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Douglas yee

Determine port size for a 1x12 cab

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I'm repurposing a 1x12 open back guitar cab to bass cab and need help determining port tube size. Its a cab I built years ago and figured it would be an easy conversion.. The cab is all 3/4" Pine box, with 3/4" ply baffle and back. Interior volume and speaker displacement is

1.19 cu. Ft. or 33.2 l volume

.079 cu. Ft or 2.25 l displacement

I plan to use standard 2" port tubes for the conversion. Is there a formula to determine how many and what length the ports should be or does it matter that much at all?

Thanks for the assistance.

 

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There are many considerations involved, so many that you need to use a speaker modeling software program, like WinISD, to determine the correct port sizing..

http://www.linearteam.org/

If you don't have the driver Theile/Small specs you can't use modeling software. If it's a guitar driver it won't work anyway, for a variety of reasons, any more than you can use guitar strings on a bass.

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As Bill said, there are many considerations involved.

First of all, think about closing up the back panel. Download and install the latest version of winISD -  see Bill's link.

Then post the information you have on your chosen 12" bass unit. Once the model and/or it's details (Theile/Small parameters) are
visible, then a number of people on here can let you know whether it's suitable for bass and suitable to fit in a 30litre enclosure.

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Without the TSP's of your intented 10" driver,  the modelling  just wont work in the software.

Have you any ideas of drivers yet ?    You could try Blue Aran for  Beyma drivers. They're pretty damn good. I;ve used  4 of the 10" drivers before in a couple of 2x10 cabs, helped by the speaker gurus on Bass chat

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And the names of the scientists were A. (Albert) Neville Thiele (Australian; definitely not Theile) and Richard H Small (born in the US).

Edited by itu

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Sorry I am having a hard time finding data for the speaker.

The speaker is NOS, from SWR factory close out I bought from a local builder. The speaker its self is 10" 8 ohm, 250 watt Eminence Megoliath. That's all I could find out for now. I went to the Eminence website but no info is available. I emailed their tech for a spec sheet but they are closed for a holiday today here in the US.

 

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Sorry to disagree with the general consensus that this is so complicated, but you really don't need the driver parameters to port your cabinet. Don't use 2" ports - they are too small. Use either 2 x 70mm ports (standard drainpipe size) or preferably 1 x 100mm port. The length in both cases should be about 180mm.

The Eminence Megoliath is described as a bass guitar speaker. So you have a good chance that it will work, as 30 litres is a reasonable size of cab for a typical 10" bass guitar driver. Without the TS parameters you won't be able to predict the driver's performance in the cab, but as you have never used speaker modelling software, you are unlikely to have a clue what to look for anyway.

Just port the cab and see what it sounds like. I wouldn't expect the driver to handle its rated output and I would expect the solid pine cabinet walls to resonate at volume. Just suck it and see. Good luck!

Edited by stevie
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I would be tempted to do it properly, buy a well known driver ( recommended ) like the Beyma, which will have the TSP's and ask here for help with Win ISD. Thats just what i did. Guesswork is fine, but why not go the whole hog ?

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I don't disagree, fleabag, but I suspect the OP just wants to make use of the bits he has in his possession.

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Stevie, you are correct in assuming I want to use what I have available. I'll ditch the smaller ports for the larger one you mentioned and see what up. I play bass for fun and had the cab that wasn't being used. 

Basically this cab is just for home play through so it will never be maxed out. I just needed it to sound good enough.

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Understood - but you're doing the right think by asking the cab gurus.

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I was going to ask what you wanted to use the cab for. The problem with putting a speaker in any old cab is that you could get an unusual frequency response and that the power handling could be affected at low frequencies. It looks like the SWR Megiliath used some version of the Eminence Alpha 10, maybe just rebadged or maybe with a few tweaks and the original cab gave it about 45litres per driver or thereabouts. Here's the spec sheet

http://www.eminence.com/pdf/Alpha_10A.pdf

Putting a speaker into a smaller than optimal cab will do three things potentially, You'll lose some of the bottom end with the cab rolling off fairly high. To compensate you'll get a bass boost just above that point, if you are lucky it will warm up the sound and you may not miss the deep bass in a small room. Generally you get better power handling in a small cab but that is frequency dependant it will be better at some frequencies than others depending upon the exact tuning you achieve.

I'd tune the cab to around 50Hz which is the resonant frequency of the bog standard Eminence Alpha but that's off the top of my head, I haven't modelled it. Two ports of plastic drainpipe around 15cm long will get you in the ball park, but if someone comes along to run winISD for you or you try inputting the Eminence specs and doing it yourself then take their figures as mine haven't been checked. I'm playing with a new band this week and need to get on and learn their set.

Good luck, have fun and if you don't push it too loud you shouldn't go far wrong :)

 

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So I received this spec sheet from Eminence. I may have to return the speaker for something else as it does not appear to handle to my amp power, ([email protected] [email protected])

    101109A





 

   

Re    4.10    Ohms    fs    46.7    Hz

Le    0.80    mH    Mms    35.3    grams

QM    11.95

QE    0.33        Cms    0.330    mm/N

QT    0.33        Rms    0.866    N*sec/m

Xmax    3.00    mm    Vas    62.0    liters

Pmax    150    Watts    SD    366.1    cm^2

Bl    11.27    Tm   

Coil Diameter    2    Inches    EBP    139.7   

Gap Height    0.313    Inches    Magnet Weight    38    ounces

Efficiency    1.82    %    Winding Width    0.550    inches

            SPL    94.6    dB 1W-1m

 

Thanks for all the help everyone. 

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Your amp power is moot, you'll never run the amp at anywhere near full power. Besides, with 3mm xmax that driver will fart out at well under 100 watts anyway. Tune the cab to 50Hz Use two 2" ports 5 inches long.

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Bill Fitzmaurice, so as long as I dont run the amp near full power with my cab set up as you stated I won't have to worry about damage to the amp itself? 

Edited by Douglas yee

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Certainly not.

By the way, it would be easier to comment if you'd specify your amp by type and name. It tells more about the principle of the amp (tube, solid state) and so on.

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Hello Douglas,

Having put your supplied figures through winISD I would say you have a workable solution. The finished speaker will be more than loud enough for home practice or even a jam in the garage with a mate. That level of loudness will be achieved by only putting in 50 watts! So your amp will be cruising nicely.

If you can get one round plastic pipe of about  3" internal diameter and 2.8" long, that will do for the port. Alternatively, two 2" ports of 2.5" internal diameter will work with a length of 5.1".

In both cases the tuning is at 59Hz.

If you can confirm the port sizes you have to use, (your pipes in the USA may be different to ours) then I can tell you the required length.

Edited by Balcro

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Find a decent thickness cardboard tube, say out of the middle of a roll of carpet. Make a hole to suit and experiment with different lengths until you get the sound you like best.

Front and back ports sound different. Back port is good for a bedroom blaster played near a wall or corner. Test it where you will play it most.

Works every time.

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