Bad phasing has several bad effects on the overall sound. Specifically with speakers, think of a simple stereo
system with just one
speaker on the left and one on the right. Now imagine a
bass instrument center
stage (phasing effects are more noticeable at low frequencies). If it's mixed at the center, you should have equal amounts of
bass from both sides. But if one of the speakers is wired backward, it will be pushing while the other is pulling - the sound from the two speakers will cancel out, causing the
bass to sound really weak when you're exactly in the middle between the two speakers.
Now imagine you're playing a recording of a full
orchestra. Instruments at the center of the
stage will be weak and lacking
bass, while instruments at the sides (where the sound only comes through one
speaker) will appear to be much louder - like there's a hole on the middle of the
orchestra. With proper phasing, the instruments appear in their proper places, instruments in the center are at the appropriate levels and the
bass is clean and powerful.
Proper phasing lets the whole
system work at its peak efficiency. With the speakers in
phase, the sound from them adds - most of the
power you put into the speakers gets to your ears. If they're out of
phase, it subtracts: you can dump a lot of watts into them and they can be working really hard, but the sound is weak.
For multi-way
speaker systems, phasing is important to
flat, smooth
frequency response. Crossovers, the gadgets that aim the
bass at the woofers and the treble at the tweeters, don't make the transition from one
speaker to another instantly: there's a small band of frequencies surrounding the nominal
crossover frequency where the sound goes to both speakers. For example, if the
crossover frequency from the
subwoofer to the main cabinet is 150 Hz, frequencies between about 100 to 200 Hz will be going to both speakers. If the sub and main are out of
phase, the sound from the two in that frequency range will cancel, scooping out that frequency band just as if you pulled the 150 Hz
fader on your graphic EQ all the way down.
With microphones, there are situations where being out of
phase can be good. For example, a band has two singers, each using fairly good technique, singing close-in on an SM-58. What will happen if the two SM-58s are out of
phase?
Well, because each singer is close to just one
microphone, there's no cancellation of the signal you want - the voice. But sources farther from the mics, like band instruments, floor monitors and
feedback from the mains, will tend to be picked up by both of them and, because they're out of
phase, tend to be cancelled out.
Some sound engineers will go as far as to put two mics on each vocal
mic stand, one to sing into and one just below/behind it, where it won't get much of the singer's voice, connected out of
phase, to cancel background noise and
feedback.
But this only works for close vocal mics. For distance miking, where, for example, a stereo pair needs to
pick up a vocal chorus of several people, the mics should be in-phase for best sound quality.
John