  This paper introduces an acoustic probing technique to estimate the storage
time and firmness of fruits; we emit an acoustic signal to fruit from a small
speaker and capture the reflected signal with a tiny microphone. We collect
reflected signals for fruits with various storage times and firmness
conditions, using them to train regressors for estimation. To evaluate the
feasibility of our acoustic probing, we performed experiments; we prepared 162
tomatoes and 153 mandarin oranges, collected their reflected signals using our
developed device and measured their firmness with a fruit firmness tester, for
a period of 35 days for tomatoes and 60 days for mandarin oranges. We performed
cross validation by using this data set. The average estimation errors of
storage time and firmness for tomatoes were 0.89 days and 9.47 g/mm2. Those for
mandarin oranges were 1.67 days and 15.67 g/mm2. The estimation of storage time
was sufficiently accurate for casual users to select fruits in their favorite
condition at home. In the experiments, we tested four different acoustic probes
and found that sweep signals provide highly accurate estimation results.
