Labrinth type port in bottom of enclosure ?

[quickshift]

Hello

I'm looking at building a vented sub using an Audio Drive 12-300SB for HT. So far everything models out OK in WinISD to give me a 45l enclosure tuned at 25Hz which gives me a decent FR, more than enough SPL and sufficient power handling. But... I need to use a 32" port and length of the box is only 20". I can easily build a labrinth type port into (under) the floor of the box with the port exit at the back of the enclosure but wanted to get some advice please. I can't find anything specific about this type of arrangement so was wondering what the potential problems are in doing this and if it's actually a good idea in the first place.

I've attached a quick diagram of what I'm looking at doing.

[Ap]

Problems can arise from resonances in the port itself, I think the recommended max is a 1:4 ratio (area/length) to reduce port resonances. The alternative is to use a passive radiator.
Andrew.

[quickshift]

OK, thank you for your reply

Is port resonance worse than using a straight port if I'm using a folded one?

I get the 1st port resonance at about 280Hz, I guess this is dependent on the output of the driver at this frequency which should be -43dB or thereabouts.

[Ap]

I woudlnt think it would be worse, just different, you may end up with resonances for each section of the pipe - hard to tell without building & testing. You could dampen the port/line hmmm now we are heading into TL terrirtory.
A passive radiator would be the simplest approach + also easier to build.
Andrew.

[pinkmouse]

This is the perfect example of when to build a prototype box out of cheap materials to test a concept.

[rabbitz]

I did one as an alternative to a PVC port and had no problems. The only limitation is you cannot readily change the port length but that is usually not a problem.

http://www.darcheraudio.com.au/rzaudio/rz52/12xls.htm

[consort_ee_um]

The original scheme you proposed would work but as the volume goes up and the port velocity increases you woiuld get turbulent airflow, hence chuffing
Rabbitz scheme is an improvement because the mitres in the corners help the air turn the corners when it is flowing at high speed
Google for Collo and long ports to find alternative ways of doing this.
Really it depends how loud you want to listen.

[quickshift]

Thanks again Ap, if I can't get good results with a ported box, I'll look at using a PR.

pinkmouse, almost everything I build is a prototype using cheap materials, somehow I usually end up keeping them though I'll be using spare MDF I have kicking about so won't cry too much if it doesn't work out OK.

rabbitz, excellent, thank you. Some significant improvements over my idea there. I can build in curved corners easily as I've got loads of filite and fibreglass resin left from my last effort. I was wondering about port length. If I built in a kind of sliding end I could get +/- 5Hz tuning without having the end of the port stick out too far. Nice work!

Thanks consort_ee_um, I'll get googling for more ideas. I don't need this to go very loud TBH, just nice and low without straining itself too much. I think I'll see about 12 m/s airflow in the ports which I guess is fairly significant so curved corners it is.

[rabbitz]

consort_ee_um is right saying have a look at Collo's work on ports. He's a local guy and has put in a huge effort to gather this information. I've been over to see some of his stuff and experiments.... he has some good ideas.

http://www.subwoofer-builder.com/

[micb]

This driver is proven day in day out in a larger and lower tuned enclosure. i.e. the BK Monolith design.

Your 45L tuned to 25hz is too small and tuned too high IMO.

Go for around 90L tuned to 20hz, it will play flatter and be more efficent.