In my prior post on Choosing a Crossover Frequency, I discussed the fundamental role played by a Subwoofer in any good, Home Theater system, and the equally important choice of allowing that Subwoofer to handle the lowest bass frequencies which would otherwise be played through the Main speakers.  This task of "steering" bass from the various Main speaker channels to the Subwoofer is the job of the Crossover processor -- part of the "Bass Management" system in your Audio Video Receiver (AVR).

The Crossover does not act like a switch, with bass suddenly cut off to each Main speaker and sent instead to the Sub.  Rather, the Crossover rolls into effect over a range of frequencies -- typically one Octave (factor of 2 in frequency).  So for example, an 80 Hz Crossover -- the most common choice -- actually rolls into effect from 80 Hz down to 40 Hz.  At 80 Hz, all the audio in the Main speaker channel is still coming out of the Main speaker.  At 40 Hz, all the audio is being sent to the Subwoofer.  And IN BETWEEN, the Main speaker and the Subwoofer SHARE the job of producing the audio output!

So in that critical range of in-between frequencies, a given Main speaker and the Sub are playing the SAME audio at the SAME time.  But the Main and Sub are not located at the same spots in your room, and the differing designs of the Main and Sub undoubtedly introduce different delays in how the electrical input signal to each turns into motion of their speaker cones -- creating the audio you hear.  The upshot is the audio waves arriving from the Main and Sub may very well NOT reach your ears at precisely the same instant of time!  The sound which SHOULD have blended perfectly from the two of them will not do so.

Technically, this timing mismatch is called Phase error, and it can be a real problem!  If the two wavefronts are a full 180 degrees out of Phase, for example, they'll CANCEL each other out!  That is, the result of improper Phase matching is anemic sounding bass.

In this post, we'll talk about how to get your Subwoofer(s) into proper Phase with your Main speakers.