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phase....what and why.

Discussion in 'AV Receivers' started by Craig Forsythe, Mar 2, 2004.

  1. Craig Forsythe

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    I tried doing some searching to answer this question...I'm sure its covered somewhere...alas....

    I "think" I more or less understand what phase adjustability is, but it sure would be nice if someone had a link to "Phase for Dummies".

    Why do most plate sub amps have adjustable phase tuning and pro amps do not? It phase adjustment something only needed down in the low hz area and thus skipped by pro amps? Or are pro amps so well designed they just don't need the ability?

    Taking, say, an SVS sub for example, all their powered subs have a phase knob, but the high end pro amps for the unpowered models (Samson,Crown) don't. I'm missing something, I'm sure.

    thanks in advance!
     
  2. Tim Hoover

    Tim Hoover Screenwriter

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    For pro-sound rigs, phase can oftentimes be adjusted at the mixer, before sending a signal to the power amps. Pro-sound amps are just that - amps...and dedicated onboard sub amps will usually have a better featureset that is tailored to that specific application, instead of a relatively barebones pro amp that is designed for multiple applications.
     
  3. Ben.T

    Ben.T Agent

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    Phase matters at all freq but you will notice it (phasing out) the most at lower ones (bass).

    (I'm probably not going to make sense without pictures but here goes...)

    This is because sound is AC, thus if you can picture what a sine wave looks like at one freq with the wave going positve and negative... basically if you have more than one driver playing the same freq but they are pointed 180deg oposite of each other, their postive and negative swings of that sine wave cancel each other out.

    It's mainly only noticable when you haev overlapping bass, which happens to some extent no matter what crossover freq you choose to drive the sub at vs your main speakers. There are other considerations but that is the primary one.
     
  4. Wayne A. Pflughaupt

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    Wayne
    If this were true, then if you have the speakers in the doors of your car, when you turn on the radio you would get nothing. Quite the contrary – when you pan the balance control from one side to straight-up, the sound gets noticeably louder and bass increases.

    Craig, I wish there were indeed a “phase for dummies” link. It’s a complicated issue and I don’t totally “get it” either. It has to do with time alignment between the sub and mains, and the slopes of the crossover play a role, too.

    The crossover is actually a large contributing factor, which is why a lot of powered subs and plate amps have phase controls. In fact, you won’t find a plate amp or powered sub with a phase control unless it has a built-in crossover. And that’s also why pro amps don’t. As Tim noted, they’re just amps.

    For some reason pro systems don’t concern themselves with phase. Perhaps it’s because generally they are hopelessly out of time alignment to begin with. Or because it would be impossible for time-align the system for everyone in a crowd.

    Regards,
    Wayne A. Pflughaupt
     
  5. Tim Hoover

    Tim Hoover Screenwriter

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    Wayne, I've seen some live-sound boards with phase control on them. It's not a continuously-variable control, just a phase invert switch. In addition, many large acts DO use time-alignment in their sound rigs. Many RTAs have this feature built in, as well as some standalone devices...
     
  6. Ben.T

    Ben.T Agent

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    hmmm I see you point but I know I've had real life situations where two drivers facing opposite directions canceled each other out (turning one driver around remedied the problem)... taking your car door example, I wonder if the fact they are playing in stereo had anything to do with it, I wonder if they were playing in mono if that would have an effect.

    /shrug

    oh ya, phase can get complicated, that we agree on :b
     
  7. Brad Wood

    Brad Wood Stunt Coordinator

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    I hate to hijack a thread, but wanted to add my .02.

    To echo ben's response to Craig's question, pro amplifiers don't usually have phase control because in most pro systems, phase is adjusted elsewhere.

    To answer some of Wayne's concern's:
    -Hmm. I’ve never seen a mixing console with a phase control

    Mixing consoles do have phase control, but not on the outputs. Any good live console will have a phase inversion switch at the top of the input strip. Some high end recording consoles will have variable phase control on the inputs. This is usually used when using multiple mics on one source (such as a guitar amp or kick drum) to ensure phase addition rather than cancellation.

    For some reason pro systems don’t concern themselves with phase. Perhaps it’s because generally they are hopelessly out of time alignment to begin with. Or because it would be impossible for time-align the system for everyone in a crowd.

    As a live sound engineer, I take a certain degree of umbrage to this response. First, sound engineers, at least the ones I've worked with, work meticulously to time allign any system they work with. Time allignment starts with the correct crossover program for the loudspeaker system they're using. This is usually done by the engineers at the speaker manufacturer (EAW, JBL, V-Dosc, etc) and loaded into a digital crossover (XTA, BSS, etc). These programs deal with the proper phase, eq and time allignment for the particular speaker system. When on-site, sound engineers use distance calculations and their ears to time allign multiple speaker clusters. While it's difficult to time allign to every seat in the house, it's not impossible and it should be the goal of any good sound engineer.

    As for the discussion of time-allignment, don't confuse it with phase. While there certainly is a time component to phase, time-allignment in pro sound can refer to time allignment of drivers in a speaker cabinet, or time allignment of multiple speaker systems. For example, time alligning the mains to the stage sound, the wraps and delays to the mains, etc.

    Craig, phase can be confusing and there are a number of terms that shouldn't be confused with each other such as phase, time allignment and polarity. All similar, but all distinctively different. If you want to look into it, there are a number of great articles out there all over the web on the topic of phase.

    Again, sorry to hijack, I just need to stand up for my fellow sound guys.
     
  8. Craig Forsythe

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    More information is never hijacking! [​IMG]

    I've learned lots just from this bit.

    I don't know if its helped me determine why someone would buy a pro-amp/sub combo instead of a same-wattage plate-amp/sub but at least I understand better why proamps don't think to include it.

    Is phase really just something your average guy (me) won't notice? Or more to the point, wouldn't know enough to recognize it? :b

    Positioning of my future sub made someone comment that phase adjustment might be needed, only reason this entire thing was even brought up)
     
  9. LanceJ

    LanceJ Producer

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    Craig:

    Phase issues definitely CAN be something Regular Joes [​IMG] can notice. And can be bad enough to make one think their amp, woofers & even tweeters are damaged.

    1) Bass--if the left speaker is wired backwards compared to the right speaker, depending on how large the room is bass output can be affected a lot. That's because one woofer is moving out while the other woofer is moving in during the same bass note and they try to cancel out each other's sound. Or to demonstrate mathematically:

    (+1) + (-1) = 0

    If you think a stereo system is out-of-phase, flip the wires on ONE speaker only & listen to see if the volume of bass has been improved. >>> Facing them a few inches from each other makes this much easier to hear.

    Satellites & subs can suffer phase problems too because the electronics powering them might not be wired the same way (i.e. polarity) or their physical locations in relation to each other can cause subtle phase problems too (well actually, this is partly a timing issue but I'll keep it simple).

    2) High frequencies--these are a little harder to hear but also cause problems, in particular screwing up a speaker system's imaging abilities. The best way I've found to detect this problem is to stand in front of the two speakers and slowly move side to side. If they are out-of-phase you'll notice the high frequencies changing in volume as you move. It can sound quite weird sometimes, almost like the sound is "inside" your head or at other times, as if someone quickly stuck a wad of cotton in your ears (specialized phase tricks are used to generate all manner of sonic effects, including virtual surround sound and "floating" instruments). But to confuse things, if a speaker has limited horizontal dispersion abilities (i.e. the beaming effect most often heard from large & badly designed cone tweeters) this changing-volume effect can also appear even if they are wired correctly. Then you'll just have to check the wires themselves for correct polarity (remember as has been mentioned already, sound signals are AC in nature and the positive connection wire doesn't nessarily need to be connected to the positive connector on the speaker for proper sound output).

    LJ
     

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