Transmission Line (T-Line): Where to start measuring port length?

[qObsession]

I was just reading the linespeakers.org site! Thanks for taking the time to pass along this resource, planet.

 

[GM]

You're welcome! After a quick look-see at the other forum, some more tidbits:

Theoretically ideal driver, vent offsets are at odd harmonics, but the vent/horn terminus opening causes a downward shift, hence the distances are further down than a closed box's 1/5, 1/3rd, etc., but both the driver and vent should be at a vented pipe's odd harmonics. You can figure it out using Hornresp's

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'path' slider in 'chamber' or just use these as proven audibly 'close enough':

Z = [L]x ~ 0 [top], 0.217, 0.349, 0.424, 0.561

Of course the first one doesn't apply to any ~zero area [CSA] terminated pipe/horns and if your desired ear seated/whatever driver height doesn't line up close enough to your design, suggest you change the alignment tuning [pipe's length] by adjusting Fs to get it to line up with one of these offsets.

Vent offset only, depending on box HxWxD ratio: Z = [L]x ~0.651, 0.714, 0.848, 0 [bottom]

corner folds impact on axial length

Air Column Resonance

There's a small book's worth more, but not one to 'show off'. 😉

 

[sockbite]

Acoustically speaking, a pipe of given length resonates at a slightly lower frequency than is given by a bald axial length & taper ratio due to the exit boundary condition. For untapered pipes of square or circular cross section, a decent approximation is to take tuning frequency as being the pipe length, plus ~0.613 * internal radius added to that length. The ratio tends to vary as the aspect ratio of the pipe increases due to friction.

how does the correction change if using a tapered cross section

 

[David McBean]

Assuming that you are referring to the transmission line itself being tapered so that the cross-sectional area of the outlet becomes smaller, then the required end correction will be shorter.