# EQ sub to LPF of LFE?



## ToBeFrank (Feb 27, 2010)

Since the LFE channel can go up to 120Hz, should I be EQing the sub to 120Hz rather than to my 80Hz crossover for the mains?


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## glaufman (Nov 25, 2007)

Theoretically, no. At the same time as your AVR diverts main channel bass <80Hz to the sub, it should divert LFE > 80Hz to main channels.
As a practical matter, however, it's rumored that many AVRs don't.
Even so, IF you wanted to sacrifice the localization to fix this, you would increase the crossover to 120Hz instead of simply EQing it flat to 120Hz. Since a xover isn't a brick wall but a gradual rolloff, the truth is your xover will still be sending some content >80Hz to the sub, and if you EQd the sub flat to 120Hz, that content would get blown out of proportion.


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## ToBeFrank (Feb 27, 2010)

glaufman said:


> Theoretically, no. At the same time as your AVR diverts main channel bass <80Hz to the sub, it should divert LFE > 80Hz to main channels.


If that is the case, why would I even need a LPF on the LFE? It is my understanding that since the LFE is a separate track limited to 120Hz, the only time it would get redirected to the mains is if I tell the receiver I don't have a sub. Otherwise the LFE track only goes to the sub and anything filtered by the LPF of LFE is simply gone.



> As a practical matter, however, it's rumored that many AVRs don't.


This seems like correct behavior to me if I have the sub turned on in the receiver.



> Even so, IF you wanted to sacrifice the localization to fix this, you would increase the crossover to 120Hz instead of simply EQing it flat to 120Hz. Since a xover isn't a brick wall but a gradual rolloff, the truth is your xover will still be sending some content >80Hz to the sub, and if you EQd the sub flat to 120Hz, that content would get blown out of proportion.


If I EQ the sub to follow a 120Hz 24dB/octave LPF (by setting the mains crossover to 120Hz for EQ, then setting it back to 80Hz after EQ), shouldn't the frequency response still properly follow the 80Hz LPF for the mains tracks and also the 120Hz LPF for the LFE? I don't understand how it would get blown out of proportion.


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## laser188139 (Sep 19, 2009)

I think what Greg was indicating was that, if you equalized to a curve of 120Hz while the AVR crossover was set to 80Hz, you would be elevating the content above the crossover... not what you want. 

You're right, if you want to equalize the sub for the content above 80Hz in the LFE channel, as with content in the main channels just above the rolloff where the sub is still contributing, you could set the AVR crossover to 120Hz, equalize to that curve using REW, and then adjust the AVR crossover back to the preferred value, e.g., 80Hz. 

I suspect that most people using an external equalizer don't go to the trouble. Better to use whatever capacity is available in the region where it matters most. I have verified that the Audyssey automated equalization in my AVR does adjustments up to 162Hz on my sub channel, even when the crossover is set to 80Hz.


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## glaufman (Nov 25, 2007)

ToBeFrank said:


> If that is the case, why would I even need a LPF on the LFE?


That's why. You generally want to HPF the mains and LPF the LFE. Generally you set these to the same frequency. That's what makes a xover. That's why many AVRs simpyl have a setting labelled Xover, and don't give you individual controls. the "Mains Crossover" isn't a crossover with an LPF. LPF+HPF=Xover.


> It is my understanding that since the LFE is a separate track limited to 120Hz, the only time it would get redirected to the mains is if I tell the receiver I don't have a sub. Otherwise the LFE track only goes to the sub and anything filtered by the LPF of LFE is simply gone.


If your manual states that this is how your equipment operates, that's fine. But in general, you don't want any more LFE content above 80Hz going to the sub than you want mains content above 80Hz going to the sub, for the same localization reasons.


> If I EQ the sub to follow a 120Hz 24dB/octave LPF (by setting the mains crossover to 120Hz for EQ, then setting it back to 80Hz after EQ), shouldn't the frequency response still properly follow the 80Hz LPF for the mains tracks and also the 120Hz LPF for the LFE?


I suppose.


> I don't understand how it would get blown out of proportion.


I thought you were talking about leaving the xover at 80Hz, and EQing the sub to match a 120Hz curve.


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## glaufman (Nov 25, 2007)

laser188139 said:


> I think what Greg was indicating was that, if you equalized to a curve of 120Hz while the AVR crossover was set to 80Hz, you would be elevating the content above the crossover... not what you want.


How'd you beat me to that post?


> You're right, if you want to equalize the sub for the content above 80Hz in the LFE channel, as with content in the main channels just above the rolloff where the sub is still contributing, you could set the AVR crossover to 120Hz, equalize to that curve using REW, and then adjust the AVR crossover back to the preferred value, e.g., 80Hz.
> 
> I suspect that most people using an external equalizer don't go to the trouble. Better to use whatever capacity is available in the region where it matters most. I have verified that the Audyssey automated equalization in my AVR does adjustments up to 162Hz on my sub channel, even when the crossover is set to 80Hz.


The other thing is, this really isn't any different than eq'ing with an 80Hz xover to an 80Hz target curve.


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## ToBeFrank (Feb 27, 2010)

laser188139 said:


> I think what Greg was indicating was that, if you equalized to a curve of 120Hz while the AVR crossover was set to 80Hz, you would be elevating the content above the crossover... not what you want.


Got it and that makes sense. Definitely not something I would do.



> I suspect that most people using an external equalizer don't go to the trouble. Better to use whatever capacity is available in the region where it matters most. I have verified that the Audyssey automated equalization in my AVR does adjustments up to 162Hz on my sub channel, even when the crossover is set to 80Hz.


Fortunately, I've got plenty of headroom. After the BFD and Audyssey my sub volume knob is only at 40%. I've also found that Audyssey does a much better job when I get the frequency response really nice before I run it.



glaufman said:


> That's why. You generally want to HPF the mains and LPF the LFE. Generally you set these to the same frequency. That's what makes a xover. That's why many AVRs simpyl have a setting labelled Xover, and don't give you individual controls. the "Mains Crossover" isn't a crossover with an LPF. LPF+HPF=Xover.


From everything I've read when researching Audyssey, this isn't correct. I know that Chris from Audyssey states that the LPF of LFE should always be set to 120Hz and that manufacturers shouldn't even give the option of changing it from that. The reason is because you could lose content since the track can go up to 120Hz.



> If your manual states that this is how your equipment operates, that's fine. But in general, you don't want any more LFE content above 80Hz going to the sub than you want mains content above 80Hz going to the sub, for the same localization reasons.


Perhaps I'm a purist, but if a BluRay contains 120Hz sound in the LFE track, I want to hear it, not drop it. I know it's localizable, but I want to hear it as it was intended.



> I thought you were talking about leaving the xover at 80Hz, and EQing the sub to match a 120Hz curve.


Yep, laser cleared that up.



> The other thing is, this really isn't any different than eq'ing with an 80Hz xover to an 80Hz target curve.


I don't see how as I'm EQing 80-120Hz. I've just finished this up and both the 80Hz LPF and the 120Hz LPF now follow the target curves very well. My mains+sub response is also better.


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## ToBeFrank (Feb 27, 2010)

BTW, I hope I'm not sounding ungrateful or argumentative. The help is much appreciated.


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## laser188139 (Sep 19, 2009)

ToBeFrank said:


> ... I don't see how as I'm EQing 80-120Hz. I've just finished this up and both the 80Hz LPF and the 120Hz LPF now follow the target curves very well. My mains+sub response is also better.


Certainly equalizing above the crossover at 80Hz should smooth the transition from sub to main, and give a better combined response. 

I expect Greg is right, that it should come out the same, whether you raise the AVR's crossover, smooth the curve, and lower it back, or equalize directly to a target curve with a rolloff frequency of 80Hz. When I've played with the Filter Tasks options in REW, it will find peaks above the crossover at 80Hz, assign filters, and optimize gains and Q factors. It may be easier to see what is happening the way you did it, though, as the target curve is then flat over the range you intend to equalize. 

I'm glad it sounds better,
Bill


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## glaufman (Nov 25, 2007)

ToBeFrank said:


> Fortunately, I've got plenty of headroom. After the BFD and Audyssey my sub volume knob is only at 40%. I've also found that Audyssey does a much better job when I get the frequency response really nice before I run it.


If you or Audyssey is applying a lot of boost anywhere (including the channel trims), there may not be as much headroom as you think even at 40% on the master volume. 


> From everything I've read when researching Audyssey, this isn't correct. I know that Chris from Audyssey states that the LPF of LFE should always be set to 120Hz and that manufacturers shouldn't even give the option of changing it from that. The reason is because you could lose content since the track can go up to 120Hz.


I'm not an expdert on Audyssey, so that may be, but that would be a mistake IMO. But it really depends on whether this LPF is a true LPF, or is part of the xover. If it's a standalone LPF, then by all means set it at max. But ideally it would have processing to take any filtered content and redirect it to main channels. Why would anyone want to simply filter out and lose content?


> Perhaps I'm a purist, but if a BluRay contains 120Hz sound in the LFE track, I want to hear it, not drop it. I know it's localizable, but I want to hear it as it was intended.


I agree, I want to hear it too. But because of the localization issues I want to hear it through main channels. Again, just because it's in the LFE channel doesn't mean it's intended for the sub. There's a reason it's called an LFE channel and not a SUB channel.


> I don't see how as I'm EQing 80-120Hz. I've just finished this up and both the 80Hz LPF and the 120Hz LPF now follow the target curves very well. My mains+sub response is also better.


You can EQ up to 120Hz without doing what you've said by leaving the xover at 80Hz and EQ'ing to the apprpriate target line in REW. Same thing, but as Bill pointed it it's probably easier your way to preserve SN ratios and the like.


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## ToBeFrank (Feb 27, 2010)

glaufman said:


> If you or Audyssey is applying a lot of boost anywhere (including the channel trims), there may not be as much headroom as you think even at 40% on the master volume.


That's interesting. I didn't know that. How would I know if I don't have enough headroom?



> But it really depends on whether this LPF is a true LPF, or is part of the xover. If it's a standalone LPF, then by all means set it at max.


That was Chris's (the Audyssey guy) point. The LPF on the LFE is just that, not a xover.



> But ideally it would have processing to take any filtered content and redirect it to main channels. Why would anyone want to simply filter out and lose content?


Agreed.



> You can EQ up to 120Hz without doing what you've said by leaving the xover at 80Hz and EQ'ing to the apprpriate target line in REW. Same thing, but as Bill pointed it it's probably easier your way to preserve SN ratios and the like.


Now I understand what you're saying. Makes sense.


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## glaufman (Nov 25, 2007)

ToBeFrank said:


> That's interesting. I didn't know that. How would I know if I don't have enough headroom?


There's a chance you'd already know. There's a chance bad things would happen. What are your trims at? How much boost is being applied where? Chances are good that you're ok, didn't mean to scare you. Well, maybe I did a little... 


> That was Chris's (the Audyssey guy) point. The LPF on the LFE is just that, not a xover.


That's a shame. I wonder if this (as with some other things) is left open to the implementation by the MFR, and could therefore vary with MFRs? For instance, one common problem with Audyssey is that it often sets speakers to "large" meaning full range when most often they should be set to "small" ... Audyssey has been quoted as saying that this is something that's left up to the implementation, it's the MFRs fault, not Audyssey's... so...

If you're in contact with him, maybe you could ask "why?"


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## laser188139 (Sep 19, 2009)

ToBeFrank said:


> ... How would I know if I don't have enough headroom? ...


That is a much harder question to answer than it is to ask. Most people indicate that it is obvious when you reach it, as the sub sounds obviously different when something in the system becomes non-linear. 

Audyssey equalization limits its gains and cuts to +9dB and, I believe, -20dB. You can see this for yourself by measuring the AVR's sub LFE line out directly, comparing Audyssey Off to Audyssey On. Be sure to remove the mic calibration file when taking preout measures: if you don't this will be obvious when the Audyssey Off curve is not flat away from the crossover. 

In addition to its equalization, Audyssey offers DynEQ which increases the bass and high treble levels so the sound is perceived identically as the volume is reduced below reference. In my case, at the 75dB levels normally used with REW, I observed an additional 12dB boost in the bass below 20Hz. The actual amount varies depending on the volume level of the AVR. 

Some people have observed voltage clipping at the output stage of their AVR or the input stage of their sub. Obviously the easiest way to observe clipping is with an oscilloscope. Using REW to generate a sine wave well below the crossover and with Audyssey Off, you can measure the voltage on the LFE lineout and correlate this a volume setting on your AVR. Line out levels are nominally 1V, so you can calculate at what volume level you might be going beyond this. (I thought about using the REW scope function as an oscilloscope for this, but it is very likely the soundcard line in starts clipping before the receiver does.) The sub manufacturer should be able to give indication of how much voltage its input stage can accept. 

REW drives one or two channels. If you have a multichannel setup, don't forget that adding speakers will add content to the sub channel. With only two channels, the LFE content predominates in determining peak levels, but by the time you get to a 5.1 setup, the other channels could theoretically contribute as much as the LFE channel. 

As regards the real question, of what level the sub accepts, this may be measured in independent reviews you find trustworthy. Typically these are outdoors or anechoic near field measures, so you need to adjust these based on room gain and how far you sit from the sub. You can, of course, try to take your own measurements. Ilkka used to do these for popular subs, and described his methodology here. In particular, his max output level and power compression test illustrates how one can raise the sweep level in +5dB steps, until the frequency response curve stops increasing linearly with the input/volume level. Someday I may try to do this myself, but so far I've been reluctant to push my sub that hard. 

In the DIY sub arena, another suggestion is to measure the excursion of the sub directly, correlate this with sound/voltage levels, and calculate the maximum level possible before the excursion limits are reached. 

I think that's all the points I've seen on this subject. Good luck,
Bill


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