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It is believed that by tapping on a watermelon and listening to the feedback sound, one can deduce whether the watermelon is ripe and tasty or not. This belief seems to be very common in some areas of the world.

Some references which mention it:

whataboutwatermelon.com

Some say a ripe watermelon will produce a hollow sound, while a “thick” or “solid” sound indicates a watermelon that’s not ripe or too ripe. Others say a hollow or “tight” sound is bad, and your watermelon should instead sound “firm.” Still others say a ripe watermelon should produce a B-flat sound. (What?!)

scotlynn.com

Unlike other watermelon suppliers who use the “tapping” method of quality control, Scotlynn uses the most advanced x-ray machines available.

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There are a few papers assessing methods and results of acoustic measurements to determine the quality of watermelons:

  1. Diezma-Iglesias et al evaluated "[d]ifferent acoustic parameters...for spectral characterisation: resonant frequency, maximum amplitude of the spectrum and band magnitude (BM) of the acoustic spectrum" and found that "[t]he BM parameters...were the best indicators of internal quality, especially the BM encompassed between 85 and 160 Hz (BM1), which showed the minimum overlap between good and hollow watermelons".

  2. There is also a recent literature review that examines different methods for "nondestructive determination of internal quality in watermelon/melon", where they point out how acoustic methods are "greatly influenced by the angle and location of the impact on the fruit surface".

So, the answer seems to be yes, you can deduce certain qualities of watermelons if you have an appropriate device to detect and analyze the sounds and tap on the watermelon at the right angle and location.

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    Would 'ears' be an appropriate device? :) – Benjol May 13 '14 at 10:01
  • Yes, that raises the question of whether the human ear is good enough for this. – Hello World May 13 '14 at 21:23
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    Someone should write a smartphone app for this.... – Paul May 14 '14 at 5:19
  • @Benjol: Ostensibly a trained tap-and-listener is required. The human range of hearing goes down to about 20 Hz; so the sounds in question are not beyond the reach of our ears; but you have to know where and how to hit, and what to listen for. – Williham Totland May 15 '14 at 21:13
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In my experience as an assistant for a watermelon farmer, I must testify to the verity of this method at least as an approximate indicator of ripeness. The basic notion is that a higher concentration of sugar in a watermelon reflects a density which corresponds to a duller tone when thumped than a greener melon with less sugar - showing its maturity. Sometimes, when a melon is left on the vine too long, the melon will become so dense that a thump will hurt your finger.

This, combined with visual cues of size are fairly reliable means to decide it a melon is ready to be picked.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    Welcome to Skeptics! Please provide some references to support your claims. We can't accept your claim to be an expert because (a) there are too many trolls who lie, and (b) it isn't clear that an assistant to a watermelon farmer would actually know. – Oddthinking May 13 '14 at 0:31

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