Structure/air noise power ratio spectra of machine casings

A formula for approximating these ratio spectra has been developed as part of Rohr's surface‐effects ship‐noise prediction methodology. These spectra are used as multipliers of measured or predicted aero‐noise power spectra (in watts) of bare machine casings to predict the corresponding available machine structurenoise true power spectrum. The latter rarely is available. The formula developed for this ratio R is $R = (1.5/c)(ρs/ρ)t(ρ + 300)$ where c and ρ are, respectively, speed of sound in and density of the fluid medium surrounding the casing, ρ_s is casing material density, t is casing thickness and ρ is frequency in Hertz. Units consistent with the dimensionless nature of R must be used. Underlying idealized theory and results will be sketched. Also, rationale for moderately adjusting constants of theoretical results on empirical grounds will be sketched. Adjustment of the initial constant is based on results of a relevant Rohr test/analysis program. The second constant is based on a Rohr study of compartment noise prediction validity involving a tested ship. [Work supported by U.S. Navy, NAVSEA, PMS304.]