Dr. David Joseph Zartman
Zartman Inc., L.L.C.,
Loveland, Colorado

Popular version of 3pSP4 – Imaging watermelons
Presented Wednesday afternoon, May 25, 2022
182nd ASA Meeting, Denver
Click here to read the abstract

When imaging watermelons, everything can be simplified down to measuring a variable called ripeness, which is a measure of the internal medium of the watermelon, rather than looking for internal reflections from any contents such as seeds. The optimal experimental approach acoustically is thus a through measurement, exciting the wave on one side and measuring the result on the other.

Before investigating the acoustic properties, it is useful to examine watermelons’ ripening properties from a material perspective.  As the fruit develops, it starts off very hard and fibrous with a thick skin. Striking an object like this would be similar to hitting a rock, or possibly a stick given the fibrous nature of the internal contents of the watermelon.

As the watermelon ripens, this solid fiber starts to contain more and more liquid, which also sweetens over time. This process continues and transforms the fruit from something too fibrous and bitter to something juicy and sweet. Most people have their own preference for exactly how crunchy versus sweet they personally prefer. The skin also thins throughout this process. As the fibers continue to be broken down beyond optimal ripeness, the fruit becomes mostly fluid, possibly overly sweet, and with a very thin skin.  Striking the fruit at this stage would be similar to hitting some sort of water balloon. While the sweet juice sounds like a positive, the overall texture at the stage is usually not considered desirable.

In review, as watermelons ripen, they transform from something extremely solid to something more resembling a liquid filled water balloon. These are the under-ripe and over-ripe conditions; thus, the personal ideal exists somewhere between the two. Some choose to focus on the crunchy earlier stage at the cost of some of the sweetness, possibly also preferable to anyone struggling with blood sugar issues, in contrast to those preferring to maximize the sweet juicy nature of the later stages at the cost of crunchy texture.

The common form of acoustic measurement in this situation is to simply strike the surface of the watermelon with a finger knuckle and listen to the sound. More accuracy is possible by feeling with fingertips on the opposite side of the watermelon when it is struck. Both young and old fruit do not have much response, one being too hard, getting an immediate sharp response and being more painful on the impacting finger. The other is more liquid and thus is more difficult to sonically excite. A young watermelon may make a sound described as a hard ‘tink’, while an old one could be described more as a soft ‘phlub’. In between, it is possible to feel the fibers in the liquid vibrating for a period of time, creating a sound more like a ‘toong’. A shorter resonance, ‘tong’, indicates younger fruit, while more difficulty getting a sound through, ‘tung’, indicates older.

An optimal watermelon can thus be chosen by feeling or hearing the resonant properties of the fruit when it is struck and choosing to preference.

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