Pear-shaped (sounds, not airplanes)

I've heard "pear-shaped sound" ever since the 1950s, initially by a music teacher when I was in on elementary school.  The dictionary says orotund, which means full and round. 

Whether round or shaped like a Seckel, how does a sound have a geometric shape?  On an oscilloscope, it's a line, not a closed loop.  How long has this terminology been around, and where did it come from?  Dictionaries don't seem to ever have etymologies except for the primary definition.