Airframe noise generated by commercial aircraft at landing approach is a barrier to the achievement of desired noise reduction goals for the next generation of aircraft.

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Experimental noise-reduction data have been obtained on a 43 % scale model of a typical narrow-body aircraft. The acoustic performance of six sidewall "add-on" noise-reduction treatment designs has been evaluated under random and harmonic acoustic excitations and compared with predictions. A previously derived mathematical model for sound transmission into a stiffened cylindrical shell with mult Havered treatments is used for the predictions. The theory compares favorably with experimental data at high frequencies and high-surfacedensity sidewall conditions, but underestimates low-frequency noise reduction. It is shown that add-on noise control treatment designs are effective in reducing the interior noise of propfan-powered aircraft to acceptable levels.