HT school of thought

Discussion in 'Home Theater Projects' started by Dave_Gib, Jul 1, 2004.

  1. Dave_Gib

    Dave_Gib Stunt Coordinator

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    in car audio you want the music to sound like it is coming from in front of you

    so you cross your subs over as low as your midbass drivers will allow

    as far as choosing your midbass driver you are much more limited in a car than you are in a house. being able to put an 8" woofer in the door and still angle it where you want, physically having enough room for the magnet to clear the window in your door, or having kick panels big enough for the speaker, and still clear the clutch pedal

    but in a house you have much more room, many people have full range front speakers way more capable than what you find in a car

    So why do so many people cross their HT subs over at 80 Hz?
     
  2. ThomasW

    ThomasW Cinematographer

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    Because that's the THX standard XO frequency, and it's a XO frequency found in many, many, receivers and pre/pros.

    To ensure there's no hole in the FR, the main speakers need adequate response one full octave below the XO point. That means for a 80Hz XO point, the mains need good output to 40Hz. Frequently main speakers don't have good output lower than that.
     
  3. Mark gas

    Mark gas Second Unit

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    Because in a car it's hard to put a sub upfront. If you could put a sub up front you would cross it higher.
     
  4. BruceD

    BruceD Screenwriter

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    I know Thomas can explain this a whole lot better.

    I always wondered how well the receivers 80Hz xover worked since the THX designed 80Hz xover requires main speakers with a 12dB/octave rolloff starting @80Hz. This characteristic 12dB/octave rolloff (slope) is provided by THX certified speakers or sealed (acoustic suspension) speakers with a -3dB of 80Hz.

    Many of us (me included) certainly don't have speakers that meet those criteria (not even close for me). I have tower mains with a -3dB of 32Hz.

    While this may be less of a problem for typical surround speakers (less low-bass anyway), it seems to be one of the major problem areas for smoothly blending sub(s) with main speakers (at least for me it was).

    Since the low-pass half of the xover (for the sub) is defined as a 24dB/octave slope in the receiver (required for THX certification), the high-pass half (especially for the main speakers) needs to also be a 24dB/octave slope for the total response (sub+speakers) to be smooth across the crossover boundary, i.e. a consistant SPL level through the crossover boundary like the 80Hz example we are discussing.

    The problem is that the receiver only has a 12dB/octave slope for it's high-pass (required for THX certification), expecting the speakers (especially mains) to provide the other 12dB/octave rolloff that sums with the receiver's to a total 24dB/octave slope. This just isn't the case for most of the speakers many of us use.

    Many times this typically means we get a hump (SPL increase) over some range of frequencies before, through, and after the crossover, causing excess boomy bass over some range of frequencies. In other cases you may get a frequency hole (SPL decrease) because the speaker is rolling off faster than 12dB/octave.

    I'm using part of the strategy Thomas recommended, doing the xover to the sub at least an octave above the -3dB of the mains, but I'm not using the prepro's internal xovers to the sub-out jack.

    Instead, I send all re-directed bass and LFE to the main speakers (mains=large, sub=no) and put a separate electronic 24dB/octave L-R crossover (Marchand) between the mains and sub(s). This way I get a symmetrical (high-pass and low-pass) 24dB/octave xover. The benefit is it also works for 2-channel doing it this way. By the way I do the xover @60Hz.

    Wish more prepro/receiver manufacturers offered a true 24dB symmetrical xover as an optional selection in the menu.
     
  5. ThomasW

    ThomasW Cinematographer

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    I should add that like Bruce, I don't use the LFE out from my pre/pro.

    I use Marchand active XO's with 24dB/octave slopes
     
  6. Wayne A. Pflughaupt

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    Another reason is because you generally want only one source generating low frequencies. The more speakers you have running into the low frequencies, the worse bass response sounds.

    Regards,
    Wayne A. Pflughaupt
     
  7. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I've always been wondering if the LFE channel on Dolby and DTS tracks were engineered to purposely go to a separate subwoofer in which the user can adjust "only" the subwoofer channel.

    If you have full range front speakers, you can't adjust the subwoofers phase, filters, volume, and XO as easily as if you had a separate subwoofer with an amp that commonly has these features.

    Not to mention one of the biggest benefits of having a separate subwoofer is the placement relative to the room and listener. With a full range speaker you can't tune your bass response by playing with speaker position as much.

    If I had to choose the crossover frequency of the subwoofer, I'd choose around 80hz because it is in this range of frequencies in which the sound interacts the most with the room. This means I can position the subwoofers so the sound can be more balanced for the 80hz and below range.

    If I crossed over my sub at a very low 30hz, then my mains would be playing a lot of the bass, and I wouldn't be able to position them in the room for sound because I need them in a certain position for imaging.

    Crossing the subwoofer too high, and then you'd have to choose drivers than can sound good doing so. This will probably sacrifice the subwoofers deep bass capabilities or cost more.. so it's really about trade offs.

    80hz is fine by me. It's supposed to have a borderline directional sound at that frequency. Use dual stereo subs, and you don't have to worry about directional upper bass.
     
  8. Darren_T

    Darren_T Second Unit

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    I personally cross my mains over at 60Hz... It depends on the extension capabilities one has in their mains. I find that 80hz is too easy to localize in my room. And since my mains have good extension down to about 50Hz I cross them over at 60Hz which relieves my sub of the extra burden above that therefore improving it's performance... this is my opinion of what I've heard in my room. I'm not making "factual" claims.
     
  9. Aaron Gilbert

    Aaron Gilbert Second Unit

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    I'm going to buck the trend of both conventional and unconventional thinking. My system actually sounds the best with all 'large' speakers set as such, and with the front L/R bass duplicated by the subwoofer. My front L/R and surround L/R speakers (in room, of course) have relatively smooth response to about 30Hz, rolling off slightly below that, before dropping off sharply below 20Hz or so. My surround back speakers are only smooth down to about 40Hz, dropping off quickly below. While my duplication of front L/R bass in the subwoofer is primarly so the subwoofer works while playing CD's, this mode still sounds better than setting the front L/R to small.

    The primary benefits I've noticed by running the speakers as large are:

    - There is a better blend between the subwoofer and satellites.
    - The room is filled more evenly with bass, i.e. there are less spots with big peaks or nulls. The bass just seems more enveloping, as if it's coming from all over the room (which actually it is). [​IMG] Note that the subwoofer alone is not bad in this regard, just that now it's better.
    - Overall system dynamic range (for bass) is noticeably better, as is maximum output. Distortion at high levels is reduced. Note that these benefits will not be attainable by many systems. It's a result of my front L/R and surround L/R each having two 7" woofers. At the listening position, I've measured output from a single front speaker at 107dB at 20Hz. Combined, my six large satellites (center is small) can move fully 96% as much air as my subwoofer.

    All that said, someday when I have a giant room and less limited funds, I'd love to use external crossovers as Thomas and Bruce do. In fact, my dream would be a 7.7 speaker setup, with an external crossover and subwoofer in every channel. [​IMG]


    Aaron Gilbert
     
  10. Wayne A. Pflughaupt

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    Aaron, have you ever considered:

    Assuming for the sake of argument that both fronts and rears have flat response down to the limits stated, this means that four speakers are generating down to 40Hz, but only two of the four speakers are significantly generating down to 20Hz

    Since total output is cut in half between 40-20Hz, this means that response is sharply dropping below 40Hz and is probably down several dB at 20Hz.

    Theoretically, you’d have much flatter low frequency response if only the fronts were generating the lows.

    Of course, the sub’s output factors into this, too.

    Regards,
    Wayne A. Pflughaupt
     
  11. Aaron Gilbert

    Aaron Gilbert Second Unit

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    Wayne,

    I don't follow you here. Why are only two speakers generating down to 20Hz? My front L/R and surround L/R are identical, it's only the surround backs which lack below 40Hz. Further, why is total output cut in half between 40-20Hz? For that to be true, half of the sub 40Hz bass in any given soundtrack would need to be in the surround/surround back channels. Even if I followed your assumption that only two satellites are producing from 40-20Hz, I don't see how this can possibly be the case. My center channel bass is routed to the subwoofer, there's a lot of bass in the 0.1 channel, and there is certainly more bass in the front L/R than in the surrounds. I have all the surrounds set to large for that occasional movie which does put strong bass in the rear channels, for instance Attack of the Clones, Underworld, Independence Day, etc.

    I'm not saying your theory is wrong, I just don't see where it's coming from at the moment. [​IMG]

    Cheers,

    Aaron Gilbert
     
  12. Wayne A. Pflughaupt

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    My bad, I didn’t notice that you have rear “backs” in addition to rear “surrounds.” But the theory still works.

    Let’s say that each additional speaker gets you another 3dB of gain, output, SPL or whatever you want to call it. In other words, starting with say, the LF speaker with a reference output of 84dB: Adding the RF gets an additional 3dB, and adding the LR/RR surrounds gets another 6dB. Kick in the back surrounds (assuming two of them since you referred to them in the plural) gets you 6 more dB. So, your reference output is now at 99dB.

    However, at 40Hz the rear backs sharply roll out. That means only four speakers are now generating between 40-20Hz, so response below 40Hz has dropped 6dB.

    Theoretically low frequency response would be flatter if only the four that go down to 20Hz were carrying the lows.

    Of course, even this would assume that the four had perfectly symmetrical placement in the room. If one is closer to a corner than the others, its low frequency response/roll-off point is not the same as the other speakers that are not. Same if one speaker is say, close to an open doorway. And so on.

    Bottom line, people who think everything sounds better with multiple full-range speakers are usually shocked to see how bad things really are when they take some measurements.

    That’s not to say it can’t be done, but it's usually very difficult and requires hours of experimental placement, equalizing, etc.

    Regards,
    Wayne A. Pflughaupt
     
  13. Greg Monfort

    Greg Monfort Supporting Actor

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    Hmm, FWIW here's my take on this:

    Adding a source nets you up to ~ +3dB in the BW where they are acoustically very close (< WL/pi). To get ~ +6dB requires four closely coupled drivers, then eight for +9dB, etc., with the effected BW shrinking with increasing number of sources, so the theoretical dB gain = 10*log (number of sources). With speakers spread around the room and the LFE being at a higher level though, it won't be accurate enough, especially over a several octave BW.

    Instead we need to sum each channel's SPLs at the listening position:

    10*log (10^(dB1/10)+10^(dB2/10)+10^(dB3/10), + etc.).

    Assuming I did the math right (always iffy with me, so one of you math whizzes might want to double check me) and all the channels (six in this case) except the LFE are set to the same SPL, which is set 10dB higher, I get +12.04dB gain over whatever the reference is. Dropping one of the channels off-line other than the LFE only represents at most a 0.28dB loss, which of course won't be noticed.

    Note that the LFE channel's gain needs to be at the end of the summing chain since it will only add it's output occasionally for impact. Also, even though the CC is set to 'small', it needs to be in the chain since its LF BW is routed to the LFE channel, ergo if all the channels are set to 'small' then the sub needs to be able to handle 10*log (number of channels) over the LFE reference chosen.

    GM
     

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