COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Douglas DC3 Series Aeroplanes AD/DC3/27 Crew Compartment Acoustic NK Treatment - Restoration Applicability: All models operating in Papua New Guinea. Requirement: Carry out suitable sound attenuating treatment of the crew compartment to achieve a sound pressure level not greater than that experienced in DC3 aeroplanes in their original condition. Note: Measurements of the sound pressure level in a number of DC3 aeroplanes in their original condition show that 90 db (re 0.0002 dyne/sq.cm.) is not exceeded in any of four octave bands 300-600, 600-1200, 1200-2400 and 2400-4800 cycles/second when in flight at normal cruise power settings, the measurements being at a point midway between the pilots' heads. Compliance: Forthwith.