I had been posting on Facebook’s Transmission-Line forum, and I forgot to update this one. I will likely do a youtube video. The cabinet is finished. It seems to work pretty well.
Acoustic transmission lines were once designed for organs. The cabinet may be the first transmission-line guitar speaker cabinet, and it likely is the first bifurcated transmission-line cabinet. Also, it may be the first transmission-line with a T-line stuffed with reticulated foam.
[Anti-rat screens were added, as this is Silicon Valley, and they love the roofs and palm trees we provide them. The crows love our lawns.]
Subjectively, the cabinet seems to be more natural the a standard ported bass-reflex cabinet. Although my old ported cabinet was tuned to 50Hz, the new cabinet seems less reactive. Some of benefit may be that it is a full-ranged driver with no cross-over. The cabinet has more bass than an open combo amp usually has.
I had to repackage the Class-D amplifier and canned effect unit. (below) The amplifier puts out up to 35 watts max, through now a single channel. The combination goes loud enough that the police would likely come to the coffee shop if, I turned it up to 11, while playing outside. I do want to do some more high’ish volume testing.
This is a strange rig, as I am running the Class-D amp on only one channel. The other input is pulled down with a 1k resistor, currently, but I want to put a DPDT switch in so that the amp can be used for standard audio use. Compared to the old stereo/dual-mono setup 6-1/2" triaxial setup, the new speaker is louder, but a lot of that is from the Jensen Blackbird 40, with it’s fortified Alnico magnet, which I suspect is anisotropically optimized, which is pretty efficient. It’s Fz is high compared to a Hi-Fi speaker, as it is a full-ranged “guitar” speaker.
The completed speaker is a guitar speaker, and as so, the accurate reproduction of uncolored sound–is not a strong design requirement. LOL!
[As you may know, guitar speakers suffer from edge-modes caused by radially stiff uncompliant surrounds that are instead not strong enough in circumstance. They have stamped steel baskets, which not only resonate but also impede sound waves. They often have small-diameter voice coils, which make coupling the coil to the cone problematically weak. What do they do right? They have paper cones, and so, when they like all speakers do distort, they do so in a way pleasing to the ear. Oddly, the curve on this driver is pretty smooth, well, for what it is.]
Because there is no free-lunch. The coupling between the driver and the TL in my design could and should be better. The cabinet is heavier than a combo cabinet. Using tubing sections instead of stacked plywood radius–made this cabinet very time-consuming to build. In other words, if I had CNC’ed out the radiused corners, it would have been much easier to assemble. Also, my quicky-poo box joints were not the best. The grill cloth was too old and creased to use. The grill fasteners could be a bit tighter. The plywood I got from Home Depot was terrible with lots of voids. Some core wood was no stronger than walnut shells.
Getting into the nity-gritty… The transmission line, like most, is a modified transmission line made with walls that are slightly taped to avoid standing waves.
Guitar speakers are an anachronism, in that they were designed in the 1940’s and 1950s to be used in pre acoustic-suspension sound system. The cabinet of the day was a simple baffle-plate, whereas the back wave will interfere with the front wave causing constructive/reconstructive interference, from which is a non-ideal, non-Infinite tends to null out the bass, once again, because the back wave meets the front.
But the problem is: that nice flexible full-ranged cone is generally not the best to put in a sealed (acoustic-suspension) cabinet. AFAIK, bass reflex as well as ported cabinets are not great with impulse response. In both, the bass is reinforced from a standing wave.
The best solution would probably be: an infinite baffle with the back of the driver facing your neighbor’s yard. After a wavelength or so of the lowest reproduced note, you don’t have to worry about the front wave mixing with the back.
So, I wanted to try a transmission line cabinet because, if made decently, it will attenuate most of the back wave.
I am not an expert on transmission lines, yet I think that one of the biggest problem with them is: the stuffing is generally not an exact science, and is easy to cause variations, which makes studies on them difficult. That’s why I tried the reticulated form: it’s homogeneous. Though the walls of the cabinet and transmission line are (mostly) lined with rubber-backed, indoor-outdoor carpet.
I also have a cart handle on the cabinet, now. It was a wheel/handle thing that someone tossed out. I have to get a better pic in sunlight, with the handle.
