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A silicon carbide disk was sintered from 2090" to 2190°C in 25°C steps. After each sintering step, the disk was examined using a precision acoustic scanning system to determine acoustic attenuation and velocity. The bulk density was found to vary nonmonotonically with sintering temperature. The density varied as much as 10% from its value at 2090°C during the sintering process. Local density fluctuations occurred in an organized and history-dependent way. These local density fluctuations varied up to ?7% of the bulk density and were made visible by acoustic attenuation and velocity mapping. [Key words: silicon carbide, sintering, density, porosity, ultrasonics imaging.] ERAMIC processing is being investiCgated extensively by an international effort. A major goal of current processing research is to control the amount, size, uniformity, and distribution of porosity in ceramics. Considerable effort is being made to control porosity variations by modifying the ceramic powder processing that is done to form green or unsintered ceramics. Milling techniques, powder-size distributions, binders, pressing pressures, etc., all have an effect on the final poresize distribution of a sintered ceramic. There are relationships between the type of powder processing and the final microstructure of the sintered ceramic. '** We will show that there exist large local density fluctuations that occur within the bulk ceramic during sintering. These local density variations are historydependent and can be tracked or followed during the sintering process.

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The forward propeller of a model counterrotation propeller was tested with its original aft propeller and with a reduced diameter aft propeller. Noise reductions with the reduced diameter aft propeller were measured at simulated cruise conditions. Reductions of up to 7.5 dB were measured for the aft propeller blade passing tone, and reductions in the harmonics were also measured. Reductions in the interaction tones were observed as the result of the reduced diameter aft blades no longer interacting with the forward propeller tip vortex. Significant reductions in the total noise at each harmonic were observed. The chief noise reduction at each harmonic came from reduced aft propeller-alone noise, with the interaction tones contributing little to the totals at cruise. Total cruise noise reductions, as much as 3 dB at given angles for the blade passing tone, and as high as 10 dB at some of the harmonies were observed. These reductions would improve the fuselage interior noise levels and represent a cruise noise benefit for using reduced diameter aft propeller blades.

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