Short vs. Long MLTL

[rdf]

Question for the Martin King crowd: what design goals drives the preference for long lines? The reason I ask is that for the two drivers I have for MLTL projects - pairs of TB W4-657s' and a Jordan JX92s' - the Mathcad sims always point to a short line as the best balance of LF excursion control, port 'junk' reduction and low pass band ripple. By short I mean 22" with a 3" L x 1.5" D port for the TB's, which results in bass more impressive anything that size has a right to push. The sim's in the .rtf attached. Short lines also push the 10th (?) harmonic port emmissions into a range which is easily attenuated with wall damping without affecting the primary port resonance. The Jordans look like they'll fall in the same range.
Yet people seem to prefer long, tapered line projects. Am I missing something obvious?

[rdf]

Ack! RTF's not allowed. Here it is in zip:

[GM]

Originally posted by rdf
Question for the Martin King crowd: what design goals drives the preference for long lines?

To force a driver to a ~max flat response below what the driver's specs indicates, with the side benefit of being able to use an even shorter vent.

The reason I ask is that for the two drivers I have for MLTL projects - pairs of TB W4-657s' and a Jordan JX92s' - the Mathcad sims always point to a short line as the best balance of LF excursion control, port 'junk' reduction and low pass band ripple.

Yep.

Yet people seem to prefer long, tapered line projects. Am I missing something obvious?

Well, reverse tapered is just another form of mass loaded vent and a positive taper is a horn variant, ergo another form of mass loading the mass loading vent. The problem though is that to keep peaking at Fb to a minimum you have to move the driver to offset it and/or add more stuffing, to some extent defeating the point of the longer pipe. Still it has its uses in some apps.

GM

[GM]

Originally posted by rdf
Ack! RTF's not allowed. Here it is in zip:

Hmm, I have somewhat different specs for this driver that yields considerably different results. Are yours measured?

GM

[rdf]

Originally posted by GM


Hmm, I have somewhat different specs for this driver that yields considerably different results. Are yours measured?

GM

They were indeed. Two drivers were measured using an old Auratone cabinet for the test box and the gen load function of an Audio Precision One+ for impedance measurements. Repeated attempts yielded consistent results, though it's possible they were consistently incorrect. The sim used an average of two W4-657sb versions. I recall the impedance of driver in cabinet was close to predicted, though I spot measured as a sanity check and didn't write down the results.
Did you measure different?

[rdf]

Originally posted by GM


To force a driver to a ~max flat response below what the driver's specs indicates, with the side benefit of being able to use an even shorter vent.
GM

I guess that's what I'm getting at, a short line does the same with fewer downsides. I tried Martin's BR sheets on a cube with the same internal volume and port dimensions of this line and it sim'd with less bass extension. Line action is occuring in MathCad. However, I found there's a point of diminishing return. You get deeper bass with long lines at the expense of increased excursion in the mid and upper bass as well as pushing port harminics lower. It seems to me the optimum tuning achieves deep bass and keeps excursion in the 'power range', say 500-100 Hz, managable. The figures are gone but one sim of a Jordan kept excursion at no more than 15% above a .707 Q closed box to almost 32 Hz with a -3 dB point in the high 30's. It was 24" long.
Short vents are also at a disadvantage when considering port harmonics, long ones attenuate them better. So I'm just wondering if shorter lines are "tweakier" or something and why they're generally avoided.

[GM]

Originally posted by rdf


They were indeed..........Did you measure different?

I did not measure any, just have some posted by Al? (Creative Sound?). Anyway, I calc B*L = 5.6451 N/A, pretty far off from your 5.1, so I have to wonder about their accuracy.

Of course once the stuffing is fine tuned in-room, odds are they will sound fine whether they are 'optimum' or not.

GM