Nondestructive Measurement for Maturity of Muskmelons by Analysis of Acoustic Signals

Abstract

We had showed that the impulse waveform of acoustic signals induced by impact to a muskmelon was transmitted along the surface of equator with uniform velocity. This velocity varies according to melon maturity. This paper deals with the development of the system for determining transmission velocity. The system consists of 2 microphones, an A/D converter with the simultaneous sample-and-hold circuit and a computer. Calculation of the lag time from the two microphone signals using cross-correlation techniques was very repeatable. Transmission velocity was determined by dividing the distance between the microphones by the lag time. Experimental results were as follows: (1) The microphones should be located on the equator 45°-90°from impac point. (2) Optimum microphone-to-sample distance was found to be 1-4mm. (3) For an A/D conversion rate of 100kHz, a minimum of 512 data points were needed to calculate the accurate lag time by cross-correlation techniques. (4) Edible muskmelons varied in hardness from 250 to 700gf (3mm plunger method). (5) Correlation coefficient between transmission velocities and fruit hardness was found to be 0.832. The transmission velocity of edible muskmelons was ranged from 37 to 50m/s.