An ApproachAvoidanceGradient effect discovered by Peter Hall of LimpinWood. 
* Observe any cloud of midges at dusk. 
** Sing a pure tone to them and the cloud will expand. 
** Lower the tone and they contract. 
** Oscillate it and you can dance with them. 

Not to be confused with the HallEffect
*** http://cs.auckland.ac.nz/~peter-f/FFTPfiles%/Hall.pdf