In the image, you see an oscillogram of a percussive tone.    From Wordnik.com.  [Recently Uploaded Slideshows] Reference

If you drew a line surrounding the upper half of the oscillogram, you could call it the envelope of the sound-a graphic displaying the level as a function of time.    From Wordnik.com.  [Recently Uploaded Slideshows] Reference

"And here's the oscillogram, sir, with the flattened curves.    From Wordnik.com.  [Plasma Monster]