Interesting study. From the
FAA's Noise Levels for U.S. Certificated and Foreign Aircraft, U.S. Certificated Propeller Driven Small Airplanes (14 CFR Part 36, Appendix F).
The Cessna 337H with the TSIO-360-C (the only model tested) is 78.6 dBA.
Did the airport you mention have posted noise restrictions?