This app is a speaker polarity tester. Find out if your speakers are wired correctly (in phase or out of phase). Test your surround speakers, stereo speakers, PA speakers, in fact any speaker with wiring terminals on the back. This speaker polarity tester is made by the same people who built the Audio Toolbox, and the polarity tester in that unit was known as the most accurate and reliable polarity tester available. Now that technology is available in your iPhone.