Transmission Line Loudspeakers: The Ultimate Domestic Hi-Fi Loudspeakers?

Given its ability to produce unbelievable amounts of bass for its size, are transmission line loudspeakers qualify as the ultimate domestic hi-fi loudspeakers?

By: Ringo Bones

As of late, there has been discussion on how come hi-fi audio designer Arthur Radford’s electronics – especially his EL34 based audio power amplifiers like the Radford STA25 - get more recognition in comparison to his transmission line loudspeakers which despite being manufactured 50 or so years ago can still manage to hold its own in a side-by-side comparison to more recently manufactured hi-fi loudspeakers. Arthur Radford did a lot of work, both theoretical and practical, on transmission line loudspeaker design during the 1960s.The Radford Studio 90 which was also sold in the U.S. in kit form during the late 1960s as the Audionics TL90 – a ¼ wavelength 3-way transmission line loudspeaker equipped with Radford sourced drivers and crossover circuit. The Radford Studio 90 was the first commercially manufactured hi-fi transmission line loudspeaker based on A.R. Bailey’s papers and there are even larger models based on the same design. Even though they were a bit pricey when they first entered the market, enough of them were sold in the United States to establish a “cult following” of this legendary loudspeakers. Despite their excellent subjective sound quality, why are transmission line loudspeakers relatively rare in comparison to competing designs? But first, here’s an introduction on the theoretical and practical principles of transmission line loudspeakers. 

Then and now, designers of closed-box / acoustic suspension and vented / bass reflex loudspeaker systems have long been able to make heavy use of computers, while transmission line systems had been designed by cut-and-try methods. Transmission line loudspeaker systems are more complicated to design because of their inherent distributed-parameter nature. By way of comparison, sealed / acoustic suspension and vented / bass reflex loudspeaker systems can be treated as a much simpler, lumped-parameter acoustic systems. Also, there is still a dearth of research literature regarding transmission line loudspeaker design to guide the prospective designer. But to those adventurous enough, they should check out Quick & Easy Transmission Line Speaker Design by Larry D. Sharp and those “white papers” published online by John Wright of TDL. 

Transmission line loudspeakers are known for their deep, powerful bass and the way they grip a room, filling it with a full scale performance. But they have their downsides too, notably the difficulty of tuning the line, which appears to be the blackest of arts in the audio engineering world. A practical transmission line has two main effects that are advantageous to loudspeaker design. Firstly, the sound venting from the end of a quarter-wavelength transmission line is 90-degrees out of phase with the signal from the back of the cone which drives the line, which itself is 180-degrees out of phase with the front radiation. This ensures that sound radiation at lower frequencies is progressively moving into phase and it will add, rather than subtract, to the forward low frequency response. Also a practical transmission line loudspeaker is lined with acoustic felt and wool, so that a majority of upper bass and midrange energy is absorbed before it reaches the end of the line and the outside world. Lower frequency bass energy gets through, making low bass apparent. In electrical engineering terms, a transmission line port is a low pass filter with a 270-degrees plus phase shift. 

An area where transmission line loudspeakers receive criticism is midrange coloration. To overcome this, various transmission line loudspeaker designers over the years adapted a number of sensible design features. If the transmission line vent length is made exactly ¼-wavelength in length, it tends to act like a tuned pipe which introduces coloration. Making the transmission line slightly shorter, a technique later adapted by John Wright of TDL during the late 1960s, reduces this coloration without adversely affecting bass quality.

In some transmission line loudspeaker designs where the line is folded quickly behind the cone, the strongly reflected sound wave from the cabinet wall can give that “clatter-type” coloration in the midrange. For this reason, in newer transmission line loudspeaker designs, the rear wall of the cabinet is kept as far back as possible and fitted with a generous covering of soft, absorbent carpet felt. 

One great thing about a transmission line loudspeaker is that once the cabinet dimensions have been optimized and fixed, it can be subtly tuned to give the bass quality you want in your listening room just by altering the amount and density of long hair wool in the line. This makes transmission line loudspeakers extremely versatile for home constructors, which combined with their subjective performance explains why they’ve been making a comeback since the 1990s hi-fi boom. 

Another great advantage, or possibly the greatest advantage, of transmission line loudspeakers – and probably why they’ve returned during the 1990s in conjunction with the zero negative feedback single-ended-triode audio amplification Renaissance of the period - is their highly damped impedance curve. Impedance varies little across the entire audio spectrum – especially in the upper bass and lower midrange region in comparison with vented / bass reflex and closed box / acoustic suspension loudspeakers. This makes transmission line loudspeakers much, much easier to drive with zero negative feedback single-ended triode audio amplifiers and other low feedback vacuum tube audio amplifier designs.