Why does music sound pleasant?

When you pluck a guitar string, what you hear is the trace left in the air by a string moving back and forth hundreds of times a second. When that trace reaches your ear, it becomes a note. But what makes music music isn't the individual notes; it's how they sit together when they're played at the same time. Some notes sound peaceful side by side, others sound tense. Why?

The journey starts with what sound is: vibration, frequency and amplitude. Then you'll discover why a man and a child singing the same song sound as if they're singing "the same notes": the octave. After seeing the inverse proportion between string length and frequency, you'll arrive at the harmonious ratios the Pythagoreans found with the single-stringed monochord.

In the sound lab you'll set two tones yourself and see how the waves fall into order as the ratio gets simpler. It works with the sound off, too. By the end, you'll be able to explain why a chord sounds pleasant through both physics and culture.