Making cents of sounds

A vending machine frustration inspired a science project

When coins get old or dirty, vending machines may not recognize them. A student decided to instead identify coins by how they sound.

When coins get old or dirty, vending machines may not recognize them. A student decided to instead identify coins by how they sound. 

J J/Flickr (CC BY-NC-ND 2.0) Image has been cropped.

PITTSBURGH, Pa. — Some people encounter something irritating and do nothing but whine. But when Yousuf Siddiqui got annoyed at vending machines rejecting his coins, the 16-year-old didn’t get mad. He got scientific. His results now show how vending machines might be able to recognize which coins are dropped into them solely by the sounds they make.

“Vending machines are very annoying to me,” Yousuf complains. “The machines don’t recognize the coins. They are always throwing them back out.” Vending machines identify coins based on their size, shape, magnetism and other factors. But as coins are used, dropped and generally dented and dirtied, it becomes harder for the machines to recognize them. And when such machines spit back his coins too often, Yousuf ended up late to class.

Instead of just getting frustrated, he thought about how he might change a vending machine so it would dispense his bags of chips more reliably. The results of his testing won the sophomore at Al-Amal School in Fridley, Minn., a trip here to the 2015 Intel International Science and Engineering Fair. Sponsored by Intel and run by Society for Science & the Public, this competition has brought students here from all over the world to show off their science fair projects. (SSP also publishes Science News for Students and this blog.)

Yousuf’s first step was to consult with his mentor, Sener Ozonder. He was a doctoral student at the University of Minnesota, and is now a physicist at the University of Washington. “He slept on it and came back with an idea,” Yousuf recalls. “He said, ‘we recognize birds by the sound they make, why not coins?’” After a little additional research on the site Physics Stack Exchange, Yousuf found that it could be done, and decided to test the idea.

The teen took a LEGO robot and set it to dropping coins over and over again. The robot dropped the coin from 15 centimeters (5.9 inches) above a table. Each time, it fell through a paper tube to make sure the coin landed exactly on its edge. “If the coin lands flat it has to land heads or tails,” Yousuf explains, and they have different things on the faces.

Another challenge was what type of surface on which to drop the coins. He tried a glass table, but it absorbed the sound “and it messed with my data,” the teen says. “Then I tried a metal table.” He thought the metal might amplify the sound. In fact, he found, “it amplified it too much.” Finally he tried a wooden table. “And that turned out just fine.”

The robot dropped denominations from pennies to silver dollars. Each time, Yousuf recorded the pinging sounds on his smartphone. Then, he analyzed each sound with a computer program to graph each coin’s sound waves. Each of these spectrograms included the frequency — the number of sound waves in a specific period of time. It also mapped the amplitude — or intensity of the sound waves. These two measurements together formed each coin’s signature.

Every single coin had its own unique signature, the teen found. So no one penny was exactly like another. But among coins of a particular denomination, each type of coin had a fairly common sound signature. In this sense, quarters will make a very different sound from any penny or dime.

Yousuf says he hopes vending machines will adopt the use of sound spectra to help identify coins. With that, getting a snack might never make him late for class again.

MAKING CENTS Yousuf Siddiqui, of Al-Amal School in Fridley, Minn., shows off his experiment that dropped coins to fingerprint the sounds they make.

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Power Words

(for more about Power Words, click here)

amplitude  A measure of the height of a recurring wave in some signal, water or beam of radiation. In sound, wave amplitude corresponds with intensity — loudness or softness.

computer program  A set of instructions that a computer uses to perform some analysis or computation. The writing of these instructions is known as computer programming.

frequency  The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

magnet  A material that usually contains iron and whose atoms are arranged so they attract certain metals.

Society for Science and the Public (or SSP)  A nonprofit organization created in 1921 and based in Washington, D.C. Since its founding, SSP has been not only promoting public engagement in scientific research but also the public understanding of science. It created and continues to run three renowned science competitions, including the Intel Science Talent Search (begun in 1942). SSP also publishes award-winning journalism: in Science News (launched in 1922) and Science News for Students (created in 2003). Those magazines also host a series of blogs (including Eureka! Lab).

software  The mathematical instructions that direct a computer’s hardware, including its processor, to perform certain operations.

sound wave  A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.

spectrogram  A visual (or graphical) representation of a spectrum. That spectrum can map sound waves or other types of electromagnetic radiation. A sound spectrogram graphs sound frequency (pitch) over time; and its amplitude (or volume) through the intensity of color.

wavelength  The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Editor’s note: This post was updated at 10:15 a.m. on May 18, 2015, to include a link to Ozonder’s new position at the University of Washington and a link to the Physics Stack Exchange.

Bethany Brookshire was a longtime staff writer at Science News Explores and is the author of the book Pests: How Humans Create Animal Villains. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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