In this study, we experimentally estimated the sound speed of silica nanofoam as a function of the density through acousto-optic measurements. He–Ne laser light at the wavelength of 632.8 nm was emitted through the sample in the direction perpendicular to the propagation of the ultrasound. The diffraction of the light wave by the ultrasonic longitudinal waves with frequency in the range of 500–2,000 kHz agreed with the Raman-Nath theory. The sound speed estimated from the Raman-Nath diffraction angle varied in the range of 55–178 m/s for the sample density of 100–300 kg/m³. The measured sound speed almost agreed with the sound speed calculated from the density, bulk Young’s modulus and Poisson’s ratio. This implies that the low sound speed might be attributed to the reduced bulk elastic constant owing to the structure and might not be affected much by air confined in the porous structure.

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In this study, we attempted to evaluate air-conditioning sounds in a vehicle from the viewpoint of sound quality. First, we carried out an experiment to select suitable words for evaluating air-conditioning sounds in the compartment of a vehicle. The subjects of the experiment selected several words that were appropriate to represent images of the air-conditioning sounds from a list of adjectives. As a result of a cluster analysis of the experimental data, the seven evaluation words “quiet,” “refreshing,” “heavy,” “wide,” “muddy,” “violent,” and “dry” were selected as the evaluation words. Next, we carried out a subjective evaluation experiment by the rating scale method. The subjects of the experiment evaluated each presented sound in seven degrees for the seven evaluation words. As a result of a factor analysis, the air-conditioning sounds were found to be represented by three factors: “rough,” “space,” and “refreshing.” Finally, we carried out an experiment to evaluate the air-conditioning sounds using psychoacoustic parameters such as loudness and sharpness. As a result, we found that the rough factor correlated with loudness and space and refreshing factors correlated with sharpness.

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In this experimental study, we conducted measurements using a 128-element ultrasonic array system. Ultrasonic images generated by the conventional migration method were compared with the Envelope method, a promising high-resolution imaging technique. To determine the image quality of an ultrasonic measurement system that uses fewer elements, we selected 64, 32, 16 and 8 elements from the total 128 elements used in our imaging process. The results show that in homogeneous environments the Envelope method can produce clearer images than those produced by the conventional method even when using a small number of elements, suggesting that the introduction of the Envelope method could be an effective way of reducing the number of elements employed in array systems, while maintaining high image quality.

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