Sub and LFE volume

Discussion in 'Archived Threads 2001-2004' started by Joe Barefoot, Aug 29, 2002.

  1. Joe Barefoot

    Joe Barefoot Stunt Coordinator

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    I've assumed this but would like input...In case I'm wrong. Bare with me....

    My pre/pro has a sub level and an LFE cut level. I can calibrate the sub to a given SPL reading by either adjusting the volume knob on the sub or by using the pre/pro. So I can turn the subs volume up with it's knob and decrease the sub level on the pre/pro to get the same SPL reading as before any adjustment. For example, I can get the same SPL reading with my subs volume knob turned half way up or 3/4 way up. I do this by using the pre/pro to attenuate the sub when it's volume knob is 3/4 up.

    Now, since my LFE cut is at 0dB, shouldn't I get more LFE with the subs volume knob set at 3/4, rather than 1/2. Stating that another way, with the sub pre out attenuated more, there is a greater relative difference of the sub and LFE. Therefore, for a given LFE signal, the SPL will be higher with the pre/pro sub level attenuated (and the volume on the sub turned up).

    I know I'm doing kind of a crappy job trying to explain this, but it seems to be an important concept for those who are not getting all of the LFE that they expect.

    I hope this helps someone. Or if I'm wrong, someone point it out and help me! :b
     
  2. Joe Barefoot

    Joe Barefoot Stunt Coordinator

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    OK, no responses in 3 days. Let me put it like this. (This assumes the LFE cut, if you have one, is set to 0dB.)

    1) How do you make your LFE louder?.... Turn the volume knob on the sub up.

    2) How can you turn the volume knob on the sub up without making the sub sound pressure level too loud (or out of proportion to your other speakers)? Turn your sub output on your pre/pro down (eg, attenuate it to -10 dB).

    3) By turning the subs volume knob up, and turning the sub out on the pre/pro down (attenuating it), and leaving the LFE cut on the pre/pro at 0dB, one can get a balanced sub level and more LFE volume. To get even more LFE, turn the subs volume knob up more and attenuate the sub out on the pre/pro more to properly calibrate the sub.

    The pre/pro has no idea where the subs volume knob is. So if the LFE cut is set to 0 dB, the same LFE signal is sent out from the pre/pro to the sub, whether the sub out on the pre/pro is attenuated or not. The sub and the LFE are two independent signals and can be calibrated independently through the pre/pro.

    Clear as mud? Again, if there is any disagreement, I'd like to hear it. I'm here to learn (and get my system sounding awesome). The reasons I responded to my own post:

    1) I think this is important for home theater
    2) I've never seen it explained anywhere in my reading
    3) If I'm wrong somewhere, I want someone to point it out.
    4) I'm going crazy and starting to talk to myself.
     
  3. Bill Kane

    Bill Kane Screenwriter

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    hi joe,
    well it's been a holiday weekend here in the States!
    there's not too much I can say that you dont know...but I think you're over-engineering this a bit...
    I believe the LFE "cut" is adjustment for DD (my Yamaha also has a Dts LFE set) that accounts for the original mastering and adds 10dB to the final output in DD; all rcvrs are spozed to provide this for DD. So typically we set it to max and forget it! Guy Kuo of Avia also suggests setting Dts LFE to max, typiclly +10dB and only adjusting it down if resultant Dts LFE sounds too boomy. HERE is an excellent post by Guy on these issues.
    Yes, the dual adjustment of rcvr's sublevel and the Sub's own amp gain control is a two-handed exercize. I think tht within these two ranges, however they are set on respective scales, when you achieve the subwoofer SPL you want, say boosted +5dB over the Main "reference" of your choice, that's the "loudness" you are looking for.
    So the two factors then are optimizing the low-level subwoofer (SubOut) voltage from the receiver and setting the sub amp gain at that amp's particular optimum. Moderator Vince Maskeeper advocates setting the sub amp as high as possible, but says he's coming from a "pro amp" school.
    One may find the receiver line-level voltage quite robust near the bottom of its range (my Yamaha sub level is set for -18 on a scale of -20 to 0 and my sub amp ends up one position past midpoint).
    So I look at the sub's manual first to see the suggested start-out level. SVS says start at 3/4.
    Then to play the middle-man game, I want my rcvr sub level to end up no more than halfway up its scale (1) knowing I'm getting "pretty good" voltage IAW that rcvr's parameters, and (2) I've set the sub amp gain as high as possible without forcing the rcvr level too low on its scale.
    Having the rcvr sub level in the middle allows convenience room while seated to adjust it up or down w/ the remote according to the media being played. I leave the sub amp gain control untouched after SPL set-up.
    When you say you are not getting all the LFE you expect, do you refer only to the .1 of 5.1 DD, the Low Frequency Effects, or do you also lump LOW BASS under the crossover frequency that you have in place?
    bill
     
  4. Joe Barefoot

    Joe Barefoot Stunt Coordinator

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    Hi Bill,

    To answer your last question, I'm getting plenty of both...sub crossover and LFE. I'm just trying to figure this out and help others.

    I agree with most of what you have said, but I believe we differ on one point (maybe not).

    The sub crossover and the LFE are two separate signals. The only thing that they have in common is the speaker that they are played through...the subwoofer.

    What you are saying, in essence, is that when you attenuate (or boost) your sub out, that the LFE is attenuated (or boosted) also. I don't think this is true.

    I first noticed this when I got my first SVS. I was playing with the subs volume knob to try to get the "auto on" to work correctly. I noticed that when I had the volume knob turned up, and the sub out attenuated, that my LFE was more prominent (or at least I thought so). That got me to thinking....

    It is more confusing, but as I said in my first post, with the volume knob turned up and the sub out attenuated, there is a larger relative difference in the sub out and the LFE. So if the sub is calibrated to it's proper level, the LFE will be louder....because of that greater relative difference.

    I emailed this to Tom V and he thought it was "an interesting theory" and would like to test it. To be honest, this has been a subjective finding. I need to sit down with a test disk and see if the LFE is actually louder by doing what I am proposing.

    Thanks for the input, and any more is welcome!

    PS I've read that post by Guy several times, and yes, it is an excellent post.
     
  5. Bill Kane

    Bill Kane Screenwriter

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    Joe,

    You might be onto something here. Perhaps it's pre-pro/rcvr dependent, or pertains to the sub amp's gain control design.

    OK, we know that the LFE chl as mastered is spozed to brickwall at 120Hz and that DD theater reference-calibration peaks are 115dB. Our bass management diverts all bass to the sub with appropriate rolloffs in the 80-90Hz range, which surely accommodates LFE since these generally are the LOW-LOW booms/rumbles etc.

    Then there's the auditory effect that lower frequencies become more difficult to perceive at lower volumes (hope I haven't oversimplified this). Fletcher-Munson Curve?

    It has been my assumption (I know, never assume) that as the Master Volume is decreased, the overall subwoofer output level for bass and LFE decreases in tandem with our mains, et al.

    Now to your observation: That the LFE signal "operates" independently of regular sub-120Hz or sub-80Hz bass signals in the terra incognita between rcvr subout level and sub amp gain level. Hmmmm.

    What amp does yr SVS use? What receiver?
    How do you manage media to get bass and LFE playing long enough to test or audit the relative loudness levels?

    Pls share whatever TV relates!

    bill
     
  6. Joe Barefoot

    Joe Barefoot Stunt Coordinator

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    Well, so much for my theory. This is the response from Ovation/AVIA. It appears that the sub and LFE levels are connected. I highlighted the part that I thought was most important to blow me out of the water. I'll forward this along to Tom V and see if he agrees...........


    Hi Joe

    The LFE adjustment in the decoder is NOT the level of the LFE channel sent out the subwoofer jack...

    The LFE channel is a channel just like all the others. Well, its a little different but not in so far as what we are concerned with here.

    Just as each channel needs to be balanced so that they are all at the correct level to one another, so to must the LFE channel be at the correct level.

    The "catch" (if you will) is that the LFE channel, unlike all the others, really does not have its own speaker. Sure 99% of the time it comes out of the subwoofer but in reality it is sharing that subwoofer with bass from all the other channels.

    When you calibrate your subwoofer level, you are doing so in relation to the 5 main channels. Specifically, you are getting the bass from those channel even with the treble coming from the speakers.

    Woa! If the level of bass from the main channels playing through the sub is correct, then the surround sound processors just needs to add the LFE channel to the bass it took from the main channels and shove it down the same subwoofer output and it too will be at the right level. Bingo.

    When you play your little game of lowering one subwoofer control and raising the other, the LFE channel comes along for the ride. If the sub level is correct, so is the LFE channel.

    Why does your SSP have an LFE trim (as I like to call it)? It is NOT for setting the level of the LFE channel since its level is fixed relative to the bass from the mains, and you set that when you set the subwoofer level. The LFE trim changes the level of the LFE channel relative to the bass from the main channels. "0" is correct. The ONLY time you touch that control is if your subwoofer cant handle all that bass. You can "relieve" it a little by cutting back how much LFE goes along for the rid.
     
  7. Bill Kane

    Bill Kane Screenwriter

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    whew! another thing not to have to worry about...

    Since the Dolby Digital encoding scheme masters at -10dB for LFE, and our AVR chipsets automatically add 10dB back in decoding, we know to set our receiver DD LFE trim to 0dB, typically the maximum provided on the trim scale range.

    A separate question is where to set Dts LFE trim if you have it on the AVR. I do, and my scale is -10 to +10dB. In an earlier query to Guy Kuo, he suggests also settiong the max here for Dts DVD tracks and 10dB below this for Dts music. I have this question also pending with Brian Florian on the avsforum.
     

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