Do transmission lines need stuffing?

guy48065

Member
2018-09-27 5:20 pm
The other day I was browsing on Youtube and ended up watching several construction videos on transmission line speakers & subs. I noticed that most builders there didn't use any damping materials in the box at all.
Is that wrong--or another "it depends" answer?

I also noticed a rare few tapered the line from the driver to the port and I couldn't think of a rationale for doing so. One was even CNC routed so there was no box volume other than the volume of the line. Solid MDF with a "carved" channel from driver to port. Is that an ideal load for the driver?

Some of these projects apparently were "quick & dirty" boomers for car audio.
Set me straight on what's correct for audiophile results.
 
The other day I was browsing on Youtube and ended up watching several construction videos on transmission line speakers & subs. I noticed that most builders there didn't use any damping materials in the box at all.
Is that wrong--or another "it depends" answer?

It depends. ;) Application specific, although damping is generally required to kill harmonic modes & reflections; potentially less of an issue for subwoofers given the limited BW involved for this application, but not something to ignore. A high quality design uses the smallest amount of material to achieve a given set of results, but a certain amount is almost always needed.

I also noticed a rare few tapered the line from the driver to the port and I couldn't think of a rationale for doing so.

Terminus rather than 'port' per se. The tuning frequency of a QW line is a function of axial length and taper. An expanding line (horn) has an Fp higher than a straight pipe of the same axial length; a contracting pipe has an Fp lower than a straight pipe of the same axial length. The extent depends on the extent of expansion or taper. Quarter Wavelength Loudspeaker Design Resonances of open air columns

One was even CNC routed so there was no box volume other than the volume of the line.

Yes, a straightforward QW pipe, rather than a chambered. They are the most common sort.

Is that an ideal load for the driver?

Depends on how you define an 'ideal load' I'm afraid. QW / TLs are no different to any other load: there is no magic bullet, no universal 'this is the perfect design', just an infinite variety of compromises you select from in order to acheive a given set of results.
 
Ye olde approach: Build the TL straight or tapered, length=1/4W at desired cutoff frequency, stuff it almost completely (probably because of the horrible results of just leaving it un-stuffed). Pretend that you've created something extraordinary.

Car audio approach: Build the TL straight, un-stuffed, length=1/4W Fb, basically ending up with the equivalent of a "bass whistle", and likely one-note low bass enhancement, which many in the car audio field think is "good". YMMV.

Modern approach: Build the TL with a taper, the amount of taper depending on the bandwidth that you're aiming for (higher taper = greater usable bandwidth), and the minimum particle velocity at the terminus that you're prepared to accept. Include the impact of stuffing (minimal for subwoofer duty, more for full range duty) in the sim so you're not disappointed by the results when you build it.
 
It depends. ;) Application specific, although damping is generally required to kill harmonic modes & reflections;

Historically (volume fill) damping was absolutely required to kill the undesirable upper harmonics of the quarter-wave line. Modern tools have turned up new ways to kill some of the unwanted harmonics (ie Zd and mass loading) as well as moving the unwanted harmonics up & down in frequency. The TL still needs damping but it can often be just a lining of the box.

dave
 

guy48065

Member
2018-09-27 5:20 pm
The set in my avatar was custom made by Unity Audio. The TL sub has a 12” driver at each end of the line and had no stuffing. I added some poly fill behind each driver (what I could reach) but couldn't really tell a difference.
I have no idea how many baffles are in there but I'm pretty sure they're parallel.
 

TBTL

Member
2013-10-08 12:26 pm
Assuming it is desired to make the box behave as a resonator which only resonates at the fundamental:

Stuffing can be placed such that it affects harmonics to a stronger degree than the (desired) fundamental. Note that stuffing is most effective at the velocity maximum of the mode, while it is least effective at the velocity minimum. If you search for 1/4 wavelength resonators, you will see that the fundamental and harmonics have different patterns of velocity maxima and minima across the line. Stuffing material which is positioned near the closed end works well, with respect to this aspect.

Positioning the driver some distance away from the closed end is another technique to suppress harmonics. Some of the harmonics are not energized with specific positions. Software that simulates transmission lines can help find these positions.
 
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