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.
"I received your CD transfer of the LP recording of my piano teacher a few days ago. I must tell you I am very pleased with the result as the LP was very old, had pops and scratches but your CD transfer captured all the nuances and artistry of my teacher. As I can afford it, I will be sending you more of my LPs to your excellent company. Thank ..."