Best method for determining subwoofer phase

Discussion in 'Speakers & Subwoofers' started by MuneebM, Jun 9, 2004.

  1. MuneebM

    MuneebM Supporting Actor

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    I used Avia to check if my HSU STF-2 is in phase, but I'm not sure I've got it right. Avia says that if the test tone sounds "balanced", then its in phase and if the lower frequencies are more audible OR if the test tone drops significantly in level then its out of phase. Well here's my dilemma, with the STF-2's phase switch set to 0, the lower frequencies are more audible indicating out-of-phase, but when I switch to 180, the test tone decreases in level by about 3-4 dB on the SPL meter but it definitely takes on a more balanced sound.

    So which is right? I'm leaning towards 180 because it sounded more balanced, despite having dropped in level by 3-4 dB. With the switch set to 0, I can really hear the subwoofer and easily separate the subwoofer from the front left speaker, which I assume means the "lower frequencies are more audible".

    Thanks

    http://66.46.69.23/sigserv/pl/index.pl?p=1
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  2. Brian L

    Brian L Cinematographer

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    MuneebM,

    Didn't like my answer over at AVS?[​IMG]

    BGL
     
  3. Lee Carbray

    Lee Carbray Second Unit

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    Also try the warble tones they provide. I found them more useful. Also when you do a sweep the level should remain constant throughout the crossover point. As you do a sweep watch your spl meter and choose the setting that provides the least SPL variation. You could also burn some sine wave test tones onto a disk and plot the frequency responce for both settings. As you can see there are many ways to skin a cat. It sounds like 180 is correct though.
     
  4. MuneebM

    MuneebM Supporting Actor

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  5. Wayne Ernst

    Wayne Ernst Cinematographer

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    Damn, I came across a method for adjusting/setting the phase a little over a month ago. Now, if I could just remember where that information was posted. [​IMG] [​IMG]
     
  6. Lee Bailey

    Lee Bailey Second Unit

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    If you use the Avia DVD, here is the info from Guy Kuo on setting up your sub:

    AVIA intentionally carries its low bass test signals on each of the five main channels independently instead of the LFE channel for the express purpose of making main channel bass reproduction more accurate. Understanding why this is important means knowing the difference between three different concepts.

    a. The signal going to the subwoofer

    b. The low bass which is on each main audio channel (left front, center, right front, left surround, right surround).

    c. The low bass on the LFE channel.

    Most "newbies" (and I think a lot of pretty well experienced home theater buffs) get confused and think that the subwoofer is used purely for LFE when it actually should also be used to help the other channels reproduce the low bass in the main channels. Indeed, it is possible to have low freq effects carried completely on the mains and nothing on the optional LFE channel.

    AVIA is designed to get the highest fidelity out of the main channels - the channels which carry the vast preponderance of sound. By placing the low bass test information solely on the main channels, one is able to isolate and check the bass management of each channel and ensure that all are being appropriately handled in a system.

    The setting of speaker size and bass management tremendously affects how bass is handled for each channel. Speakers set to "small" have their low bass routed to the bass output channel (s) which can be either just the subwoofer , the "large" speakers or some combination of subwoofer and large speaker depending on how the receiver is set up.

    We highly recommend setting all speakers to "small" because the very very low bass content down to the 20's Hz simply is not as well reproduced by most main speakers as a dedicated sub. Even if one has powered subs inside the main speakers, room placement of those speakers is rarely if ever the best location for bass reproduction. By setting the speakers to "small" you give each speaker a chance to excel in what they do best.

    If a speaker is set to "large" its low bass content will not be sent to the subwoofer output. Remember this! AVIA's subwoofer tests are on the main channels. If a particular channel is set to large, then that channel's AVIA test tone is not routed to the subwoofer (just like low bass on that channel is not routed to the subwoofer). This means that the AVIA subwoofer test behaves exactly the same way as live material to be played back on that channel and you can examine that behavior to learn how real material behaves. This also means that playing AVIA subwoofer tests for a channel set to large produces nothing on the sub.

    Speakers set to "small" have their low bass routed to either the sub and/or "large" speakers depending on how you have set up the processor to handle bass. AVIA subwoofer tones in channels set "small" behave exactly the way regular bass material in that channel does.

    So keep track of how you set your speakers and bass management. AVIA's signals will be routed exactly that way.

    It is frequent for people to find that low bass in their various channels is not routed in ways they previously expected. Listen to what the AVIA tones do and you'll discover what is actually happening.

    During calibration of your subwoofer level, you usually don't have an ability to independently adjust the strength of bass coming from each main channel. This can mean that you find the SPL reading of subwoofer tests vary depending on which channel is being tested. The most common reason for this is a difference in how the bass is being processed (one channel is large while the other is small). Since you can't independently adjust each, it is reasonable to either average for the front three channels or simply concentrate on getting the front channel right since that one carries the most work in a movie soundtrack.

    If nothing comes out of the subwoofer during AVIA tests, it's most likely because all the speakers are set to "large." That prevents routing of low bass in all the channels to the subwoofer. Only the LFE channel would play out of the subwoofer.

    The LFE channel is normally not independently calibrated. Some systems allow you to set things between -10 dB and 0 dB (normal). Ordinarily, leaving the LFE at 0 dB gives good results once the main channel bass has been calibrated.
    If LFE must be independently calibrated, use the Low Freq 6 Channel Sweep in AVIA (Title 6 chapter 29). This test has a discrete LFE channel signal to verify that LFE level is correct relative to the other 5 channels of bass content. All should read the same SPL.

    (For the overly knowledgeable ... the LFE signal is already pre-compensated -10 dB in intensity so it yields the same SPL as the other channels if LFE gain is at 0 dB. No need to worry about the 10 dB playback boost, it's been accounted for)

    Getting the subwoofer to main channel sound pressure levels correct is only the beginning. There is a critical overlap frequency range for each channel which is around the crossover frequency. At those frequencies both the main and subwoofer are involved in creating sound simultaneously. Proper blending of the mains and subwoofer requires that the sound from both subwoofer and main speakers be in phase. Otherwise the main speaker and sub cancel each other out in that frequency range and one creates a hole in overall system frequency response.

    AVIA allows detailed testing of response at the frequency crossover range for both phase adjustment and frequency selection. This can be done for each of the main channels, but usually getting it right for the center speaker is sufficient if you have already correctly positioned the left and right front speakers to be in phase with the center speaker.

    The low frequency phase tests in AVIA have acoustic energy throughout the usual bandpass region. If the subwoofer is out of phase with the speaker channel being tested, partial cancellation of some frequencies will occur and the sound will tend to de-emphasize some frequencies in the test tone. Set phase of the subwoofer to make the sound have the widest audible range of frequencies. It is impossible to do this alone. An assistant is required to flip the phase control while you listen for this to work correctly. It simply takes too long to walk up to your sub, flip the phase, and return to your seat. Instant A/B comparison is needed.

    Lacking an assistant, you can use your SPL meter by positioning it at your normal head position. Note the SPL reading as the test tone is played in each phase. Select the phase with the higher reading as that is the one with the least overall cancellation of sound between the two speakers (main and subwoofer). If your sub has a variable phase control rather than a 180 degree switch, slowly adjust the phase to maximize the SPL reading. At that point the sub and main speaker are at best phase coherence.

    If you note little or no change as phase is adjusted, make sure that the delay or distance settings for all your speakers are correct. If they are grossly wrong, the system may be delaying one signal so far that it will never come into phase.

    The choice of crossover frequency is often fixed at 80 Hz. Some pre/pro's or subs have an adjustable crossover. The warble test tones in AVIA help you find a good crossover point. The tests waver up and down in frequency but not in amplitude. They sweep through the crossover range and well below. The warble effect allows the signal to act as its own comparison level. Bass response is often "spikey" in a room with small changes in frequency making a large change in response. The warble lets you detect unevenness more easily. If you hear a big appearance/disappearance of the test tone, then the warble is sweeping through a frequency range with a large inequality in response. Play the warble tone and adjust the crossover freq to make overall response as flat as possible during the first (higher freq) portion of the test. That should be the portion which is affected by the crossover.

    It may be necessary to redo phase after adjusting crossover frequency. The two controls will interact. Once crossover freq and phase are adjusted, go back and redo the subwoofer level adjustments. They will also be affected.

    As you can see, AVIA was designed to accomplish a much more comprehensive subwoofer adjustment than previous calibration discs. The end result is better integration of the subwoofer with the main speakers and reduction of the "separate" subwoofer sound. Unfortunately, many users expect a quick and dirty method. You can indeed do that with another disc, but should you short thrift your hard earned equipment that way?. We chose to provide a more complete tool, and the end result is better subwoofer integration, but it takes time and forces users to discover what truly is happening to their bass.

    I hope this helps people realize why we didn't oversimplify and duplicate the test tone on both the LFE and main channels at an arbitrary ratio. Doing things the way we did in AVIA requires greater understanding about your equipment, but ultimately yields more accurate calibration and enlightening information than other means.

    Guy Kuo, Ovation Multimedia
     
  7. Jon Lane

    Jon Lane Stunt Coordinator

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    If you have a 80Hz freq available, there's an easy phase setting routine:

    1. Set your active crossovers (sub, receiver, pro., etc.) to 80Hz or some convenient frequency in that area.
    2. Reverse your main speaker leads to place them 180 degrees out of phase with the rest of the system.
    3. Drive them and the sub with a pure 80Hz tone (or whatever frequency you chose above.)
    4. Adjust sub phase to minimum output.
    5. Switch your main speaker back in phase.

    Think it thru and you'll realize that assuming your crossovers are set as good as can be (not as easy as it seems) this method nails the exact point at which the phase is optimized for the crossover frequency in question.

    The trick is first getting a symmetrical slope environment without ripple at the transition from mains to sub(s), and that's not always easy. If your sub has a common 4th-order (-24dB/octave) LR filter, then you want to get the sum of the main speakers' acoustic highpass and processor's electronic highpass to SUM to an equal but opposite 4th order highpass.

    To illustrate, the ideal lashup starts with a sealed satellite with a natural 80Hz highpass function, which also has a 2nd order highpass rolloff, preferably with low Q. Add in another 2nd order electronic highpass setting at the processor, again set to 80Hz and hopefully Q=0.5, and you could have a total 4th order LR highpass function overall...just like the sub's complimenting lowpass function.

    IIRC, The resulting symmetrical 4th order LR split sums to flat amplitude and flat power, and will allow this null-test method of establishing phase to work just about perfectly.

    Other combos will also get you close, but the 4th order LR is in-phase and flat so it lends itself naturally to this method of setting variable phase.

    It can't be overstated that not knowing the crossover region's behavior means mismatching the mains (set to small) and the sub. You must try and first mesh the two halves of the system into a flat overall response before phase means much. If you hang a series of overlaying response functions on top of one another (acoustical plus electronic plus another electronic, etc.) you'll almost certainly get ripple and illdefined response through the crossover region.
     
  8. Kevin C Brown

    Kevin C Brown Producer

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    I agree with the discrete test tone method. (I have CDs from Stryke and Autosounds 2001.)

    Play test tones at or near the crossover frequency. Read the SPL at your listening position. Now change the phase knob or switch on your sub. Read the SPL again. You want the highest SPL.

    I thought Avia's phase tests to the sub weren't very useful, btw. Between speakers, yes, to sub, nope.

    Also, Chesky has a DVD-V/DVD-A test disc, that in my opinion, has the best phase tests between speakers and the sub:

    http://www.chesky.com/catalog/body_c...274&CATEGORY=2

    Don't even need a meter for these. Easy to tell by ear.
     
  9. Ned

    Ned Supporting Actor

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    Is there anything cheaper than SVS's Bass interface box for us passive sub users? Does the Behringer FD or DEQ have phase adjustments?
     
  10. Richard_M

    Richard_M Second Unit

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    Hi Ned...



    I don't know about any cheaper units, but in the Behringer range the DCX 2496 has all the tools you require, including delays and variable phase, as well as xovers, and Dynamic filters.

    One issue with the DCX is it only accepts pro levels, therefore requires some attenuation at the output. Although it has +/- 15dB level controls on all in & outs.
     
  11. Kevin C Brown

    Kevin C Brown Producer

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    There actually is a "pseudo" way to change phase. And new receivers with the auto eq/distance stuff do it automatically (Pioneer, Yamaha, etc).

    You just need to change the distance setting to your sub until the phase is the best. The equivalent of a continuously variable phase knob. I remember one person saying that after the auto thing on his Denon, it set his sub as 17 ft away. It did that to get the best phase between the sub and his speakers.

    One thing that isn't clear though, is if after that kind of phase adjustment, if the sub is still time aligned to the speakers though. [​IMG]
     
  12. Edward J M

    Edward J M Cinematographer

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    I have conceptually struggled with this issue myself, and have come to the (quite possibly erroneous) conclusion that achieving the best possible frequency response (over the sat/sub transition bandwidth) at the listening position should be Job 1.

    I had all sorts of FR irregularities in the 50-100 Hz region when integrating the PB2-Ultra with both Polk RT800i and Polk LSi7's. I tamed the FR curve in the transition bandwidth with a combination of the phase control, the on-board PEQ, and the low pass filter.

    I think many filters (such as those used in PEQ's and crossover networks) impose a time delay (and hence a phase shift). And some filters do not have a constant time delay at all frequencies over their bandwidth (i.e., they will exhibit group delay) and will therefore behave differently at different frequencies.

    All that time/phase stuff gives me indigestion, so I stopped fretting over it and simply decided to shoot for a flat FR at the listening position using the available controls at my disposal.
     
  13. MuneebM

    MuneebM Supporting Actor

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    Thanks for all the responses!

    I tried the simplest test last night. I just measured the SPL of an 80 Hz test tone, which is what the xo is set to in my AVR. Turns out the 180 setting was best as that resulted in the higher SPL reading. Although Avia was right afterall, it was a little difficult to determine it with the test tone on Avia, as others have mentioned above.

    My Yamaha's YPAO feature can adjust phase by setting the distance to the sub, but even after allowing it to set the distance automatically with the sub's phase switch set to 0, the 80 Hz test tone proved that the 180 setting was better. Does that make sense?

    http://66.46.69.23/sigserv/pl/index.pl?p=13
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  14. BruceD

    BruceD Screenwriter

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    Jon and Edward really highlighted the major problem I have with most implementations of prepro bass management even today.

    Mismatched high-pass/low-pass xover implementation

    These days, I wish the prepro manufacturers would provide a menu selection for a symmetrical 4th order L-R electrical xover (low-pass and high-pass) in the prepro so we wouldn't have to deal with the mismatch of the prepro's 2nd order high-pass xover and the speaker's ?? acoustical rolloff.

    To solve this problem (primarily on my mains, towers with -3dB @32Hz), I've gone to a separate active crossover (4th order L-R) between mains and sub.

    I set mains to Large, center and surrounds to Small, and set sub to NO (prepro LFE-out left empty). With the active xover set @60Hz, and also set to SUM L+R bass, the prepro SUMs the LFE + re-directed bass and sends it to the mains.

    An additional benefit -- this config also works for 2-channel, important for me as I'm 80/20 2-channel/HT.
     
  15. Edward J M

    Edward J M Cinematographer

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    I might be going your route too, Bruce. Getting a flat FR over the transition bandwidth was a real challenge.

    I ran sweeps on just the mains with them set to small at 40, 60, 80, 100, and 120 Hz. Then I ran sweeps of just the subwoofer under the same conditions. The sub showed one peak at 63 Hz which I knew was not coming from the mains (i.e., room induced). So I killed that with the PEQ.

    The mains showed an emphasized response in the 50-100 Hz region, so I brought the sub on line and started messing with the CV phase control and different electronic xo points to see how it all interacted. Pretty fascinating stuff, really, with some unexpected anomalies at certain stages of tuning.

    I also enabled the low pass of the SVS and experimented with that at various settings. Ultimately, enabling the low pass at 120 Hz, setting the CV phase at 100 degrees, and using the PEQ to knock out the 63 Hz room peak suddenly resulted in an (almost) flat response from 200-35 Hz. Praise the Lord and Pass the Ammunition.

    MuneebM - it is more than likely that the phase setting which gives you the most reinforcement at the selected xo frequency (in your case 80 Hz), may not be the phase setting which gives you the best frequency response at the listening position. Some subwoofer manufacturer's who include a CV phase control in the feature set specifically recommend that the user deliberatly set the mains and the subwoofer partially out of phase to provide just enough cancellation to obtain a flat response.

    Whether this is good advice, or if it causes time smearing and incoherency in the bass response, I don't know. All I know is that a flat FR across the sat/sub transition sounds way better to my ears than an irregular FR, and if implementing controls like CV phase, PEQs, LPFs, and the like cause time shifts and phase changes, then I'm willing to live with them because of how much better the flatter FR sounds.

    As with everything, YMMV. [​IMG]
     
  16. Brian L

    Brian L Cinematographer

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    Bingo!

    You do what you can with the tools you have. What more can anyone do?

    Most users can't tweak Xover slopes, are unwilling to trade their existing vented loudspeakers for sealed boxes, and don't have any EQ at their disposal.

    There was a pretty long recent article in either Audio Critic (I think!) that covered all the reasons why it was basically impossible to dial in a sub/sat system in a typical user's system.

    Made me want to toss the whole thing and get a mono AM radio[​IMG]

    But then I fired up some music I liked and thought, theory be damned! I have my rig tweaked with EQ to be as flat as possible from 20hz to 800hz at the main listening chair.....which is the best I can do with the tools I have.

    And I like it, it sounds good to me, better than anything I have had previously. I know it could be better...maybe a lot better, but not with the tools I currently have to play with, in the room I have.

    But the future is bright. Has anyone looked at the DEQX products? My local NAD dealer is also a DEQX dealer, and he has been bending ny ear on this stuff. Looks like it could really be the holy grail, in that it actually functions as a loudspeaker crossover, a sub/sat crossover, room EQ...I think it even makes popcorn.

    Here is a link to his site, with some discussion of the product:

    http://forum.adnm.com/viewforum.php?...b0abf7705e4b53

    The only down side is cost. A basic 2 CH rig is $3500, and thats just for processor. Do it in a 5.1 rig, and its way north of what my budget is going to permit, not to mention that it basically bi and tri amps everything. Lots and lots of amp channel needed to do one of the rigs properly.

    I heard some B&W 603's running bi-amped (the native crossovers were disconnected) with this rig, and they sounded fabulous. Nothing to compare them to really, but on their own merits, playing some Mapleshade recordings, and the sound was wonderful.

    Hopefully there will be a trickle down affect on this stuff...and soon.

    BGL
     
  17. Jon Lane

    Jon Lane Stunt Coordinator

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    And with room EQ, we've added yet another variable to this already potent mix. There's a risk that premature EQ will add complexity to our efforts and move us away from our target.

    Electronic equalization can actually confuse the result by merely adjusting what's really an inherent mismatch in the initial setup. IOW, having a good understanding of the underlying mesh between subwoofer and satellite is the primary goal. Only when that's set correctly should we, IMHO, equalize the environment.

    The setup order I'd suggest goes like this:

    1. Research the crossover region by determining the natural responses of the subwoofer and satellites. Do not include any electronic crossovers -- defeat them all and either measure or have the manufacturer tell you the exact rollout point and attenuation rate of the sub and the sats.

    2. Using these natural acoustic slopes as your starting point, try to add just enough electronic equalization to make up a total 4th order, LR, 24dB/octave lowpass and highpass transition between sub and sats. This crossover type is flat in both SPL and power, and is so with an in-phase connection. (Normal hookup.)

    3. Having approximated this function to reasonable accuracy, do the 80Hz phase test I mentioned earlier in this thread.

    4. Adjust levels for a reasonable flat average response across at least 3 or 4 octaves, beginning with the sub's lowest frequency.

    5. Only now should you attempt to equalize room infuences away.

    You can see how if room EQ precedes setting the attenuation slopes the result probably won't give you the combo of flat SPL (what your mic reads and what you hear in your listening chair) and flat power (what the system distributes in 360 degree space.)

    It's a challenge, but it pays off in the end with a smooth, accurate response.
     
  18. BruceD

    BruceD Screenwriter

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    Jon,


    By the way, I agree that a 24dB/octave L-R high-pass/low-pass transition is a very good choice for the sub-to-main xover.

    I have never found this to be very feasible with common HT equipment. Why? Because there is typically no way to adjust the order/slope of the prepro's electrical high-pass unless you have Meridian or Theta prepros ($$$$$). Can you recommend methods to adjust the electrical high-pass slope? Fmods are not options IMO.


    I couldn't 5 years ago and this is why I chose to use a brute force method (Marchand XM9-L active xover 4th order L-R) and minimize the effect of the acoustical rolloff of the mains (towers not sats) by crossing over at least 1 octave above the -3dB.
     
  19. Jon Lane

    Jon Lane Stunt Coordinator

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    Like I said, it's not an easy problem to solve, but it does speak to the heart of the setup solution.

    Phil Marchand makes an economical way to adjust bass rollout called the Basis. IIRC, it could "prepare" a satellite to simulate a number of slopes prior to dialing in the other crossovers elsewhere in the system.
     
  20. Kevin C Brown

    Kevin C Brown Producer

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    Ahh, ever think about the common 24 db/octave low pass, 12 dB/octave high pass crossover like this?

    I believe that this is actually the best way to do it. Why? Because lets choose an 80 Hz crossover as an example. 24 dB/octave low pass means 24 dB over 80 = 24/80. 12 dB/octave high pass = 12 dB/40 Hz. => 24/80 = 12/40! Symmetrical. Obviously, dB is a log quantity, however, for small ranges, you can use a linear fit.

    THX 80 Hz is not magic. All that is happening is that the slopes of the electronic crossover *add* to the slopes of the speakers.

    This is why a lot of people [​IMG] feel that if you *don't* use THX speakers, that the crossover freqs chosen *must* be at least 1/2 to a full octave away from the -3 dB point of the speakers to make sure that the slopes imposed by the electronic crossover do not interfere (or add to) the slopes from the speakers. The 24 db/octave low pass, 12 dB/octave high pass crossover *is* designed to give flat freq response, at least electrically.

    So if a non-THX speaker has a -3 dB point of 80 Hz, ain't no way an 80 Hz crossover is going to work right. There will be a dip in response there.
     

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