What is the significance of the folding transmission line in a loudspeaker?
My guess is that not many of you have heard of a folding transmission line in a loudspeaker, much less ever heard one working. The late Bud Fried, from Fried Audio (pronounced “freed” as opposed to what one might do with eggs), was a big proponent of the design style to get bass out of a cabinet without dealing with a sealed box.
Transmission lines can be very effective to extend bass response, control the woofer’s movement and lower distortion in a speaker over a broad range giving generally better overall results than those of a simple port.
If you’re going to punch a hole in a woofer box to get lower bass, this might be the best way to do it. Unfortunately it is tough to implement because the idea behind it is to make a long tube or tunnel for the rear of the woofer’s energy to travel – long enough that by the time it gets outside it is twice out of phase – meaning it is 360 degrees out of phase as opposed to 180 degrees.
At the long wavelengths found in low bass the sound being out of time by one cycle, relative to out of phase by half a cycle, is fairly meaningless to the ear and thus you really can’t hear what’s happening and you ignore this difference.
Creating a tunnel long enough to delay the output of the woofer long enough to be in phase takes some cabinetry work and what’s usually done to make this work is called a folded path, where you make a sort of maze inside the speaker that goes back and forth until you get the correct length for the sound. Picture it like a line at Disneyland, where you have a folded line, people moving back and forth in a small space.
Transmission line loudspeakers typically have a broad mouth or opening just below the main woofer where the back pressure of the main woofer exits and supports the front of the woofer. It’s a very clever design but for cost reasons it isn’t done very often.
Paul McGowan (PS Audio)
I can point to two people who disagree.
1. John Watkinson:
“In the transmission line loudspeaker, the back wave from the woofer is delayed by guiding it through a folded pipe that causes a delay. At some frequency, the delay will be equal to half a cycle and the delayed back wave emerging from the pipe will be in phase with the radiation from the front. It is only in the case of a sine wave that a delay is indistinguishable from an inversion and we know a sine wave carries no information. In the case of a transient, the transmission line speaker destroys the waveform. The baby is thrown out and the bathwater is retained.”
http://www.theregister.co.uk/2014/07/02/feature_the_future_loudspeaker_design/?page=4
and 2. Alan Shaw of Harbeth
http://www.harbeth.co.uk/usergroup/showthread.php?2370-All-sound-to-all-speaker-to-all-men-Is-there-a-universal-speaker
Quoting Paul McGowan:
“At the long wavelengths found in low bass the sound being out of time by one cycle, relative to out of phase by half a cycle, is fairly meaningless to the ear and thus you really can’t hear what’s happening and you ignore this difference.”
I am always fascinated by this sort of thing in hi fi. I think that in a complete standard-issue audiophile system there are quite a lot of these arbitrary dismissals of quite major anomalies.
“It’s a very clever design but for cost reasons it isn’t done very often.”
Another aspect that bemuses me. If we really think that this is the way to make the best speaker, then knocking together a few pieces of chipboard to make a folded transmission line doesn’t cost a lot of money. The impression that statements like this make, is that despite hi fi equipment costing multiple thousands of pounds, the manufacturers are *still* penny-pinching. Really? Are we sure, in fact, that a lot of high end manufacturers aren’t taking a pre-penny-pinched concept (such as a small two way bass reflex speaker) and then gilding the lily to produce a fundamentally penny-pinched speaker that costs £10,000? By starting with a clean piece of paper they might have come up with something that works so much better for the same money.