Greets!
OK, though among my various mini careers was designing industrial AC electrical controls, ducting systems for Trane, Carrier, etc., and one of the golden rules was to not exceed a 9:1 ratio, so lacking more knowledge to the contrary have propagated it till proven otherwise.
Anyway, haven't reverse engineered any of Dave's Onken-like designs, only glanced/presumed, so introspect may have given the wrong impression re his designs, but having built similar with > 9:1 up to Altec's A8 small cinema speaker's 12:1 ratio on a 12 -15" driver scale for others since speaker width dominated and they all exhibited an audible difference versus my own Sd = Av pioneer's round/square 'ideal', some quite a bit in fact, so 'something's' going on acoustically with a > ~ 1.2561:1 (averaged) ratio the max acceptable horn design 'square' Vs optimal round (pg. 238 @ 248).
That said, all mine have been with low Fs, Qt, high Vas drivers, so maybe there's more to it whether a heat/AC's wide range ambient or speaker vent ducting's ~room ambient has anything to do with its optimal max ratio same as with horn 'squareness'.
I suspect that the 9:1 aspect ratio 'rule of thumb' for air conditioning ducting has more to do with structural and cost considerations, rather than resistance issues. As the duct is made wider, more support would be required top and bottom to maintain structural integrity, and to prevent panel vibration. Thicker sheeting material might also be required to add extra rigidity, further increasing cost.
(In an AC duct the air particles travel down the duct, whereas in a bass reflex port tube they simply vibrate back and forth).
I think that any audible difference in performance, directly attributable to port shape and not to some other factor, would probably be due to the different radiation impedance conditions existing at the port exits (rather than to any resistance issues in the port tubes themselves).
Many thanks for posting the link to the 'Hi-Fi Loudspeakers and Enclosures' book. I did not have a copy myself, but have now downloaded the document for future reference. The section you refer to deals with horn mouth area and shape and is concerned with optimising mouth acoustical impedance loading conditions and 'dispersion' characteristics, rather than considering resistance issues in the horn itself.
Leo Beranek gives a formula for calculating the acoustic resistance of a slit, given width, thickness (height) and length (depth). What is interesting is that the thickness of the slit (or slot) in metres has to be less than 0.003 / Sqrt(f) for the formula to apply. This means that at a frequency of 1 Hz the slit needs to be narrower than 3 mm, decreasing to 0.3 mm at 100 Hz and 0.03 mm at 10,000 Hz.
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Interesting! First time I'm aware of in all the decades I've told/posted this with 'stretched' implying 'tightly', that it didn't have the desired results, but for now have modded it to read:
(Qts'): (Qts) + any added series resistance (Rs) with 'stretched' as in 'TIGHT AS A DRUM' stretched: http://www.mh-audio.nl/Calculators/newqts.html
😉
(Qts'): (Qts) + any added series resistance (Rs) with 'stretched' as in 'TIGHT AS A DRUM' stretched: http://www.mh-audio.nl/Calculators/newqts.html
😉
Purely operator error, not picking up on the important bits/operative mechanism. Nothing wrong with the instructions!
I often serve as a bad example. 🙂 Every now and then to the point of feeling like that Nickelodeon movie "Jinxed" where the Murphys' cursed family became professional "failure mode testers".
I often serve as a bad example. 🙂 Every now and then to the point of feeling like that Nickelodeon movie "Jinxed" where the Murphys' cursed family became professional "failure mode testers".
Admittedly me and especially the older original, predates the need for in depth English translations, but it still needs improving even though this translation is more than accurate enough to my way of thinking; it too might not translate well enough in some of the various forum's dialects based on my occasional inability to fathom other's translation into anything meaningful, much less relevant, to the subject matter. 
With my personal experience it's not easy to go with transmission line design especially for high effeciency drivers, good news is certainly it compensates bass they naturally lack off in most cases, but hardship is that, it will be quite hard to make it just right or else you will feel a slow and incoherent bass, deep but weak elasticity. I think it depends what compromise you are willing to make and what preference your ears will take.
Yes, if tuning to Fs, a low Qt driver is a poor choice for a typical TL unless driven with a high output impedance amp or other series resistance.
Greets!
Certainly not the original; Hiraga's Onken/Altec 360L is considered the reference, so notice the summed TL vent's powerful peaking at Fb and mid-bass, lower mids' TL pipe harmonics comb filtering with the driver's output; IOW loosely based on Jensen's bass Ultraflex-15, pg. 12, 1:1.4 vent ratio.
As for the rest, we 'discussed' it earlier, I just forgot to post this first before moving on to the 'resistive' aspect that has dominated in recent years on the 'FR' forum, which TTBOMK isn't indicative of so called Onken alignments in general.
Indeed, according to the late, lamented JMMLC; Onken the company never made any Jensen Ultraflex type alignments, but don't recall him ever explaining why hiraga gave them credit.