Monday, February 17, 2014

Superposition, Interference, and Audio Engineering

When musicians record in the studio, the will often record each instrument separately. Each recording would be stored in some sort of loss-less format, an example of which is .wav

A portion of a .wav file shown in Audacity
I'm not sure if the .wav file is a record of pressure over time or velocity over time, but either way it's a record of the sound wave received by the microphone. Each part (vocals, guitar, bass, drums, etc) will have one or more records and the Audio Engineer's job is to use superposition to combine all those parts into one file for the consumer.

For a demonstration of sound and superposition, I found a sample set of these separate recordings for Iron Maiden's "Run to the Hills" and used Audacity to combine them into three different files:

  • The complete song sample with all the parts
  • The song sample with all the parts except for the lead vocals (this is like a karaoke version)
  • An inverted version of the instruments-only song sample.
The normal Instrumental song and the inverted song side-by-side

I can then use Audacity to re-play those files in different combinations. I like to get the students to think about:

  1. Will the inverted instruments only file sound different to the sound wave when it's right-side-up?
  2. What will it sound like when the inverted instrumental track is played with the right-side-up track simultaneously? Audacity can load multiple files and play them all at once so we can test this out
  3. What will happen if I play the complete song and the inverted instrumental simultaneously?
If you'd like to run this demonstration you just need Audacity (or something similar) and the song files, which you can download here:

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