How Loudspeakers Work 31 2.4 Transmission Line Loudspeakers A true transmission line or "acoustic labyrinth" loudspeaker sends the energy from the rear of the drive unit down an infinite pipe, where it is gradually absorbed and never heard of again. Since infinite pipes rarely fit in well with domestic surroundings, practical "transmission line" loudspeakers are a compromise. The rear radiation passes down a length of duct chosen so that it is in the correct phase to reinforce the forward radiation at low frequencies. To get worthwhile reinforcement the duct needs to be a quarter of a wavelength long at the frequency of interest, so it shifts the phase of the rear output by 90°. At 40 Hz the wavelength of sound is 8.58 metres, and a quarter of that is 2.145 metres, which is an impractically long duct unless it is folded at least twice, as shown in Figure 2.1d, where the bobbles represent absorbent material lining the duct. Transmission line loudspeakers are often described as non- resonant, but without the quarter-wave resonance (which could be suppressed by using enough absorbent stuffing in the duct) there will be no enhancement of the bass response. The low-frequency reinforcement works in the same way as a ported box, and the response falls at 24 dB/octave, once again making low-frequency response extension by equalisation in the crossover more difficult. Transmission line loudspeakers are not used in sound reinforcement because the duct greatly increases the volume and weight of the