Hi all,
I was helping a friend set up some new transmission line speakers that he built. They are a simple 1/4 wave straight line with no taper. With no stuffing, the mids sounded predictably funky and the bass was quite heavy. We ran an impedance sweep and there was a prominent spike around 180 Hz, which correlated nicely to the height of the folded line. There were other spikes also, at higher frequencies. So far, this was about what I expected. Then we added stuffing to the first half of the line starting at the woofer to the first fold. It sounded much better, as predicted. Now there is an impedance peak around 280 Hz and the higher spikes were no longer visible.
The lowest spike of 180 made sense to me, but why did it appear to move up in frequency when we added stuffing? The new peaks wavelength is about 12", which may relate to the distance from the beginning of the line to the beginning of the stuffing. Could the pressure wave be reflecting off of the stuffing? I don't know how else to explain it.
Thanks for any insight...
I was helping a friend set up some new transmission line speakers that he built. They are a simple 1/4 wave straight line with no taper. With no stuffing, the mids sounded predictably funky and the bass was quite heavy. We ran an impedance sweep and there was a prominent spike around 180 Hz, which correlated nicely to the height of the folded line. There were other spikes also, at higher frequencies. So far, this was about what I expected. Then we added stuffing to the first half of the line starting at the woofer to the first fold. It sounded much better, as predicted. Now there is an impedance peak around 280 Hz and the higher spikes were no longer visible.
The lowest spike of 180 made sense to me, but why did it appear to move up in frequency when we added stuffing? The new peaks wavelength is about 12", which may relate to the distance from the beginning of the line to the beginning of the stuffing. Could the pressure wave be reflecting off of the stuffing? I don't know how else to explain it.
Thanks for any insight...
Move the stuffing around and re-measure. I doubt the stuffing is causing a reflection. Maybe it was always there and you damped the larger resonance making the 280hz one more obvious?
For T-lines I usually use wool felt around the woofer area to absorb early midrange reflections. Then I use wool or fiberglass in various places down the line and in various densities. Pillow stuffing and foam do nothing. I’ve found that treating the first 1/3 - 1/2 of the line makes the biggest improvement in sound. Also, you can make a simple taper by putting blocks of wood or a sheet of MDF in the second bend of the line. Experiment.
For T-lines I usually use wool felt around the woofer area to absorb early midrange reflections. Then I use wool or fiberglass in various places down the line and in various densities. Pillow stuffing and foam do nothing. I’ve found that treating the first 1/3 - 1/2 of the line makes the biggest improvement in sound. Also, you can make a simple taper by putting blocks of wood or a sheet of MDF in the second bend of the line. Experiment.
It seemed pretty clear that the 180 Hz bump moved to 280, it was about the same amplitude and shape. There were four speakers total and they all behaved similarly, a few Hz one way or the other. I figure that is from variations in the stuffing process. I do not like to use pillow stuffing, but that is what he had and it seemed to do the job. I prefer natural materials such as wool or cotton myself...
I would certainly do some experiments. but they are not my speakers and my friend is happy with the results and does not really want to take them apart.
I would certainly do some experiments. but they are not my speakers and my friend is happy with the results and does not really want to take them apart.