Porting a 2x15 Cabinet

[tntfrahm]

I have a Vox T100 cabinet (from NCM) that has two 15" speakers. Not liking the sound and am wondering if porting could help (it sounds "barky"). Has anyone ported one of these? Any advice on what to install and how? Any help would be appreciated.

[DukeLeJeune]

"Barky" sounds to me like you're describing a peak or two in the midrange region. A port isn't going to help.

If it's an internal box resonance, then damping material inside the box can help. You might try stuffing the box with Polyfill, ballpark 1/2 pound per cubic foot.

Unfortunately my guess is that "barky" comes from a peak or peaks in the speaker's innate frequency response, in which case aggressive EQ might be your best bet (short of changing woofers). Just a guess, of course.

If you can give me your best estimate of the internal dimensions of the box I can suggest possible port dimensions, of course without offering any guarantees. But I don't think that's where your solution lies.

[tntfrahm]

Thanks for the input. Not sure of the internal dimensions but the external are 36" x 19" x 11.75"

[Gizmot]

Porting a cabinet (if done right) will give you more extended low end and higher sensitivity (more output with the same power input). This will probably help with the some of the sonic problems you describe. If you have more lows, your mids will smooth out.
In order to do correctly port the cabinet, you will need the Thiele-Small parameters of the drivers and the precise size of the enclosure. This will determine how big and how long the ports need to be.
Check out http://en.wikipedia.org/wiki/Thiele/Small for a discussion of this.

[tntfrahm]

I was told there would be no math. . . .

[recreate.me]

well, its math or money my friend...

I mean, you can always cut a hole see if it helps, start small, and get bigger until it sounds good lol

[Stick_Player]

That cabinet is just way too small for two 15s.

I does look fab, though.

You'd probably be better off losing one of the speakers (seal up the hole), get a copy of WinISD, look for a speaker that'll work in that size box with porting (from WinISD).

You will probably have to secure the back panel, otherwise it'll vibrate and create some hideous unwanted vibrations.

A LOT of math will be necessary to fix that cabinet. The internal volume is about 3.25 cubic feet. Unfortunately NCM does not make available the T/S parameters on its 'custom' speakers. My experience with NCM cabinets (I have two), while well constructed, are not sealed and will need additional 'help'. NCM sells based on looks, not on sound.

[DukeLeJeune]

Thanks for the info.

Okay we're not necessarily going to do this "right", but we're going to try to stack the odds in your favor as much as possible under the circumstances.

I suggest installing four round ports, each 3" in diameter by 7.5" long. Also, buy four 3-inch expandable "test plugs", from the plumbing department of the store where you buy the tubing for your ports. Make sure the test plugs will fit the ports. Larger diameter, longer ports would be better, but I don't think there's room for them. Install the ports on the back of the box, and make sure you take the location of the woofers into account; port and woofer cannot simultaneously occupy the same space.

With all four ports open, your box is tuned to about 50 Hz. This will probably give you a lot of bass emphasis.

With three ports open, your box is tuned to about 43 Hz. Less bass emphasis.

With two ports open, your box is tuned to about 35 Hz. Again, less bass emphasis.

With one port open, your box is tuned to about 25 Hz. Not much more bass than a sealed box.

With all four ports sealed, you're back where you started... sort of.

Listen for port chuffing. The port areas here are probably less than we should have, but I don't think your box can reasonably accomodate ports of sufficient cross-sectional area and length. It is possible, depending on the woofers' parameters and the signal you send to the cab, that you might get away with it. The undersized box works in favor of getting away with undersized ports, but also works against porting in general giving good results.

I'm not recommending you do this, but what I've described is a little better than a wild roll of the dice. Worst case scenario the sealed box works best, you're out fifty to sixty bucks and one Saturday, and from behind your box certainly looks unique. Unfortunately along the way you've probably hosed the resale value. So like I said I'm not recommending this, but it's your call.

[Sartori]

Before you try porting, you should try putting damping material on the inside walls of the cab. It'll probably be cheaper and easier than porting.

[billfitzmaurice]

Originally Posted by Stick_Player

That cabinet is just way too small for two 15s.

I does look fab, though.


A LOT of math will be necessary to fix that cabinet. The internal volume is about 3.25 cubic feet. Unfortunately NCM does not make available the T/S parameters on its 'custom' speakers. My experience with NCM cabinets (I have two), while well constructed, are not sealed and will need additional 'help'. NCM sells based on looks, not on sound.

+1. Without knowing the driver specs I wouldn't touch the cab, as you're as likely to make it worse as you are to make it better. If the drivers are actually well matched to the cab porting it won't make it better.

[tntfrahm]

The speakers are two Eminence Legend 15s. It does look good. The sound. . . .well. . . .

[tntfrahm]

LEGEND CB158

SPECIFICATION
Nominal Basket Diameter 15", 381mm
Nominal Impedance* 8 ohms
Power Rating
Watts 300W
Music Program 600W
Resonance 34Hz
Usable Frequency Range 47Hz-3kHz
Sensitivity*** 98
Magnet Weight 80 oz.
Gap Height 0.375", 9.53mm
Voice Coil Diameter 2.5", 63.5mm

THIELE & SMALL PARAMETERS
Resonant Frequency (fs) 34Hz
DC Resistance (Re) 6.15
Coil Inductance (Le) 0.33mH

Download PDF Spec Sheet
Mechanical Q (Qms) 5.9
Electromagnetic Q (Qes) 0.36
Total Q (Qts) 0.34
Compliance Equivalent Volume (Vas) 336 liters / 11.9 cu.ft.
Mechanical Compliance of Suspension (Cms) 0.31mm/N
BL Product (BL) 16.0 T-M
Diaphram Mass inc. Airload (Mms) 70 grams
Efficiency Bandwidth Product (EBP) 95
Maximum Linear Excursion (Xmax) 4.8mm
Surface Area of Cone (Sd) 856.3 cm2
Maximum Mechanical Limit (Xlim) 9.5mm

MOUNTING INFORMATION
Recommended Enclosure
Sealed 54-65 liters/ 1.9-2.3 cu.ft.
Vented 54-159 liters/ 1.9-5.6 cu.ft.
Overall Diameter 15.21", 386.4mm
Baffle Hole Diameter 14.0", 355.3mm
Front Sealing Gasket Fitted as standard
Rear Sealing Gasket Fitted as standard
Mounting Holes Diameter 0.28", 7.1mm
Mounting Holes B.C. D. 14.56", 369.9mm
Depth 6.5", 165mm
Net Weight 17.3 lbs., 7.9 kg
Shipping Weight 19.4 lbs., 8.8 kg

MATERIALS OF CONSTRUCTION
Coil Construction Copper voice coil
Coil Former Polyimide former
Magnet Composition Ferrite magnet
Core Details Vented and extended core
Basket Materials Die-cast aluminum basket
Cone Composition Paper Cone
Cone Edge Composition Cloth cone edge
Dustcap Composition Solid composition paper dust cap

[tntfrahm]

The above is what is in the cabinet. I'm wondering if I might be better off installing the following and running the SVT through them?


LEGEND BP102

SPECIFICATION
Nominal Basket Diameter 10", 254mm
Nominal Impedance* 8 ohms
Power Rating
Watts 200W
Music Program 400W
Resonance 35Hz
Usable Frequency Range 40Hz-2kHz
Sensitivity*** 92
Magnet Weight 38 oz.
Gap Height 0.312", 7.92mm
Voice Coil Diameter 2", 50.8mm

THIELE & SMALL PARAMETERS
Resonant Frequency (fs) 35Hz
DC Resistance (Re) 5.59
Coil Inductance (Le) 0.83mH

Download PDF Spec Sheet
Mechanical Q (Qms) 5.36
Electromagnetic Q (Qes) 0.47
Total Q (Qts) 0.43
Compliance Equivalent Volume (Vas) 91.2 liters / 3.2 cu.ft.
Mechanical Compliance of Suspension (Cms) 0.54mm/N
BL Product (BL) 10.0 T-M
Diaphram Mass inc. Airload (Mms) 38 grams
Efficiency Bandwidth Product (EBP) 75
Maximum Linear Excursion (Xmax) 6.2mm
Surface Area of Cone (Sd) 334.5 cm2
Maximum Mechanical Limit (Xlim) 10.0mm

MOUNTING INFORMATION
Recommended Enclosure
Sealed 14-28 liters/ 0.5-1 cu.ft.
Vented 25.5-62 liters/ 0.9-2.2 cu.ft.
Overall Diameter 10.08", 256.1mm
Baffle Hole Diameter 9.05", 229.7mm
Front Sealing Gasket Fitted as standard
Rear Sealing Gasket N/A
Mounting Holes Diameter 0.25", 6.4mm
Mounting Holes B.C. D. 9.66", 245.4mm
Depth 4.25", 108mm
Net Weight 8.9 lbs., 4 kg
Shipping Weight 9.1 lbs., 4.1 kg

MATERIALS OF CONSTRUCTION
Coil Construction Copper voice coil
Coil Former Polyimide former
Magnet Composition Ferrite magnet
Core Details Non-vented core
Basket Materials Pressed steel basket
Cone Composition Paper Cone
Cone Edge Composition Cloth cone edge
Dustcap Composition Zurette dust cap

[billfitzmaurice]

Originally Posted by tntfrahm

LEGEND CB158

It's a pretty good driver, but it has no business being in a sealed cab at all. It will work well in a vented cab but needs at least 3 cubic feet of net interior volume per driver. The BP102 would be a lot better in the lows, but lacks mids. That means crossing over to one or two sealed back eights at 500Hz. And you'd still have to port the cab for best results. It really boils down to a complete rebuild. I'd get rid of that cab, and shop around for something that has the sound you're looking for.

[Stick_Player]

Now you can download WinISD and run the numbers.

When you are calculating the internal volume, take into account what the speakers will displace - figure out the volume of the two cones and magnet-assemblies.

You may need to also take into account the volume displaced by any internal porting.

You can have the port tubing outside the cabinet.

That internal volume will shrink pretty rapidly.

Not sure why you are considering 10s... seems like a lot of work.

It's a pretty good driver, but it has no business being in a sealed cab at all. It will work well in a vented cab but needs at least 3 cubic feet of net interior volume per driver.

I'd go with a single 15 then. You have about 3 cubic feet. It'll still be louder than McCartney's old rig. Buy a 2nd matching cab and modify. Get a GK 700RB and mount it in an AC-100 head cab.



I mounted one into a Super Beatle head cab.



All the said, it could be a rewarding project. I built an AC100/Super Beatle sized cabinet. Yes, not a lot of internal volume, but porting and the 'most right' speakers gave me great results. My cabinet has TWO chambers, the upper has two 10s, the lower has a single 15. All the porting is in the back. And it looks very fab and gear!

[tntfrahm]

I'm in a Beatles tribute band. If I didn't need the look I'd just use my SVT with my 410HLF and be a much happier individual. Now, I have an 800RB mounted in an AC100 head (much like the photo posted by Stick Player). Yours looks great. Any photos of the speaker cab?

[KramerBassFan]

BFM, wouldn't Eminence Beta's be good* in this cab?





* Good is subjective, more like rather usable.

[sconix]

Two Beta 15 or Delta 15 would work just fine. Or the older Legend B15 all have qts around 0.6 or above and work great in smallish sealed boxes. The Legend you chose was not what you needed. 2 of these higher qts models in the T100 will be great, far better than you'd think. All have a peak before rolloff in the 2k range, so dip a pinch of hi-mids and boost tiny bit of 40hz.

[JimmyM]

Originally Posted by sconix

Two Beta 15 or Delta 15 would work just fine.

honestly, this sounds like the path of least resistance, and at least the beta and delta are designed to work in vintage cabs. i'd probably go with the beta since they're cheaper and you don't need much more power handling than you'll get with them.

or you could do what johnk_10 did and seal the top 15" hole and install a mid cone and crossover along with a 3015lf and a port.

[billfitzmaurice]

Originally Posted by KramerBassFan

BFM, wouldn't Eminence Beta's be good* in this cab?





* Good is subjective, more like rather usable.

IMO no driver would be 'good' in that cab. Even Sir Paul dropped them like a bad habit at his earliest opportunity. One fifteen would be better than two, though, as previously noted. If the look is a necessity I'd stick Vox grille cloth and logo on a decent cab.