One of the most often misused terms among audio people is “phase”. “Sound’s like it’s out of phase…” seems to be
the go to troubleshooting response when something is out of whack. My response is always the same: “Do you mean polarity?”
To which the reply is: “It’s the same thing.”
Unfortunately, phase and polarity are most definitely not the same thing. In fact they have absolutely nothing in common. Polarity is an electrical term determining the positive and negative sides of a balanced electrical signal while phase is a frequency- specific relationship of time. Both have bearing on the audible response of a loudspeaker system.
In the loudspeaker realm, polarity plays an important role in coverage and efficiency. Every loudspeaker has two (2) inputs: positive and negative. Every amplifier has two (2) outputs per channel, positive and negative. Reversing polarity on a component in a loudspeaker system has some very obvious results.
The following illustrations were all done in MAPP online from Meyer Sound. The RED area denotes the highest sound pressure levels (SPL) while the BLUE denotes the lowest.
The first image shows a pair of subwoofer cabinets (Meyer Sound 700HP) in polarity with each other. Notice the omnidirectional coverage. The cabinets are working together. The second image shows the same two cabinets with one of the cabinets polarity reversed from it’s neighbor. Now they are working against each other, creating a null where the audience would usually be.
The only change in this array is the polarity. The polarity reversal creates a null. The reversal of the balanced electrical circuit is causing one of the loudspeakers in the array to work against the other. Quite literally, when one speaker is pushing toward the audience, its counterpart is pulling away. This is where the cancellation comes from. Polarity is an all or nothing situation; only in or out, no varying degrees and it is easily remedied by a connector, adaptor, cable, or switch.
Phase is more detailed.
In the most concise terms: phase is the relationship in time between two signals. This relationship is frequency dependent. This relationship is how one audio source interacts with another. If the two audio sources arrive at a common destination (the measurement mic at FOH, for example) at the same time, they are considered “IN PHASE”. If they arrive at different time, they are considered “OUT OF PHASE” and this can be measured in degrees.
Phase can only be affected by time (delay adjustment on the signal processor) or distance (physically moving a loudspeaker). The above loudspeakers are the same Meyer Sound 700HP as in the polarity example except that one is ten (10) feet farther back than the other. Without any time correction, there is a significant phase cancellation in front of the cabinets. Using time correction the loudspeakers are now in phase with one another and are summing where the cancellation was before.
Another drastic example:
The same loudspeakers are in line physically, however there is 10ms of delay (meaning that the system processor’s output signal is delayed by 10ms causing the amplifier fed by the signal from that channel is firing later than its neighbor) on of the loudspeakers. The loudspeakers are in the same physical plane, however, they are not firing at the same time and therefore the signals generated are arriving at different times causing cancellation.
While polarity and phase are often misinterpreted, they both have drastic effects on audio systems. They can be used to create loudspeaker arrays that focus audio on its intended audience while canceling it elsewhere. However, if used improperly, polarity and phase adjustments can create a myriad of problems and create an unpleasant listening experience for the audience. Know the difference and keep your audience happy. Check out the video!
Originally posted 2012-02-05 02:31:24.