This is way out there, but lots of us have done this experiment in physics class, and no one has made the connection before Steven Waller, a doctoral researcher at Rock Art Acoustics USA:
If two pipers were to play in a field, observers walking around the musicians would hear a strange effect … At certain points, the sound waves produced by each player would cancel each other out, creating spots where the sound is dampened.
The theory is highly speculative, but modern-day experiments do reveal that the layout of the Stonehenge ruins and other rock circles mimics the piper illusion, with stones instead of competing sound waves blocking out sounds made in the center of the circle.
In support of the theory, Waller pointed to myths linking Stonehenge with music, such as the traditional nickname for stone circles in Great Britain: "piper stones." …
Waller experimented by having blindfolded participants walk into a field as two pipers played. He asked the volunteers to tell him whenever they thought a barrier existed between them and the sound. There were no barriers in the field, but acoustic "dead spots" created by sound-wave interference certainly gave the volunteers the impression that there were.
"They drew structures, archways and openings that are very similar to Stonehenge," Waller said.