If researcher Elizabeth Petitti played two musical notes from her laptop, some people would hear the notes rise in pitch, while others would hear them fall. Why the difference?
The answer may improve our understanding of how our auditory system develops and may help speech-language pathologists who work with people who have hearing impairment.
Petitti says the answer comes down to the way our brains perceive two components that make up sound: fundamental frequency and harmonics.
A note’s fundamental frequency is the primary element of sound from which our brains derive pitch—the highness or lowness of a note. Harmonics give a note its timbre, the quality that makes instruments sound distinct from one another.
Many sounds in the world are made up of these tones, whether you strike a key on a keyboard, play a note on a clarinet, or say a letter, says Petitti, who graduated from Boston University’s Sargent College of Health & Rehabilitation Sciences with a master’s in speech-language pathology.
Our brains expect the fundamental and the harmonics to be present in any given note. But when some of this information drops out, “the way you perceive the note can change in surprising ways,” says Petitti’s mentor, Tyler Perrachione, a professor at Sargent and director of the Communication Neuroscience Research Laboratory.
‘Pitch Exists Only in Our Minds’
Petitti explains that when she removes the fundamental from a tone (using signal processing software), and then plays that note, the listener’s brain automatically supplies the pitch. People’s brains deliver this information in different ways: They either fill in the missing fundamental frequency—similar to the way the brain would compensate for a blind spot in our eye—or they determine the pitch from the harmonics.
Here’s where it gets interesting: When two different tones that have been stripped of their fundamentals are played in succession, some listeners hear their pitch rising, and some hear it falling. Who’s right?
