Apples placed on a vibrating table were subjected to sine-wave vibrations at specific frequencies, and the response was observed by electronic speckle pattern interferometry (ESPI). The speckle pattern was analyzed to determine the vibration modes. The lowest resonant frequency was a longitudinal vibration and the second lowest a 0{sub left}S{sub}2 mode. Third and fourth lowest resonant frequencies were estimated as a 0{sub left}S{sub}3 and a 0{sub left}S{sub}4 mode, respectively. The fact that the response to the second lowest resonant frequency is the 0{sub left}S{sub}2 mode confirms that the elastic modulus of an apple can be calculated by the equation proposed by Yamamoto and Haginuma (1984). The influence of the surface contact between the vibrating table and the apple was examined. When the apple was bonded to the table with glue, the lowest resonant frequency greatly increased by 80, but the second, third and fourth were little influenced. The second lowest resonant frequency of an apple can be accurately determined by placing the apple directly on the table without bonding and it affords a practical measurement of fruit firmness.
展开▼
