Avatar SB112 - port tuning too high?

[bobcruz]

I recently bought a pair of Avatar SB112s. I decided to calculate the stock port tuning and came up with these measurements:

Ports: 7.5"W x 2"H x 7"D x 2
Internal dimensions of cab: 15.5"W x 22.5"H x 12.75"D = 4446.6 cubic inches or 72.9 liters.

Guesstimating a net internal volume of 68 liters, the port tuning is around 56 Hz. Modeling a stock DeltaliteII 2512 (since I don't have the parameters for Avatar's longer x-max variant) shows a 3 decibel hump at 80 Hz. But more worrisome is the risk of fartage and blown woofers since I play a 5-er and don't have a steep high-pass filter to protect the drivers below the tuning frequency.

I've plugged rectangular ports before using tightly fitting carpet covered plywood rectangles. If I plug one of the two ports, the tuning frequency drops to about 40 Hz and airspeed in the remaining port is still acceptable at mach .11. I'm inclined to plug one of the ports to eliminate the 80 Hz hump and more importantly, to protect the woofer.

Am I missing anything? Why would the port be tuned so high in the first place?

[billfitzmaurice]

Originally Posted by bobcruz

Am I missing anything?

Nope.

Why would the port be tuned so high in the first place?

To make the cab subjectively sound louder.

[rpsands]

Your estimation of the interior space of the cabinet seems to be off.

Are you including the port in the internal dimensions of the cabinet? Do not do that.

If it's 24.5" wide, should be 22.25 after just the plywood. Removing the 2" for the port, looking at 20.25.

So 20.25 changes your cab volume to gross 2.31 cubic feet (65 liters). After subtracting handles, displacement of the woofer, and any bracing/lining, your ballpark is closer to 58 liters probably.

You should then add the area behind the port to the volume of the cabinet, so that is sounding like 6.75 * 15.5 * 2.75 or 4.71 liters.

So our working figure is closer to 62.71 liters.

Running your port depth with that interior space, your tuning is closer to 51.2hz probably.

[rpsands]

Note that 51hz tuning still leaves you with about a 2.5db bump at 80hz which is undesirable but not as bad.

About the longest I think you could make your port is 13" based on the depth of the cabinet. That would probably be fine.

[bobcruz]

Originally Posted by rpsands

Your estimation of the interior space of the cabinet seems to be off.

Are you including the port in the internal dimensions of the cabinet? Do not do that.

If it's 24.5" wide, should be 22.25 after just the plywood. Removing the 2" for the port, looking at 20.25.

So 20.25 changes your cab volume to gross 2.31 cubic feet (65 liters). After subtracting handles, displacement of the woofer, and any bracing/lining, your ballpark is closer to 58 liters probably.

You should then add the area behind the port to the volume of the cabinet, so that is sounding like 6.75 * 15.5 * 2.75 or 4.71 liters.

So our working figure is closer to 62.71 liters.

Running your port depth with that interior space, your tuning is closer to 51.2hz probably.

Arggh, I did include the port in my gross volume calculation for the cab, so I'll redo the math in the morning to eliminate the port. My dimensions are correct, however, since I used a tape to measure the internal dimensions. The external dimensions are a bit off because they are from the outer edges of the plastic stacking corners, and there is a half-inch inset for the rear panel which is dadoed into the sides (similar to the baffle's inset in the front but not as deep).

[rpsands]

I'm sure your inside dimensions are correct except for the width including the port...at the very least need to remove the volume of the port (which seems to be 15.5" wide by 7" deep by 2.75" high - 2 + the 3/4" plywood of the port).

The volume behind the port is the tricky business but it's best to add that to the cab's volume from what I can tell.

Also be sure to calculate properly for the displacement of the speaker, the handles, and so on.

Anyway, lengthening the port should be the best way to improve your performance more than plugging ports. Plugging port might introduce air noise - 14" is on the low end of what I'd want for in^2 of port area for a deltalite.

[thudfromafar]

Funny, when I was experimenting with a power amp + DSP and my SB112 earlier this year, 56hz is exactly where I ended up setting the HPF....

[bobcruz]

Subtracting the port area brings my guesstimate down to 64 liters net and the port tuning at that volume is about 58 Hz. With one port plugged, tuning is 41 Hz. To get down to 41 Hz with two ports, the ports would have to be much longer than the available space, and that's more difficult to implement and not reversible, so I'll roll with a port plug for now and I can always pop it out if I get port noise. Thanks to all who posted!

[gimmeagig]

Originally Posted by bobcruz

Subtracting the port area brings my guesstimate down to 64 liters net and the port tuning at that volume is about 58 Hz. With one port plugged, tuning is 41 Hz. To get down to 41 Hz with two ports, the ports would have to be much longer than the available space, and that's more difficult to implement and not reversible, so I'll roll with a port plug for now and I can always pop it out if I get port noise. Thanks to all who posted!

That sounds interesting. When I get my cab I want to try that out as well. Could you please post a picture of the modification so I can see how far inside the port the block of wood is mounted.
The other thing I was thinking was that instead of blocking one side completely, you could also insert two triangular blocks into the port on the sides. That way the air would not hit a flat surface but it would hit a 45 degree angle before it shoots out of the cabinet.I don't know if there is any gain by doing that but it might be aesthetically more pleasing.
When I order the cab I'm also thinking about getting just a single handle at the top, because it is light enough.
That would also change the internal volume of the cab a little.
Would that be something that I might be able to hear or is the difference so small that it doesn't matter?

[bobcruz]

I'll have to take a picture of it, which might be in a day or two. I had some closed-cell packing foam that is barely compressible, and I cut a chunk just a little larger than the opening and squeezed it into place to form an airtight seal, then covered that with a thin rectangle of black carpeted wood, also held in place by friction plus a few tiny nails into the foam and the center port brace. From the front it's just a black piece of carpet that looks almost like the open side. Whatever you use, it needs to be airtight or you'll hear whistling around the edges. Angling or partially blocking the ports could lead to unpredictable results, so I wouldn't go there. The good thing about this mod is it's totally reversible, so if you don't like it you can open the port up again.

[gimmeagig]

Great, thanks.I got it.No need for the picture.I will definitely try it out.

[bobcruz]

I might take some pix anyway since I'm proud of my work, LOL. I also sprayed the remaining port openings with satin black paint for a smoother, darker look. The flat black spray that Dave and most manufacturers use inside cabinets looks too gray and dusty to me. Pix on Monday or Tuesday....

[will33]

58hz is crazy high for that driver in that volume. It's crazy high for a lot of bass guitar drivers in a lot of cab volumes. Trust your ears, they're telling you the right things.

[gimmeagig]

Originally Posted by will33

58hz is crazy high for that driver in that volume. It's crazy high for a lot of bass guitar drivers in a lot of cab volumes. Trust your ears, they're telling you the right things.

Not sure I understand what you are saying. 58 hz is high? Isn't that a low A or something like that?
Can you dumb it down a bit for me so i can understand it too?

[will33]

Originally Posted by gimmeagig

Not sure I understand what you are saying. 58 hz is high? Isn't that a low A or something like that?
Can you dumb it down a bit for me so i can understand it too?

The box tuning doesn't have to do with a particular note, like an A or something. It has to do with where the ports contribute to the overall system output and the shape/slope of the lowend rolloff. In this case, that's a high tuning, which results in a hump in response around 100-120 hz which makes the cab sound louder/"bassier" but in a band mix makes boomy sound, indestinct notes, can hurt your sense of pitch on the low notes due to there being an abundance if those frequencies in comparison to tbe middle/upper frequencies and make for generally garbled/less clear representation of what the bass player is playing. A little bit like the difference between "bedroom tone" and "band tone". You can get away with a little higher tunings if your driver is particularly well suited to smallish cab sizes, or if you always play 4 string, but still there comes a point where more lows doesn't make the bass sound better, it actually hurts it. Making everything sound like the same frequency of "boomy" rather than well defined, nice bass sound. Taken to the extreme, you can end up with a "one note wonder". Meaning you lose sense of pitch and clarity and all the bass notes start to get rumbled together. The Avatar 12's 58hz tuning isn't to that extreme, but it's on that path.

[gimmeagig]

Thanks for explaining that.So you agree that Bobcruz had found a way to improve the cabinet by blocking one half of the port?
I play a five string active Jazz and use the B string quite a bit so anything I can do to help with that especially if it is such a simple modification....

[bobcruz]

It might be an improvement but there are plenty of people playing these cabinets in stock form with no complaints.

58 Hz tuning for the stock port is a calculated number--the actual tuning is probably lower than that because shelf ports interact with the cabinet walls, acting as if they are a bit longer than they are. To find the actual tuning frequency of the unmodified and modified ports, I'd have to do the "sugar test," explained at post #9 in this thread: Cab building question about shelf port length.... I may do that this summer.

[gimmeagig]

I currently have a 112 and a 210 in my house that Dave loaned to me to try out.It's hard to really know what each cabinet does best by sitting in your room by yourself playing some licks.But I need a cabinet that sounds full,punchy and has clarity(Marcus Miller is my man) and I want to have that sound up close and good for small to medium rooms 100 to maybe 200 people max( I can always add a Bag End 15 if it gets bigger).
I think the 112 is right for that.I don't want to compromise the sound that I'm hearing while I'm playing hoping that 50 feet away it sounds better which might be the case if I bought a 210 cab.
When I get my cabinet built I'll try the port modification but,as it is, the cabinet sounds really nice at both super low volume and also cranked up and it seems to handle the low B really well also.
I don't go through as much gear as maybe some other players. I have played through Eden world Travelers since they first came out and just upgraded to the higher power model as they were put on the market and I have owned my Bag End S15XD cabinets for the past 13 years. I have a feeling the Avatar 112 is a huge step up for the types of gig that I'm playing and I can't wait to start using it in a band setting.

[kraigo]

In a tangent, I've never really come up with a good way to estimate the volume that the speaker takes up. I've taken to going through Eminence's suggested cabs for a particular driver and doing subtraction from there. What do you guys do?

KO

[bobcruz]

Originally Posted by kraigo

In a tangent, I've never really come up with a good way to estimate the volume that the speaker takes up. I've taken to going through Eminence's suggested cabs for a particular driver and doing subtraction from there. What do you guys do?

KO

I forget what I did but I think I got a volume figure for a 12-inch driver from some manufacturer that publishes the figure (it might have been Carvin) and used that since it would be close.