Ah, Apple origin stories. We've heard about the Swiss railway company behind Apple's Clock app, the inspiration for the iOS Calculator, the voice of Siri, and now, we know about the unlikely inventor of the omnipresent "tri-tone" alert sound popularized by iTunes and the iPhone.
"I was looking for something simple that would grab the user's attention."
In a blog post, media producer Kelly Jacklin explains how a friend once asked him to write a short, declarative, alert tune for SoundJam MP, an MP3 player for Mac. "I was looking for something 'simple' that would grab the user's attention," writes Jacklin, who was tasked with composing a melody for when the app finished burning a CD. Jacklin used Macintosh Common Lisp to experiment with various three and four-note sequences until he settled on one ascending note sequence he had made using the marimba, an instrument he had become fascinated with. The result was the tri-tone we've come to know — but not quite yet.
SoundJam MP was subsequently acquired by Apple in 2000 and became the basis for iTunes, which launched in 2001 featuring Jacklin's melody. Apple later used the tri-tone to signal when an application finished installing in OS X, and when a text messages arrived on iPhone. Only then did Jacklin begin to understand the implications of his work.
He posted a minute's worth of variations on the tri-tone at the bottom of his post, which is definitely worth a listen.