Repeat of question.........

[point_and_shoot]

My subwoofer has a phase invertion button on the back. When would I ever need to use this? I can't tell much difference when I listen either way.

[Blackmesa]

If you have a second subwoofer, and even so, it would be rare you would need to invert the phase. It basically adds a delay in the output so that the waves line up better and give you true constructive interference. The only reason to invert the phase is if one of the subwoofers is really far away from the other, relative to the listening position and still it may not be ideal to change the phase on one. Normally there is a variable phase control from 0-180 to fine tune the phase adjustments from one sub to the next. But if the subwoofers are equal distance to you, then there should be no phase differences between them and they should be both have the same phase shift. You also may need to adjust the phase if your main speakers dip into your bass a little. Again, this all has to do with distance from listening position.

[point_and_shoot]

Thanks!

[DrWho]

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On 6/18/2005 5:41:39 PM Blackmesa wrote:

It basically adds a delay in the output so that the waves line up better and give you true constructive interference.

You also may need to adjust the phase if your main speakers dip into your bass a little. Again, this all has to do with distance from listening position.

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Just a minor correction for emphasis...the phase has nothing to do with "delay" and the mains always "dip in the bass a little."

A single sine wave (all music is a combination of a lot of sine waves), includes both a forward and backward motion of the woofer. Typically, most sounds start by moving the woofer forward. Flipping the phase however makes it so the woofer moves backward. The way our ears work, we can't tell by hearing whether or not the woofer is moving forward or backward at the beginning so it's not a big deal if we flip the phase back and forth. In fact, listening tests show this to be inaudible (and they are easy to duplicate if you don't believe me).

However, phase is a concern when we have multiple speakers playing the same sounds. When two speakers are "out of phase", what happens is one speaker moves forward while the other one moves backwards. The two basically cancel each other out and we hear no sound. With our recievers, the mains don't just stop playing material at 80Hz and the sub take over. What really happens is the mains are gradually reduced in volume as the frequency decreases while the sub gradually increases in volume. If we do this carefully, the output from the two will result in a flat frequency response throughout the "crossover region." The crossover region is basically the range of frequencies where both speakers are playing the same material. So though phase is technically inaudible, it becomes very important in our homes where we have our mains and subs playing the same material over a narrow range of frequencies centered around the crossover frequency.

Blackmesa elluded to a point that can be a little decieving about "delay." Picture a sine wave in your head (thinking back to math in school, those fancy graphs that go up and down forever). It takes a certain amount of time for a speaker to go through one cycle of a wave. If you delay another speaker to start playing the same sounds but start it one cycle late, the two speakers can be thought of being in phase in that they are both moving in and out at the same time (and prolonged notes don't result in cancellation). However, they are technically 360 degrees out of phase and this is important because the slight time delay causes other things to happen.

Btw, it doesn't matter if you want to think of the phase dial as adding "delay" or as changing the direction of the woofer because in the end the math works out AS LONG AS you're within +-180 degrees of phase shift. Beyond that, the time delay and phase need to be treated seperately because the first speaker could be playing different material before the sound from the second speaker arrived (causing all sorts of nasty interference with different frequencies...basically muddying up the sound).

You mentioned in the other thread that it was a bit deep to think about so I'm trying to keep it simple. Feel free to ask questions and even looking around the web for some answers. There's plenty of animations and diagrams that can show you what's happening. In the end, you basically just want to turn the knobs so that you end up with the most bass possible. In reality, don't expect huge amounts of change between different settings because you are only affecting frequencies directly surrounding the crossover point.

[sivadselim]

phasing, although it is important with 2 subs, is certainly not a phenomenon reserved for that situation. more importantly, your sub must be in phase with your 2 front speakers in order to provide the proper sound.

a receiver's subwoofer distance setting can also be used to attenuate this when only a 0/180 switch is present instead of a dial that allows a variable setting between 0 and 180.

[dragonfyr]

Do we need a discussion to distinguish between "polarity" and "phase"?

[sivadselim]

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On 6/19/2005 11:35:34 AM dragonfyr wrote:

Do we need a discussion to distinguish between "polarity" and "phase"?

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i don't think so. why do you ask?

choke back the urge to regurgitate your vast knowledge of these subjects and spew it all over this thread.

[dragonfyr]

I wouldn't dare want to confuse you! Besides, you seem to be doing fine on your own!

[sivadselim]

yeah, thanks

[DrWho]

gosh, arrogance mixed with semantics

Here it is in a nutshell:

Polarity = as JBL describes it, a positive charge on the black terminal results in cone moving forwards. For speakers close together, reversing the polarity also causes the phase to shift 180 degrees (which is the reason for the interchange in terms).

Phase = Only important with two sound sources. The measurement of the relative phases indicates the destructiveness or constructiveness of the two sound waves. Two sources completely in phase double the acoustical energy, which is a 6dB increase in volume (due to the logorithmic nature of the dB scale). So if each source has a 90dB SPL (sound pressure level), then the two combined have a 96dB SPL. Two sources completely out of phase result in complete cancellation, which yields a total volume of 0dB. So two 90dB SPL sound sources become 0dB (which is sometimes noted as negative infinity). Phase anywhere inbetween 0 and 180 results in a net change in volume somewhere between negative infinity dB and +6 dB. I think two 90dB waves 30 degrees out of phase result in a net total of 93dB (I think my math is off, but the concept is there).

[Blackmesa]

Some subwoofers have only two phase settings 0 or 180. More expensive subwoofers give you a range from 0-180 degrees. So you can make minor corrections to "line up" the peaks and crests of the two sound sources that have slightly different distances from listing position. When you have two sources at different distances, they combine at different phases so you may not get the peaks and crests of the waves lining up thus not giving you the maximum constructing gain from two waves colliding. In fact, if they are 100% out of phase. (180 degrees from each other) then you'll liky get destructive interference and ultimately have less sound than a single source

This explains phase "delay"

http://ccrma.stanford.edu/~jos/filters/Phase_Delay.html