If you're listening to music right now, you can probably hear the vocalist's slight pitch-shifts, or hiss of a drummer's hi-hats, or the padded thump of a synth bed. But do you ever think about what you're not hearing? If you listen to MP3s, your music library lost some of its sonic weight during the audio compression process, but it's likely you'd never realize anything was missing. Now you will. In his project "The Ghost in the MP3," PhD student Ryan Maguire created a song made from the sounds lost when compressing Suzanne Vega's 1987 single "Tom's Diner."

Over the past two decades, the MP3 has managed to become the default audio file format while still reducing the quality of sound files, however subtly. As Maguire notes, the MP3 was first developed in the early 1990s as a way to minimize file size and make it cheaper to transfer data while remaining faithful to a song's original intent. It takes advantage of "auditory masking," a hearing limitation that makes certain tones inaudible to humans.

MP3 compression relies on "auditory masking"

But even if the human ear can't recognize that certain sounds are missing from a Suzanne Vega hit, they are. Maguire salvaged the sounds from "Tom's Diner" as it was shrunk from an uncompressed .wav file to a 320kbps MP3. He chose the song because it was the same one used in the '90s as a control to test the MP3 encoding algorithm. The ghostly, ambient track above is not even an entirely accurate rendering of the lost sounds, because it too has lost some material during MP4 video compression.

You can listen to an obvious effect of MP3 compression below using a spectrum of noise frequencies known as white, pink, and brown noise. This is what those frequencies sound like uncompressed:

And here they are compressed to the lowest MP3 bit rate of 8kbps:

There has been a push from audiophiles in recent years for the use of more complex digital file formats, and it makes sense. Despite increased bandwidths and storage capacity, most music listeners still rely on something developed in the early '90s to play their music. It's a testament to the MP3's staying power, and proof that you can't miss something you didn't know you had.