The Surprising Science Behind Everyday Sounds: Horse Whinnies and Sneaker Squeaks
Everyday sounds, from a horse’s whinny to the squeak of sneakers, often hold hidden scientific complexities. Recent research has shed light on the mechanisms behind these familiar noises, revealing surprising details about animal vocalizations and the physics of friction.
The Secret of a Horse’s Whinny
Despite centuries of interaction, much remains unknown about horses. A key discovery in 2015, by scientist Elodie Briefer, revealed that horse whinnies are composed of two distinct sounds: a low frequency and a high-pitched whistle.1
Dr. Briefer and her team investigated how horses produce this unique whistling sound. Using a compact camera inserted through a horse’s nose, they observed the vocal cords during whinnies. They found that low-pitched sounds were created by vocal cord vibration, similar to human speech. However, the high-pitched sounds were generated by tightening the muscles around the voice box, creating a small opening for a whistling sound.
To validate this hypothesis, the researchers conducted an experiment using voice boxes from deceased horses. They found that introducing helium increased the pitch of the whistling sound, even as helium had no effect on the vibrating vocal cords. This confirmed that the whistling sound originated from the constricted opening, not from vocal cord vibration.
Interestingly, horses are unique in their ability to produce both types of sounds simultaneously. While mice and rats can also create whistling sounds, they do not combine them with lower-frequency vocalizations in the same way.
Studying Squeaky Sneakers
Harvard scientist Adel Djellouli became intrigued by the squeaking sounds made by basketball players’ sneakers during a Boston Celtics game. He and his team decided to investigate the phenomenon.2
Using a high-speed camera and sound recording equipment, they slid a basketball shoe across a smooth glass plate. The recordings revealed that sneaker squeaking is related to “stick-slip” friction – a process where surfaces briefly adhere before slipping. The shoe sole experienced thousands of tiny slips and grabs per second, generating vibrations faster than the speed of sound, which resulted in the squeaking noise.
Further experiments with rubber blocks demonstrated that squeaking only occurred with blocks featuring ridges, similar to sneaker soles. The pitch of the squeak varied depending on the thickness of the ridges, and the researchers were even able to play a tune from “Star Wars” using different ridge thicknesses.
While seemingly trivial, understanding sneaker squeaking could have broader scientific implications, potentially aiding in the study of earthquakes, which also involve stick-slip friction.
Unexpected Electrical Charges
The research team also discovered an unexpected byproduct of sneaker squeaking: the emission of small electrical charges, akin to miniature lightning bolts.3
These findings highlight how even the most commonplace sounds can reveal complex scientific principles and unexpected phenomena.