Wikipedia
Transmission loss (TL) in duct acoustics, together with insertion loss (IL), describes the acoustic performances of a muffler like system. It is frequently used in the industry areas such as muffler manufacturers and NVH department of automobile manufacturers. Generally the higher transmission loss of a system it has, the better it will perform in terms of noise cancellation.
Transmission loss (TL) in general describes the accumulated decrease in intensity of a waveform energy as a wave propagates outwards from a source, or as it propagates through a certain area or through a certain type of structure.
It is a terminology frequently used in optics and acoustics. Measures of TL are very important in the industry of acoustic devices such as mufflers and sonars.