Calling cab designers - Consequence of small speaker port?

[Sparky Mark]

Attachment 206903

With LM3 for scale. Added the second port and the tone now seems more even from top to bottom with a more open bass. No tweets, but sound really good for rock with plenty of throaty punch if required. Weigh 28lbs each.

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I recently built two small 1 x 12" (Emi Basslite S2012 specs below) loaded cabs because I was curious and didn't fancy spending around £800 ($1300) to see if 12's might be for me.

By luck, rather than judgement, they sound pretty darn good and I have now played a couple of gigs with them driven by my MB LM3.

The cabs each have an internal volume of 1.45 cubic feet, the inner walls lined with 1" wadding, but the stock ports available at my local electrical store (1 off per cab) were approx 2" dia x 3" long. Now this would seem to be tuned to around 36Hz which is below the spkr resonant frequency (fs) of 48Hz, which I hope is okay?

Reading the Emi design guide for this speaker which recommends 2 off 3" dia x 5" ports, mine are woefully underported. But they sound great, and I've been playing them pretty loud in small - medium venues without any farting out.

Is the lack of porting just reducing low end performance or is it going to damage the speakers. If so, I can just plug the ports and use them as sealed cabs. (I have the plugs, but this does reduce the sound levels somewhat).

Many thanks for your advice.

Mark

Resonant Frequency (fs) 48Hz
DC Resistance (Re) 5.1
Coil Inductance (Le) 0.43mH
Mechanical Q (Qms) 5.5
Electromagnetic Q (Qes) 0.53
Total Q (Qts) 0.48
Compliance Equivalent Volume (Vas) 91 liters / 3.2 cu.ft.
Peak Diaphragm Displacement Volume (Vd) 270cc
Mechanical Compliance of Suspension (Cms) 0.24mm/N
BL Product (BL) 11.7 T-M
Diaphragm Mass Inc. Airload (MMs) 46 grams
Efficiency Bandwidth Product (EBP) 91
Maximum Linear Excursion (Xmax) 5.2mm
Surface Area of Cone (Sd) 519.5 cm2
Maximum Mechanical Limit (Xlim) 9.8mm

[JimmyM]

there's a technical answer, and there's an "if it ain't broke, don't fix it" answer. my opinions go toward the latter.

[Sparky Mark]

Thanks Jimmy, if I'm losing some bass extension then I'm not bothered, they go pretty deep, but if over excursion is a possibility, then I may be heading for a fall.

[hrgiger]

chuffing

[JimmyM]

well you'll want to hear from the cab building geniuses on here, and i'm definitely not one of them. but judging from the amount of cabs out there that have too small boxes and too small ports for the speakers, as long as you watch your volume i don't think you'll have a problem.

[hrgiger]

according to Bill, port area must be a minimum of 1/3 the cone area to prevent chuffing, so about 30" per 12. 3"x5" times 2 sounds right.

[rpsands]

Lower tuning is less likely to give you excursion problems than otherwise. Higher tuning is mostly a game of "move the point below which I don't want any content up in exchange for more sensitivity above the frequency."

So below your tuning frequency you unload rapidly but above your tuning frequency you get a significant sensitivity bump.

S2012s are fine with a tuning of 40hz, so 36hz is fine. You're sacrificing a little bass content, but I wouldn't wring my hands over it.

Now, the port area is where your design suffers. When your port area is too small and you start to chuff, your tuning frequency changes depending on how loud you're asking it to get. I believe that the tuning frequency is artificially raised as your port speed gets super high - but someone more expert would have to give some details on how that works.

I would suggest highly that you report your cabinet with at the minimum one 3" flared port, or one 4" non-flared port, in order to achieve the maximum volume each cabinet can achieve.

[Sparky Mark]

Yes, I've been trying to hear this effect, but with everything else (guitards, drums , vox etc) happening, I really cannot hear any chuffing.

[hrgiger]

chuffing is primarily found with really low frequencies (afaik.) I'm not really sure how pertinent it is with bass guitar. I was reading about it on the web a while back for one of my cabs and I mainly saw references to audiophilic home theater systems with subs.

so:

Originally Posted by JimmyM

"if it ain't broke, don't fix it"

if after awhile you find yourself lying awake at night with fears of chuffing, you can hog those things out...

[Petebass]

Wind speed, Mach, chuffing etc - they're only become a problem at high output. As a guide, try putting our hand in front of the port during a typical gig and feel how much wind activity there is at the port opening on the lower notes. You shouldn't really feel any "wind" at all, maybe just a little. If it's kinda breezy there, then consider changing the ports to a bigger diameter, or perhaps using multiples of 2" ports. Bear in mind that adding more 2" ports will make the port tube length longer so be careful you've actually got enough room inside the cab to to so.

[billfitzmaurice]

You're losing a lot of output potential tuned so low. What software did you use for modeling your cabs? 45 to 50 Hz would be better both in terms of response and displacement limited output. And the cabs are too small, I wouldn't go less than 2 cu ft net with the 2012.

[will33]

The chuffing (sometimes it can sound like a whistle....sort of) will happen right around the tuning frequency of the cab as that's where the cone calms down and the ports are providing most of the output. To much air trying to move through to small a space.....kinda starts turning the port into some sort of a flute.

A quick look at WinISD says your current size/tuning give an f3 around 66hz but because of the small box, that low tuning doesn't seem to knock down output in the lows so much as tames the midbass hump that the small box will give you. Could probably raise the tuning as high as the low 40's before they get too boomy. Sealing them up would raise the f3 to about 76hz but the slope of that roll off would be similair to what you have now.

[will33]

Sealing it would also lose about 30 watts worth of displacement limit but it'd still be in the high 90's. I'd get some cardboard tubing or plumbing pipe and fashion a bigger port. Experiment with tuning around 40 and 45ish, just don't glue it in until you've settled on one you like.

[Sparky Mark]

Originally Posted by billfitzmaurice

You're losing a lot of output potential tuned so low. What software did you use for modeling your cabs? 45 to 50 Hz would be better both in terms of response and displacement limited output. And the cabs are too small, I wouldn't go less than 2 cu ft net with the 2012.

I used WinISD beta and the Emi site recommends 1.4 cubic feet net volume for it's small vented enclosure design for the S2012.

[Sparky Mark]

Originally Posted by Petebass

Wind speed, Mach, chuffing etc - they're only become a problem at high output. As a guide, try putting our hand in front of the port during a typical gig and feel how much wind activity there is at the port opening on the lower notes. You shouldn't really feel any "wind" at all, maybe just a little. If it's kinda breezy there, then consider changing the ports to a bigger diameter, or perhaps using multiples of 2" ports. Bear in mind that adding more 2" ports will make the port tube length longer so be careful you've actually got enough room inside the cab to to so.

There is quite a lot of air pumping out of the ports, I would say 'very breezy'. When I model a second 2" x 3"L port it moves the tuming to 50Hz? So this is better. I was worried about moving the tuning above the res freq of the driver.

[will33]

Originally Posted by Sparky Mark

I was worried about moving the tuning above the res freq of the driver.

Don't worry about it. The second port would take care of the chuffing. If 50hz sounds too boomy to you, then you need to lower it by using longer pipes. If it sounds good at 50, then you're done.

[billfitzmaurice]

Originally Posted by Sparky Mark

I was worried about moving the tuning above the res freq of the driver.

Moot. What works works.
WinISD Beta is better than nothing, but just. Alpha pro gives much more accurate results, and includes both maximum power and maximum SPL charts to see the benefit of a larger box. It also allows modeling of port air velocity at maximum input power, which should be kept below 20 meters/second for best results.

[Sparky Mark]

Thanks everybody, I'll order another pair of ports, fit them, post a couple of pics, and then glory in the inevitable flood of compliments on my handywork.

[Sparky Mark]

Originally Posted by billfitzmaurice

Moot. What works works.
WinISD Beta is better than nothing, but just. Alpha pro gives much more accurate results, and includes both maximum power and maximum SPL charts to see the benefit of a larger box. It also allows modeling of port air velocity at maximum input power, which should be kept below 20 meters/second for best results.

When I started this little project I had no idea I would get such a good result. It was to test the water before buying a pair of Bergs/Aguil/Epi or such like. So I didn't really design them properly, as you say using a pretty basic/blunt tool.

The vent mach number was the one that indicated a basic design flaw (0.48 and red), but it looks like the second port could help, it's now down to 0.09 (m/s?) and green.

I may just have saved myself £1000($1600).

[Petebass]

Originally Posted by Sparky Mark

I used WinISD beta and the Emi site recommends 1.4 cubic feet net volume for it's small vented enclosure design for the S2012.

IMO those "small box" suggestions on the Eminence site are ok for PA or foldback use but too small for bass guitar. I'm with Bill on this one, a bigger box will really get there speakers singing. I have 2 of them in my garage, brand new and still in their Eminence cardboard box for which will one day build a composite 2x12 cab. I'm planning on giving them 2.4cu.ft per driver.