Ralph,
As noted by others and as you though it is interference, one of the
problems being near a source of waves (within a few wave lengths) is that
source become much more complex then when measured at a distance, as simple
rotating the tuning fork will illustrated. In physics we are always making
assumptions like a tuning fork is a point source which is invalid when your
close it it.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU