I need a bit of advice - I am mid build of some 20L ported cabinets for a pair of MarkAudio Alpair 12P’s. I have used the “DIY audio online speaker box enclosure calculator”.
The recommended port size for this 6.5" driver is dia 50mm x 130mm long.
I would much rather, If there is likely minimal impact to SQ, use some Inside diameter 35mm tube as I have a some tube and a hole cutter already? Is this going to be OK? and do I have to recalculate the length of tube to give a similar impedance?
Additional question - The TS parameters on my speaker data sheet says F0 = 42Hz but there is box for Fs (Free air resonance) and one for Fb (Resonance frequency) In my calcs I have (hopefully correctly) that the free air resonance is the critical one?
The photos shows where I am on the build - I plan to have a removable rear section.
Thanks in advance
I know 0 about speaker ports. But for what it’s worth a 35mm diameter tube has almost exactly half the cross-sectional area of a 50mm diameter one. So I’d have thought two 35’s alongside one another (or maybe they would be better spaced apart ?) would have the same air transport impedance as one 50.
VB
Graeme - Cheers thats sound good I will try twin turbo’s
Weeeeellll … as I say, I know 0 about ports, so I can’t promise that this will work. Ports involve the movement of the slug of air which is within them. I don’t know if two smaller slugs separated by the solid walls of two port tubes would move in the same way as one large slug in a single tube. But if you can risk the panel (I guess you can always plug holes if it turns out they’re wrong) then we could find out by trying it, assuming no-one on here knows the answer ahead of the experiment.
VB
It’s done now and being glued - Don’t worry the whole thing is something of an experiment - I had a new mitre saw and some old plywood and wanted to make something. I remember the Pro ac tablets had drinking straws along the ports. Some cabs have slots so I don’t expect it’s a critical factor whether there are two holes or one.
It would be worth making the port length variable if it’s not too late, as I think it will need a little experimentation to find the best tuning.
Also, it’s well worth setting yourself up with some software to measure the response of the speaker if you haven’t already - something like Holmimpulse or REW is perfect.
If you can use the latter to measure the voltage developed across a shunt in series with the speaker under test, you can use it to look at the driver electrical impedance over a frequency sweep, and you’ll find a dip in impedance where the port is resonant. You want to position this just where the driver’s bass response rolls off in order to extend the bass response, but not to generate a “hump” or “dip”, in basic terms.
Apologies if this is a lesson in egg-sucking.
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I just plugged your numbers into an online calculator here:
http://www.linearteam.dk/default.aspx?pageid=ventcalculator
Port length changed a little to 138 mm for 2x35mm ports, so you’re probably not going to be far off.
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Many thanks for doing that calc. I will look into some software once I have finished them fully. The port tubes are easily variable -They are a snug fit in the hole and can be adjusted with ease, when I said glued I meant the cabinet walls. I clamped a back on them quickly just now and had them playing music -There is plenty of bass going on with them three foot from the back wall.
I am just about to cut out the backs on my mitre saw using 19mm ply in stead of 25mm in order allow for a bit of clamping flex in the wood and make up a bit of addition internal volume to offset me not considering the volume of the port tubes or battening inside.
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Sounds good. Keep us posted. 
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Backs now cut out and fitted - Having a listen vocals sound a bit steely but overall quite decent - If you put your ear close to the speakers there is lots of sound coming out of small gaps in the joints, I think this might be stopping the ports doing their job properly because tonally they sound similar whatever the distance from the back wall. There is no damping material at all at the moment either -So obviously stuff to do still I couldn’t wait to try them thou. ?
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Did u include amp output impedance and dcr of any series choke in the xover when calcing the alignment?
Obs if it’s a SS amp and no xover, then forget that I asked 
There is no xover. Latest amendment is adding horizontal battens to clamp rear panel and help with sealing. -Also planning some xmembers inside prior to silicone sealing the holes up.
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I have now added a strengthening Xmember with an option to clamp the rear of the driver and fitted the banana sockets plus first pass at sanding one cabinet - Annoyingly I have mislaid one of the four needed I hope it turns up soon? The cab is pretty much sealed but there are a few crappy bits and blemishes I hope to improve on - this was second or third use wood.
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Update - Pure wool felt wadding ordered, Holmimpulse down loaded. Microphone incoming from Gthang and finishing advice (thanks) .
Cabinets very nearly fully sealed. Running them with no wool in ,so I can better understand the impact of wadding when it arrives . Will try to download REW tonight.
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I had so much fun building the 20L - I made a baby brother for them using more old wood - This time with PRV drivers 94 db efficient and 200w power handing.
I will give them a go tonight once they are wired up
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Gthang( Many thanks to him ) brought round a mic today and showed me the basics of HolmImpulse - The ports in the rear deffo add some bloom, It’s very nice but not too accurate - That said the FR is more affected by the speaker placement in the room than any port tube setting or even sealing therm off. So for the time being I have stuffed them up.It’s a tad boring sound compared to my usual - but I am trying to acclimatise.
The FR with port sealed is in the photo and as flat as I can muster
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It’s a lot easier to adjust everything with the mini DSP software. I’m quite pleased with the aesthetics of this arrangement.
The RAAL’s working above 3.3K and bass 15" 100hz at the moment.
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