As far as I know, the only way to measure sound level dB is with a meter and the meter will have instructions. AMA only has to tell you what the recommended limit would be. You have to read the manual for your particular meter.
Membership Manual- Page 36
http://www.modelaircraft.org/files/memanual.pdf
Leader Club Electives:1) Club has implemented and enforces a frequency-control plan
that allows all 50 RC channels to be used at its club flying site
where RC is allowed.
2) Club has implemented a school program or donates a
Aviation
3) Club conducts a review of the AMA Safety Code at a club
meeting at least once a year.
4) Club sponsors an annual public event, such as a mall show,
flying show, or demonstration.
5) Club monitors noise levels and/or institutes club field rules
such as a requirement that models be operated at a maximum of
90 db at 9 feet.Modelmagazine subscription to schools or libraries.