommendation. So many choices, so little time.

Have fun,

GM

[Fitts]

Go to a building supply store and get some sono tube. It is used for pouring concrete posts. It is real heavy duty paper tube and comed in large diameters.

[Albert Allen]

Thanks David!