Dear community,

Considering the acoustic intensity vector output in Abaqus Analysis User's Guide>>Analysis Procedures, Solution, and Control>>Analysis Procedures>>Acoustic and Shock Analysis>>Output. I would like to know what \sigma represents, and would also appreciate it if you could direct me to where it's stated in the manual.

From my knowledge, \sigma always represents some form of ratio like radiation efficiency, but I'm not so sure in this case. I have attached a picture for reference.

Thanks in advance