The imaginary part of the acoustic propagation constant. The commonly used unit is the radian per second or unit distance.
"It's a miracle. When I took a look at my very moldy old 45 rpm record that my college singing group recorded almost a half-century ago, I thought I'd never be able to hear our golden voices again (including the fact that I no longer have a record player). But I sent you the record figuring that the worst you would do is send it back to me with ..."