Subaru Boxer Rumble - Exhaust Explained

What gives the Subaru WRX STI its iconic boxer exhaust rumble? What's the difference between equal length headers and unequal length headers? What are the advantages of equal length headers? The reason the WRX STI sounds so unique is due to the unequal length headers. The exhaust pulses do not line up evenly before entering a single exhaust pipe to exit to the rear. The uneven exhaust pulses result in a fluttering sound rather than a smooth even exhaust note which is characteristic of equal length exhaust headers. This is very similar to crossplane crankshaft V8's versus flatplane crankshaft V8's, where flatplanes have a much smoother tone.