What is the principle behind telling whether a watermelon is ripe by its sound?

The unbearably hot season has come around again, and refreshing, delicious watermelon has become the “standard fixture” of summer.Can you really judge a watermelon's ripeness by tapping it and listening to the sound?This seemingly simple method, what principle does it actually involve?

Researchers have studied “telling melons by sound,” and the research shows that tapping a watermelon by hand and judging its quality by the sound has a scientific basis. The testing principle of this method is that the propagation of sound waves in a medium is affected by the medium's density and texture; therefore, the relationship between the sound waves obtained by tapping a watermelon and the watermelon's quality can be worked out. In this way, without cutting the watermelon open, it is possible to analyze the sound waves obtained by tapping it and test its ripeness and internal quality.

Specifically, the structure of a watermelon is divided into the rind and the flesh. During the ripening of a watermelon, the rind's hardness and elastic modulus change, that is, the rind of a melon with ripe flesh is more elastic and gives a slight bounce when tapped. In addition, under the action of cellulase, the cellulose inside the flesh is degraded, and the watermelon's sugar content and its own structure also change, so the tapping sound will naturally differ. Treating a watermelon as an elastic multi-layered spherical structure, when people tap a watermelon and it vibrates and produces sound, analyzing the resulting acoustic signal gives the following results: when an unripe fruit is tapped, its acoustic wave amplitude reaches its maximum and then decays sharply, showing an irregular decay waveform. For properly ripe and overripe fruits, the maximum amplitude appears at a certain moment after tapping, and the waveform is symmetrical above and below, showing regular decay. The concrete manifestation of acoustic waves is vibration and echo, so when tapped, a crisper sound means an unripe melon, and a relatively dull tapping sound means a ripe melon.

Simply put, unripe watermelon flesh has higher density, so it sounds crisp when tapped, while ripe watermelon flesh has relatively lower density, so it sounds duller when tapped, and the vibration frequency produced after a watermelon is tapped falls within the range the human ear can perceive and distinguish. Therefore, when buying a watermelon, gently tapping different watermelons lets you hear different sounds. When tapping a watermelon produces a crisp “dang dang” sound, it may be an unripe melon; one that produces a muddy, heavy “peng peng” sound should be a ripe melon; one that produces a “pu pu” sound may be an overripe melon. In fact, besides tapping to listen to the sound, there are many ways to tell whether a watermelon is good or bad. For example, the following “four looks”:

1. Look at the circle at the bottom of the watermelon; the smaller the better. Conversely, the larger the circle at the bottom, the thicker the rind and the less tasty it is;

2. Look at the watermelon's head, the so-called stem end. A watermelon with a curled stem end is usually very sweet;

3. Look at the watermelon's stripes; the neater the stripes, the better;

4. Look at the watermelon's color; it is best to pick a green one, and avoid picking a misty white one.