phase

SPEAKER PHASE

Here is a thread (from the Hometheater Forum) about speaker phase, to which I replied. Other members of the forum said that my explanation was very good, so I thought I'd keep it and post it on my web site. If you have any other questions about phase, please email me, to let me know.

Bob McLaughlin
       Pittsburgh, PA, USA
       Member
       [34 posts]

       Member Since:
       08-15-2000
                              posted on September 07, 2000 10:40 AM CST (US)

I was using the THX audio setup on my new Evil Dead 2 DVD, and one of the tests was to see if the speakers were in
phase or out of phase. I could hear the differences on the test, so I believe my speakers are set up correctly.

                           Could someone explain what is happening, scientifically, when speakers are "in phase" & "out of phase"? I am a newbie
                           but I've heard these terms before, I just am interested in what is really happening to the sound when speakers are in and
                           out of phase.

       RicP
       NJ, Overlooking the Hudson
       Member
       [400 posts]

       Member Since:
       03-23-2000
                              Re: Explain "in phase" & "out of phase" please?
                              posted on September 07, 2000 10:43 AM CST (US)

Hi Bob,

                           I'll take a crack at it. In-Phase means both of your speakers cones are pushing out air in the same direction at the same
                           time. Out of phase means that when, say, the left speakers cone is pushing out, the right speakers cone is retracting. This
                           gives the sound a very non-directional feel and is usually a bad thing.

You can correct phase problems by switching the connectors on your speakers, making sure + goes to + and - goes to -.

If I'm wrong on any of this, please someone chime in!!

                           ------------------
                           Ric Perrott
                           My Theater
                           No Animals were harmed to create this message...
                           However a great number of electrons were somewhat inconvenienced.

       Kieran Coghlan
       Long Beach, CA USA
       Member
       [165 posts]

       Member Since:
       10-27-98
                              Re: Explain "in phase" & "out of phase" please?
                              posted on September 07, 2000 01:01 PM CST (US)

                           Phase is a HUGE topic, one that could take up volumes of text. What RicP said is true, but barely touches the surface.
                           Phase is used in many, many aspects of audio encoding and processing. If it weren't for the effects that phase has on how
                           we hear stuff, we wouldn't have all the nifty surround dsp modes, and heck, we couldn't tell if some sound was coming from
                           behind us in the first place!

Some basics: First look at this chart -

                           Shown are three basic waveforms, each some amount out of phase (measured in degrees) from each other. The blue curve
                           is a simple sine wave. The magenta curve is 90 degrees ahead (leading) the blue curve (this waveform is also known as the
                           cosine function). The yellow curve leads the magenta by 90 degrees and it leads the blue by 180 degrees. So, the yellow
                           and blue are completely OUT OF PHASE from each other. What this means, is that if the two wave forms were sounds
                           played simultaneously, they would cancel each other out, and there would be NO SOUND. In fact if you sent two perfect
                           sine waves, each exactly 180 degrees out of phase from each other, to the same speaker, the speaker cone wouldn't move
                           at all. You can see this on the chart, by taking any two corresponding values from the blue and yellow curves, and adding
                           them together: the sum is zero.

                           So, sometimes your speakers will be sent sounds that are completely out of phase, or partially out of phase from the Left
                           to the Right speakers. The reason you need to test to make sure your speakers are hooked up properly is so that when
                           sounds are meant to be out of phase from left to right, they will be, and when they're meant to be in phase, they will be.

Incidentally, the new sound format from DTS, their "discreet 6.1 dts-es" uses the principle of phase cancellation in order to
provide 3 fully discreet surround channels. Read this article for more information on that.

                           ------------------
                           -Kieran
                           My HT Page

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