Two things most people don’t care to understand: physics and how the heck your car works. But you’re different. Why, you probably already know that phase cancellation occurs when a sound source is delayed slightly (by a real-world reflection, or in recording and mixing), so that two coherent waveforms of opposite phase are superimposes and cancel each other out. (Er, in plain english: one wave’s crests cancel out the other’s troughs and vice versa.)

Now, did you know this principle is what keeps your car’s exhaust from making a racket?

How Mufflers Work [Howstuffworks]

Basically, the muffler is a chamber designed to create lots of echoes, and thus lots of destructive interference.

See an extended discussion above, plus some variations on the design used in luxury automobiles (think active cancellation, as in noise-reducing headphones). And, of course, this is exactly what doesn’t happen when your remove a muffler and get that ear-splitting noise.

Got other candidates for acoustic science in the real world? Let me know!