Wired Science

High School Students Use Soundwaves to Test for Sweetness

Everyone has thumped a watermelon at some time in their life. Old timers will tell you that  a ripe one makes a deep thud, while others may tell you to listen for a resonating ping. As part of a project for the annual Lemelson-MIT Inventeams contest, in which about a dozen schools nationwide collaborate on inventions that they design and build, students at West Salem high school in Oregon devised a way to thump watermelons, look at the sound signatures, and correlate them with sugar content.

As a watermelon flexes, it produces many notes that together sound like a “thump”, similar to playing a chord on a piano. Scientists look at the spectrum of frequencies that are present in a process called a Fast Fourier Transform (FFT), which translates the thump into individual notes, together with how hard each note is played. The graph to the right shows many peaks, each corresponding to different notes from a single watermelon thump. There are two strong peaks visible at around 40 and 360 cycles per second. 

Another method examines how quickly each note dampens out. Interestingly, students discovered that the FFT signature of a watermelon was not as important in determining ripeness as was the duration of the sound. Studying over one hundred melons, students discovered that the more sugar present, the longer a melon’s thump would resonate. Other scientists have found similar results.

So the next time you thump a watermelon, remember that what sounds to you like one thump, is actually many frequencies mixed together. If these students are correct, you should be listening for the sound to hang around awhile for a sweet tasting watermelon.