Two procedures for adjusting as-measured test-day spectra to reference day conditions - the Society of Automotive Engineers' (SAE) Aerospace Recommended Practice (ARP) No. 866A (866A) and a procedure utilizing pure-tone absorption equations, developed in support of the International Organization for Standardization's (ISO) 9613-1 and the American National Standards Institute's (ANSI) S1.26-1995, and refined for application to one-third octave-band data- were evaluated and compared. The ISO and ANSI pure-tone absorption equations are identical to each other. The 866A procedure historically has been used for atmospheric absorption corrections for Federal Aviation Regulations (FAR) Part 36, Noise Standards: Aircraft Type and Airworthiness Certification and similar regulations of the International Civil Aviation Organization (ICAO). The ISO/ANSI procedure analyzed herein is under consideration for inclusion into harmonized regulations. This document presents a comparison of data corrected to several distances using the two different methodologies.