How Do I Diagnose Phase Problems in My System?

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Q I can’t seem to get clear dialogue when playing some movies and TV shows and am forced to used closed captioning. I believe my problem is caused by improper phase. My home theater setup consists of five speakers plus dual subwoofers that have a polarity switch and phase knob. I’ve heard that the proper way to adjust phase is to play a sine wave at 80 Hz (same as my crossover setting) and then turn off all speakers except the left or right speaker and its adjacent sub. The next step: adjust polarity and phase until you measure the highest SPL level at the main listening position. Am I going about things right? Any other suggestions to improve dialogue intelligibility in my system? —Amit Alcalay

A It’s unlikely that phase problems are affecting dialogue intelligibility in your system. First, the polarity/phase adjustments you describe are intended to minimize frequency response dips in the region where the main left/right speaker output blends with the subwoofer’s output (80 Hz, in your case). Second, the frequency range of movie dialogue rarely extends down to 80 Hz — even if the actor delivering it happens to be Ving Rhames (Mission: Impossible, Pulp Fiction, etc.) — so the output of your subwoofers shouldn’t affect dialogue clarity.

Where a phase issue could create problems is if the speakers are wired incorrectly. A basic system check would confirm polarity by verifying that all speakers are wired with the positive (red) output on the amplifier connected to the positive input on the speaker, and the negative (black) output connected to the speaker’s negative input.

You could then use a test disc like Disney’s Wow or any Blu-ray or DVD with the THX Optimizer (I use Pixar’s Toy Story ) to check for proper phase between the various speaker pairs in your system. For example, to evaluate phase with your main left and center speakers, in-phase pink noise is played, followed by out-of- phase pink noise. The first tone should sound clear and focused, the second diffuse. If the results you hear are reversed, then one speaker in the pair is wired incorrectly.

Another factor that can reduce dialogue intelligibility is center-channel level. Even when the center speaker’s output is calibrated to match other speakers in your system, the soundtrack and sound effects in some movies can obscure dialogue when listening at a reasonable level. To fix this, you should temporarily boost center speaker level for certain content. Another option if your receiver provides it is to use a dynamic range compression mode such as Audyssey’s Dynamic EQ/ Dynamic Volume or THX Loudness Plus that works to maintain a consistent level among all surround channels at various volume settings.

drny's picture

The likely culprit of the muddled unclear dialogue is the crossed wire connection. Just as Al describes. Positive speaker wire out on amp/ receiver must go into positive speaker wire input in the center channel.
There are probably two or more speaker channels with crossed polarity to cause unintelligible dialogue.

Oh Yeah, my clear and sound pun was intentional.

Ednmod's picture

I had the same problem. My center channel is a PSB Image 9C which I bought in 2000. My problem was I had a bad tweeter. Once I replaced it it was night and day. To test your tweeter remove it from the speaker and connect it's terminals to a AA battery, if the tweeter is good you should hear a distinctive or loud clicking sound. If it is dull or nonexistent the tweeter is bad.

Hope this helps.