The effective sound pressure taking into account both positive and negative pressures in a system. The squares of the compression and the refraction of a wave are added and the average taken.
"Thank you so much! I have received my new CD transferred from a very old 45rpm record. It is the accompaniment to a selection of trumpet solos that I loved playing 55 years ago! Simply listening to the CD by itself brought tears to my eyes, as I found I was also “hearing" my solos as well from memory. I’m practicing and will soon be back ..."