transmission line box (TML)
Transmission line (TML) is a design principle of loudspeaker boxes, respectively bass reflex boxes. The principle of the transmission line is based on the fact that the woofer is supplied via an air duct, the transmission line. The air duct prevents reflections on the inside of the cabinet, which counteract the movement of the diaphragm. With the transmission line principle, the airflow at the rear of the diaphragm is in phase with the movement of the loudspeaker.
The length of the transmission line is crucial for the lower cutoff frequency because it prevents acoustic feedback in the loudspeaker cabinet. This means that at a phase shift of 180° the transmission line is at least as long as half the wavelength, which corresponds to 1.7 m at frequencies of 100 Hz. However, the cross-section of the air duct must be at least equal to that of the loudspeaker diaphragm. In addition, it is equipped with highly absorbent materials for better absorption of sound.
The advantage of the transmission line principle is that with small box sizes, low tones are strongly emitted. That is why this construction principle is used for woofers and subwoofers.