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  • Why does phase matter?

    I'm not referring to absolute electrical phase; this is obvious. I'm referring to acoustic phase and how the drivers relate with each other...

    I'm curious, when you guys design a xover, how much do you pay attention to the phase in the both the sim, as well as the final? My reason for asking is because I've had some say it matters a lot, and some say not so much. The worst thing I've seen was a reverse null, and this was actually due to having the polarity off on the tweeter. I've also played with running my 18w wide open (no low-pass filter) with a Transducer Labs tweeter (I had a 2.5k ish HPF on this). I got a reverse null that (IIRC) went from about 4khz to 6khz (I could be wrong on the exact frequencies, it was a while ago that I did the experiment). So obviously this null was the 2 drivers cancelling each other out. Now aside from that, and aside from the phase measurement's vertical lines looking similar near the xover points, what else do you look for in phase? I've never seen them look bad that I know of.

    Now for me personally, I obviously pay attention to it, but I've honestly never seen anything in either the preliminary or the final design that caused me to raise an eyebrow while I'm designing (the phase always looked great). Am I just lucky? Or is this something that is rarely an issue? Or... am I not looking at the phase correctly?

    Input here would be greatly appreciated ! Or even better, does anyone have a picture of a bad phase, as well as a pic of a good phase response so we can see the correlation?

    And lastly... if you do have a phase issue in a design, what do you do to correct it?

    Edit: I also wanted to add this questions... Wouldn't it show up in the frequency response if the phase was causing a problem anyhow? If not, why not? And also if not, can someone give an example of when the response looked good but the phase didn't? What does this sound like?
    "The ability of any system to produce exceptional sound will be limited mainly by the capability of the speakers" Jim Salk
    "Audio is surely a journey full of revelations as you go" JasonP

  • #2
    Re: Why does phase matter?

    Phase will affect the summation between drivers as measured on-axis, but will also affect the polar pattern. As an illustration, try playing with phase in XDir to see what I'm talking about:

    http://www.tolvan.com/index.php?page=/xdir/xdir.php

    The off-axis lobes will reflect off the ceiling, floor, walls etc. and contribute to the sound.
    Francis

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    • #3
      Re: Why does phase matter?

      L/R filters (theoretically) sum +6dB, meaning at the Fc, both drivers are -6dB down and as "in phase" as they can be (and, of course, we're talking about "acoustic phase" here, which is the raw driver affected by the electrical pass filter).

      Butterworth filters are designed to (optimally) sum +3dB, meaning they're -3dB down at the Fc and are actually 90* out of phase (or, 90* in phase, depending on your POV).

      Before sim software, I think the gurus probably KNEW they did not have perfect acoustic rolloffs (as "designed" by the filter type, as in all the examples in Vance's LDC), but had to make that assumption. Today (just as you probably know that there're an "infinite" number of box models for any given driver - esp. w/vented boxes, NOT really a few "discreet alignments" as shown in the LDC) I'm sure that many completed designs (even some very GOOD ones) have drivers that are -4dB or -5dB down at the Fc and are SOMEWHERE between 0* phase misalignment (meaning, perfectly "in phase") and 90* out.

      If I'm doing a design (let's say a 2-way) and don't like the phase alignment, 1st I'll reverse the tweeter, THEN (if that's not good enough) I'll change the order of either the HP or LP filters (or BOTH) which will make the phase error (at Fc) smaller or larger. Once it gets large enough, you can flip the tweeter polarity.

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      • #4
        Re: Why does phase matter?

        I always try to have my drivers phase align, but not at the expense of much else. I can't honestly say I hear a correlation between good phase alignment and good sound in my designs, but I will say this: when my phase alignment is really good, that means I have nice, smooth roll-offs on both drivers with no weird peaks or dips in the transition range (it's difficult to align jagged phase curves). I CAN correlate smooth roll-offs to good sound in my designs and I'm wondering if this is really what is making people think good phase alignment is important beyond what it does to the frequency response.

        Dan
        _____________________________
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        • #5
          Re: Why does phase matter?

          About the only phase issue that I can think of, where it is debatable as to whether or not there are merits - or an "audible difference" is "linear phase".

          When not takling specifically about linear phase, whenever you are talking about the acoustic phase where two drivers overlap in their pass bands, the issue is not open for debate - there is a concrete, clear, measurable affect when combining the output of two drivers which changes significantly as your phase moves further in or out of acoustic alignment (considering both phase and "time alignment").

          If two drivers are time aligned, but their phase shifts at differing rates, the resulting effect will typically be far less significant than "reverse phase" where you find a clear and deep null. But, there still could be some loss in quality. Unless the effect is dramatic, you'll probably get a different answer from each person you ask to listen to your speakers and tell you what they hear if you have two similar crossovers, with overall mostly similar frequency outputs, measured on axis, but one crossover giving you better phase alignment than the other.

          I've read about tests where one pair of speakers at a DIY listening event had two different crossovers, both with similar FR, both with good phase tracking between drivers above and blow x-over point, but one "linear phase" and one not. It seemed that about half the people preferred one way and half the peopled preferred the other way even though there was not a large difference in either FR or power response between both X-overs.
          "...this is not a subwoofer" - Jeff Bagby ;)

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          • #6
            Re: Why does phase matter?

            Originally posted by Chris Roemer View Post
            Butterworth filters are designed to (optimally) sum +3dB, meaning they're -3dB down at the Fc and are actually 90* out of phase (or, 90* in phase, depending on your POV).
            There is a bit of mix up with this. All individual Butterworth sections (low and high pass) are down 3db at Fc. Odd-order Butterworth sections are out-of-phase by 90db (uncorrelated) and sum flat on-axis with off-axis peaking. Even-order Butterworth filter sections are in-phase (with one inverted driver for 2nd, 6th, etc. like L-R) and sum to +3db at Fc.

            dlr
            WinPCD - Windows .NET Passive Crossover Designer

            Dave's Speaker Pages

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            • #7
              " To me, the soundstage presentation is more about phase and distortion and less about size. However, when you talk about bass extension, there's no replacement for displacement". Tyger23. 4.2015

              Quote Originally Posted by hongrn. Oct 2014
              Do you realize that being an American is like winning the biggest jackpot ever??

              http://www.midwestaudioclub.com/spot...owell-simpson/
              http://s413.photobucket.com/albums/pp216/arlis/

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              • #8
                Re: Why does phase matter?

                Matt- it's not the 'vertical lines' that matter. The vertical lines are the same physical point on a tubular 3D grid that have been plotted on a 2D plane.
                Everything else in the acoustic phase plot is what needs to track relatively well. Some prefer more tracking than just at the xover when optimizing sims, others prefer to track as much as possible, outside the xover region or not. I agree with DanP that smoother rolloffs to contribute to a closer phase alignment. It is possible to have flat FR and have poor acoustic phase alignment. All this said- flat FR is more important than phase alignment, but phase should be considered along with the FR.

                I will also say that changing the Q of the xover components will change your phase alignment. Making the coil increase and the cap decrease (for example) is a change in Q with keeping the same xover Fc. On a flat baffle, one-order difference asymmetric slopes have better phase alignment as well.

                Some differing slope examples:
                BS2:


                BW1 (Tapers to 3rd, RARE!):


                BW3/BW5:


                BW3:


                BW4:


                BW5:


                BW6 (SXO, tweeter tail was not extended):


                LR8:


                LR6 (tweeter tail not extended):


                LR4:


                LR2/LR4 3-way:


                Hope that helps,
                Wolf
                Last edited by Wolf; 12-31-2015, 08:06 PM. Reason: Added rest of post...
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                *InDIYana event website*

                Photobucket pages:
                http://photobucket.com/Wolf-Speakers_and_more

                My blog/writeups/thoughts here at PE:
                http://techtalk.parts-express.com/blog.php?u=4102

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                • #9
                  Re: Why does phase matter?

                  Yes- I didn't get his point somehow. I stand corrected.

                  I still say that smoother rolloffs make aligning phase easier though. That still stands.

                  Later,
                  Wolf
                  "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                  "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                  "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                  "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                  *InDIYana event website*

                  Photobucket pages:
                  http://photobucket.com/Wolf-Speakers_and_more

                  My blog/writeups/thoughts here at PE:
                  http://techtalk.parts-express.com/blog.php?u=4102

                  Comment


                  • #10
                    Re: Why does phase matter?

                    Smooth is definitely good.
                    craigk

                    " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

                    Comment


                    • #11
                      Re: Why does phase matter?

                      The ones you're showing as BW3 or BW5 don't have phase that correlates with 3 or 5. Those even have sharp reverse connection nulls. That is not odd-order BW behavior. True odd-order BW behavior should be uncorrelated, 90 degrees out-of-phase. Reversing the connection should only change the position of the vertical lobes. The woofer transfer functions have more of elliptic style shape, a very deep notch. This is adding a lot of excess-delay to the woofer which is primarily responsible for the phase result. It's just not showing up in the SPL curves, being down in the stop-band. Notice also that the summed response is around 5db+ at Fc due to the phase change.

                      dlr
                      WinPCD - Windows .NET Passive Crossover Designer

                      Dave's Speaker Pages

                      Comment


                      • #12
                        Re: Why does phase matter?

                        I usually just dial in the slope and Fc until the knee/slopes match in final to see what they look like.
                        Let me ask this- If I have a 3rd order elec on a planar, and it has a response between 2nd and 3rd order inherently, then it comes out 5th order acoustic or not? The midrange had a 12dB rolloff, and I added a 3rd order to it. That has to be 5th order.

                        Are you just saying it's not BW? That's what the rolloff matches.
                        Later,
                        Wolf
                        "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                        "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                        "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                        "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                        *InDIYana event website*

                        Photobucket pages:
                        http://photobucket.com/Wolf-Speakers_and_more

                        My blog/writeups/thoughts here at PE:
                        http://techtalk.parts-express.com/blog.php?u=4102

                        Comment


                        • #13
                          Re: Why does phase matter?

                          Originally posted by Wolf View Post
                          I usually just dial in the slope and Fc until the knee/slopes match in final to see what they look like.
                          Let me ask this- If I have a 3rd order elec on a planar, and it has a response between 2nd and 3rd order inherently, then it comes out 5th order acoustic or not? The midrange had a 12dB rolloff, and I added a 3rd order to it. That has to be 5th order.

                          Are you just saying it's not BW? That's what the rolloff matches.
                          Later,
                          Wolf
                          It would be much easier to demonstrate these crossover types using textbook slopes with no offsets. You need to start there if you want people to understand the differences. PCD will that very easily.
                          Click here for Jeff Bagby's Loudspeaker Design Software

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                          • #14
                            Re: Why does phase matter?

                            Two years ago I held a seminar that focused on taking and understanding measurements and on general crossover design and optimization. Maybe it's time for seminar #2 with a focus on understand phase response, diffraction, and ....... Could be of some interest.
                            Click here for Jeff Bagby's Loudspeaker Design Software

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                            • #15
                              Re: Why does phase matter?

                              More or less the reason I posted them is that Mattsk8 wanted some examples of what good modeled acoustic phase looks like, and I just labeled the types to differentiate them and why they looked different.

                              And yes- a workshop on phase and such might be interesting.....
                              Wolf
                              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                              *InDIYana event website*

                              Photobucket pages:
                              http://photobucket.com/Wolf-Speakers_and_more

                              My blog/writeups/thoughts here at PE:
                              http://techtalk.parts-express.com/blog.php?u=4102

                              Comment

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