The instantaneous spreading apart of air particles during the negative going half cycle of a sound wave. The opposite of condensation.
"I was completely blown away by the quality of the work you performed on these very old (70+ years), home-made 78rpm records. I had not tried to even play the records for decades because the last time I tried it was impossible to hear much of anything but the scratches. I realize that a couple of them were not able to be fully restored - that ..."