long external port idea

[dwk123]

As Nigel says, "It's a fine line between clever and stupid", and I'm not sure which side I'm on with this.

I have a couple TB-W8-740C drivers I got intending to use in a Tuba, but shelved that idea. The AutoTuba is pretty much useless in a home setting, and while the Table Tuba response looks great for music, at almost 8 cu ft it doesn't seem like good use of space unless you NEED sensitivity.

So, it's either ditch the drivers or come up with something. We all know the problem - they need tiny boxes to work vented, and don't really have enough Vd for sealed.

So, how about this: use three REALLY long ports outside the enclosure, and arrange them like the Room Lens diffraction device. I'm not completely sold on the Room Lens, but there is a lot of very positive anecdotal opinion out there (see John Risch's knock off here http://www.geocities.com/jonrisch/a3.htm)

This setup seems to give you either a double or tripple whammy:
- small cheap subs mean you can have multiples, which as Geddes indicates is almost always better (I *might* be able to get 4 in my room if I can make them flat-ish to hug the walls)
- diffraction of first-reflection points
- (maybe) helmholtz resonator (first port resonance) near the first floor-ceiling mode.

As an example, in WinISD Pro I tried a 0.85 cu ft box tuned to ~27 Hz with three 2" ports each 39" long. First port resonance is 173Hz, which matches a room resonance at ~180 that I have. Vent noise seems to stay manageable up to xmax (~100W or 102dB@30Hz)

Another option is 1 cu ft tuned to 19 with 3 1.5" ports the same length, but you give up some efficiency and output. Vent noise becomes a problem below 30Hz, but since this is for music I'm not too concerned.

So, any merit to this idea? Waste of time? Yeah, I'd probably be better off just using my two NHT1259's, but where's the fun in that? :-)

[Lothar34]

I've got a pair of W8-740C in my car. Anytime I modeled a ported box, I couldn't use a port less than about 2.5" diameter. I ended up with 10.9L and a 31" port that was 1.25" x 4.25" rectangular.

[BassAwdyO]

Use a larger box and you wont need such a long port for the low tuning... If a small box is essential, just go with a PR.... doesnt sound like you need anything more than a cheapo 12" PR anyway(if a 2" port is sufficent down to 30hz!)

[Lothar34]

IIRC it would go well with the Dayton PR +~300g in a 12L box.

[dwk123]

Quick update - I haven't shelved plans for the 'room lens' port arrangement, but for now I threw together a stopgap. Two 740C's in a 2 cu ft box tuned to ~20Hz with a single 4" port (came out to about 33"). Looks completely ridiculous with the port sticking out the top, but I'm pretty darn impressed on initial listening tests.

I'm probably the only guy here that's using 15" midbass drivers and 8" subs.