Imagine a tuba stuffed into a rectangular box. The tuba’s mouthpiece is mounted to the rear side of the woofer. That is the concept of the Transmission Line Cabinet. In a transmission line, the sound wave from the back of the woofer is channeled down a long pathway filled with a fibrous bundle of wool or another synthetic material. Transmission line cabinets can be designed to have a smoother and more extended low frequency output than a corresponding vented enclosure given the same internal volume.
In EA’s Transmission Line Cabinets very low frequencies exit the end of the transmission line. This can extend the low frequency response one-half octave below the fundamental resonance of the driver.
In EA’s Transmission Line Cabinets, the back wave of the woofer does not bounce off an interior wall and radiate back into the room through the thin cone of the woofer as in most sealed or ported enclosures. These multiple echoes color the sound. Without these multiple echoes the EA Transmission Line Cabinets produce an extremely clean and accurate sound.
There is less pressure in a transmission line to excite strong enclosure resonance as in a sealed or ported box. In sealed or ported cabinets, enclosure resonances can usually only be controlled, not eliminated as in EA’s Transmission Line Cabinets.
The additional output provided by the transmission line is accomplished by modifying the cabinet impedance (load seen by the amplifier). The impedance of a cabinet varies with frequency. The nominal impedance is generally 4 or 8 ohms. This is the impedance of the cabinet at a fixed frequency (250Hz is a sensible frequency to use since it will give the same result whether the cabinet is a closed box, vented or a transmission line). The impedance of the vented 2×10 cabinet at 41Hz (open E string) is 11 ohms and the impedance of the transmission line 2×10 cabinet at 41Hz is 9 ohms. The cabinet volume and drivers are identical but the amplifier is now supplying 22% more power at 41Hz to the transmission line cabinet.
Finally, the Transmission line cabinets can be tuned over a broad spectrum where the vented enclosure is tuned over a narrow spectrum. Using the same two cabinets in the previous example, the impedance of the vented 2×10 at 31Hz (open B string) is 20 ohms and the impedance of the transmission line enclosure is 9 ohms. At 31Hz the amplifier is supplying 122% more power to the transmission line enclosure. This would account for the additional 4dB additional output that the transmission line cabinet has over the vented cabinet.
Together, the extended low frequency response, the lack of multiple echoes from the inside of the box, the elimination of wall resonance, cabinet impedance and cabinet tuning account for the extremely clean, well controlled and powerful sound in all of EA’s Transmission Line Cabinets.
Currently EA is the only manufacturer of Transmission Line Cabinets for Musical Instrument applications. The widespread use of transmission lines in musical instrument cabinets is not evident due to the added design and engineering complexity. The manufacturing of a transmission line enclosure entails the use of jigs or the more preferred method of using a Computer Numerical Control (CNC) router to machine grooves in the panels to precisely locate the various internal baffles. Currently there is not an off the shelf program or set of tables for designing transmission line enclosures due to the complex factors of a transmission line cabinet.