The firmness of a tomato can be estimated nondestructively from the measurement of its resonant frequency . The purpose of this research was to investigate the reliability of this technique in relation to the global shape and the internal structure of a tomato . The dynamic behaviour of spherical and non-spherical tomatoes was studied first by experimental modal analysis . Mode shapes , excitation techniques and measurement points for obtaining high impulse responses are compared . The ef fect of the internal compartmental structure of a tomato on the responses at dif ferent excitation points was studied . From the measured frequency response functions , the second resonant frequency , with an ''oblate-prolate'' synchronized motion of the fruit , was selected for further use . The corresponding mode shape incorporates the largest deformations for the small impact excitation used . The best repetitive results with greatest discrimination from other modes are obtained by impacting and measuring the response at the equator , at points 180 Π from each other , for both spherical and non-spherical tomatoes . Impacting and or measuring the tomato on fleshy or juicy parts of the tomato resulted in a small change (less than 0 ΠΈ 5%) in the resonant frequency . During ripening , on the other hand , the resonant frequencies of all mode shapes decreased gradually . A testing procedure based on measurements of the resonant frequencies , namely the acoustic impulse response technique , is proposed .