The continuous fading of the sound across successive 'windows' (openings of the jar) implies there is a single persisting sound, not a sequence of newly created ones.

To develop this line of thought and make it vivid, imagine a vacuum jar which has the property of immediately creating a vacuum upon closing the lid, and of immediately recalling air upon opening the lid. Take now a sounding object like a tuning fork at 440Hz and have it vibrate, supposing that the vibration fades and becomes inaudible after 10 seconds. What you hear is an A that becomes feebler and feebler until it disappears. Now, place the tuning fork inside the jar, have it vibrate as before