Narrative:

During legal aerobatic practice and as a result of noise from a fixed pitch propeller at 3200 RPM (high mach), noise carries a great distance and is sometimes presumed to be at a lower altitude by as much as a factor of 7.

Google
 

Original NASA ASRS Text

Title: PTS1 PLT JUSTIFIES NOISE CAUSED BY AEROBATIC MANEUVERS IN THE VICINITY OF 2WA4 ARPT.

Narrative: DURING LEGAL AEROBATIC PRACTICE AND AS A RESULT OF NOISE FROM A FIXED PITCH PROP AT 3200 RPM (HIGH MACH), NOISE CARRIES A GREAT DISTANCE AND IS SOMETIMES PRESUMED TO BE AT A LOWER ALT BY AS MUCH AS A FACTOR OF 7.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.