Hi guys,
I'm stumped and I need your technical expertise. I attended a Cedia Home Theater Design function a few days ago and the Cedia instructor told us that the maximum dynamic peaks for each main channel in a home theater set up is 105 dB's and the subwoofer is 115 dB's (assuming you calibrate to 75 dB's at the listening position).
Actually have a thread on Audioholics as well :
Reference level - Page 3 - Audioholics Home Theater Forums (so far I'm not understanding the logic of some of the members)
I asked the instructor what if BM was utilized. If you redirect all the main channel bass (105 dB peaks per ch) to a subwoofer that must also voice LFE content, surely that 115 dB figure is no longer valid. He said that it shouldn't make much difference.
To ME that makes zero sense. Surely the required output and overall burden placed on the sub would increase if handling LFE + directed bass rather than LFE alone ?
I know this is a subwoofer forum but I'm hoping there are some bass management gurus here that can shed more light on this situation for me. You guys have helped me out before. I need to know (and understand) that I'm on the right track. Thanks.
I'm stumped and I need your technical expertise. I attended a Cedia Home Theater Design function a few days ago and the Cedia instructor told us that the maximum dynamic peaks for each main channel in a home theater set up is 105 dB's and the subwoofer is 115 dB's (assuming you calibrate to 75 dB's at the listening position).
Actually have a thread on Audioholics as well :
Reference level - Page 3 - Audioholics Home Theater Forums (so far I'm not understanding the logic of some of the members)
I asked the instructor what if BM was utilized. If you redirect all the main channel bass (105 dB peaks per ch) to a subwoofer that must also voice LFE content, surely that 115 dB figure is no longer valid. He said that it shouldn't make much difference.
To ME that makes zero sense. Surely the required output and overall burden placed on the sub would increase if handling LFE + directed bass rather than LFE alone ?
I know this is a subwoofer forum but I'm hoping there are some bass management gurus here that can shed more light on this situation for me. You guys have helped me out before. I need to know (and understand) that I'm on the right track. Thanks.
Log math, go figure.... 115db is ten times higher than 105db.
Even if you redirected EVERYTHING through that subwoofer,
would not be enough to kick it up from 115 to 116.
"Shouldn't make that much difference" was a correct answer.
Even if you redirected EVERYTHING through that subwoofer,
would not be enough to kick it up from 115 to 116.
"Shouldn't make that much difference" was a correct answer.
I think you need to check your math. If you sum 3 main channels (105 dB peaks) + 115 dB peaks the net sum would be roughly 120 dB's (in phase). 5 channels of 105 dBs + 115 dB's would be 123 dBs (in phase). You are saying that if the sub handles redirected bass from all channels + LFE that the levels wouldn't increase above and beyond 115 dBs ? How did you reach that conclusion ?
You're thinking it's additive, like when streams group together to form a river. It's not, more like a battle of the loudest. (huh...this is hard to explain without charts...) Think about a mixing board, if I've got 5 channels each with 105db at 0 on them (i'm trying to simplify, i know i should be using V) and group them to the mains at 0, i still only hear 105db, but if one of the channels has a 115db peak on it then i hear that sound at 115db while the rest of the sounds are being produced at 105.
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Hi,
My 2-cents of thoughts:
First: The LFE channel is NOT a "sub-woofer” channel designed with a lower but equal SPL headroom like the other channels L,R,C,Rs and Ls are identified with.
I find the quoted statement here below quite in order but the calculations conclusions made here-above is not at all conclusive.
I think You need to reconsider your assumptions on how to interpret this concept when calculating HT system levels.
Here is a IMO a link to a 2-page trustworthy article of what Bass-Management is:
Bass Management and the LFE Channel | Ultimateavmag.com
b 🙂
The "reference" is 105/115dBSPL peaks at your seat with a 0dBFS peak input signal. Distance and room gain must be factored into the equation for a clear picture of what the sub is really doing.
75/85dBSPL is not the "reference," just the measuring stick. A mile is still a mile when measured with a yardstick, or a meter stick.
You have coherent and incoherent electric signals. Most summed signals from different channels will be in-coherent to a large degree. During testing the only time it was close to being coherent, near a 0dbFS situation was between the R & L Mains, or a Center to R or L combo.
Electronic signals carrying multiple frequencies of sound at the same time are tricky. You have to think a about the infinite variations of amplitudes each individual frequency could be at, along with it's position in time(phase,) which are summing together to create the single level, at any one point in time. Now sum two together, and depending on the mixture the overall signal level may not increase more than a 1/4dB.
Also, adding in the fact that the DD processor itself only allows 5dB of headroom before clipping on the LFE channel throughput for the added BM, and you can see why someone would say it doesn't make much difference.
All that said, the burden on the subwoofer does go up when BM is engaged. More so in the intermediate SPL areas than at the absolute peaks though. If your sub could handle "reference" levels with no clacking in your setup without BM, it should live happily with BM engaged for the most part.
This assumes the proper use of a HP where needed, as the below 30Hz content seems to create the biggest burden on the sub with BM engaged, as there is a lot of low frequency content in the front mains absent from the LFE track, that the sub is now tasked with.
75/85dBSPL is not the "reference," just the measuring stick. A mile is still a mile when measured with a yardstick, or a meter stick.
You have coherent and incoherent electric signals. Most summed signals from different channels will be in-coherent to a large degree. During testing the only time it was close to being coherent, near a 0dbFS situation was between the R & L Mains, or a Center to R or L combo.
Electronic signals carrying multiple frequencies of sound at the same time are tricky. You have to think a about the infinite variations of amplitudes each individual frequency could be at, along with it's position in time(phase,) which are summing together to create the single level, at any one point in time. Now sum two together, and depending on the mixture the overall signal level may not increase more than a 1/4dB.
Also, adding in the fact that the DD processor itself only allows 5dB of headroom before clipping on the LFE channel throughput for the added BM, and you can see why someone would say it doesn't make much difference.
All that said, the burden on the subwoofer does go up when BM is engaged. More so in the intermediate SPL areas than at the absolute peaks though. If your sub could handle "reference" levels with no clacking in your setup without BM, it should live happily with BM engaged for the most part.
This assumes the proper use of a HP where needed, as the below 30Hz content seems to create the biggest burden on the sub with BM engaged, as there is a lot of low frequency content in the front mains absent from the LFE track, that the sub is now tasked with.
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revboden said:
It is additive. Have you heard of Pan's Law ? The rest of your post I didn't follow.
soho54 said:
Okay, but I am assuming the signals are coherent (read : Pans Law) and we are talking about bass information below the x-over point. Two electrical signals that combine in phase will net 6 dB's. The same with acoustical sources. I've done the math. If you take all three main channels + 115 dB (LFE peak) you net 120 dB's of peak level that is required. If you add 2 additional channels then this requirement is even higher.
What are you talking about ?
There shouldn't be any difference in peak level between BM engaged or disengaged. The only difference is that the sub will have to handle all redirected bass + LFE. If that means an extra 5-6 dB's of additional headroom, it's pretty darn significant to me. That isn't to say that you will require 120 dB's all the time but that it's the maximum possible peak level requirement from the sub at reference level with BM engaged (assuming a flat calibration).
The instructor basically gave me the impression that the sub requirements would remain the same with or without BM. That simply makes no sense. You can either share the burden by using 5 full range speakers handling bass content in their channels and a sub for the LFE specifically or you can tax the sub by asking it to handle all the redirected bass which is summed with the LFE and then low pass filtered. There is no free lunch with the BM approach. My instructor wasn't even aware that deep bass existed in the main channels. You want deep bass, especially in the main channels, then let me introduce you to the movie 'Pulse'.
Well I don't know about that. If the sub requires 123 dB's at reference level (with BM engaged and calibrated flat) that's a pretty big pill to swallow.
I'm not talking about HP filters. I'm talking about reproducing everything in those channels, as per the standard. If you truly want to make sure you have all your bases covered, you wouldn't include HP filters to protect the subs. We are talking full dynamic impact at reference level. It would seem to me that the requirements placed on the sub are extremely high. Not that I would attempt to play back at reference at these levels.
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Pan's Law isn't quite the same thing. Pan's Law deals with the output from the loudspeaker. Originally, a stereo pair.
If you play a signal of a single keyboard note at a set voltage on one speaker you get a set SPL. If you pan the signal dead center, and apply the same voltage to both speakers you get up to 6dB of extra total SPL in room, as you now have two sources playing the same exact signal.
If you instead paned the keyboard note to one side, and played a note from say a sax at the same voltage on the second speaker there is no real increase in SPL, as the two signals are not exactly the same.
In a BM situation you are not doing this. You are taking the two signals, and adding them back together. If they are exactly the same you get ~6dB extra if they are at the same amplitudes. If they are at different amplitudes you get less of a signal increase over the larger input.
If you sum the keyboard, and sax tracks together you won't get much of a signal level increase at all, as the frequency make up is different.
Soundtracks are more like the latter, as the LFE information is not a carbon copy of the bass on the other tracks. It has its own frequency makeup in its own time domain. Likewise, the other channels are rarely ever doing the same things at exactly the same times, other than some panning across the front three.
OdBFS is the digital ceiling.
The first step in the processor is to reduce the LFE channel by -5dB. If BM is active the re-routed signals from the main channels are reduced by -15dB, and summed into the LFE channel.
Thus, there is only a 5dBFS window above the standard LFE ceiling before clipping the DD processor. The +10dBFS that is talked about happens after the processor in the analogue domain. Where the LFE is bumped up +15dB.
Dolby Section 3.5: http://www.dolby.com/uploadedFiles/...PDFs/Professional/46_DDEncodingGuidelines.pdf
Umm, yeah there is. The peak levels your sub will see will be different. Are you saying there is no increase in SPL now when channels are summed? 😉
So the sub will need to play higher SPLs, but the peak level will remain the same? Think that through again.
Now you are saying the peak level will be higher?
It doesn't, and never will.
Yes, I was talking about them to qualify my statements before someone comes along to provide an example of a setup where it isn't valid without the conditional additives.
You seem a bit too confrontational about this. I was just trying to help. Have fun.
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soho54 said:
I am assuming amplitudes that are the same and in phase.
But what I am saying is that the peak level in the room shouldn't change. It should net the same result. My whole concern (my whole argument essentially) was the signal that the sub 'sees' once redirected content + LFE is summed and low pass filtered. It will take a world class magician to convince me that a sub handling LFE and one handling LFE + redirected bass won't require different capabilities. Clearly the requirements are steeper if using BM for the sub.
In theory, the peak level should remain the same either way, in the room. Obviously the subwoofer will have to shoulder the brunt of this vs each main channel handling it's own bass information but either way, BM or no BM, if done right, should net the same peak level.
Just because the bandwidth exists doesn't necessarily mean the mixer will make full use of it. Some soundtracks might require 108 dB peaks at reference. Some higher. Some lower. Then you have some like WOTW that require over 120 dB's and take everyone by surprise. 🙂
Nope. 😀
dB Pascals
115.00 11.246827
105.00 3.556559
105.00 3.556559
105.00 3.556559
105.00 3.556559
105.00 3.556559
Summed Value dB = 123.24
If there was a simultaneous 0 dBFS peak in all 5 speaker channels and in the LFE channel, and if they were all routed through the same transducer (i.e., the subwoofer) in order to avoid phase cancellation issues, they will sum to 123.24 dB. Actually if you route the redirected content to multiple subs, that's even better. But the fact is, you need one HECK of a capable sub in order to do this.
Yes I seem confrontational but that's because I've been trying to defend my position all day. 🙂
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I'm sorry, but I am not sure what you are trying to get across now.
The FP contained this, "I asked the instructor what if BM was utilized. If you redirect all the main channel bass (105 dB peaks per ch) to a subwoofer that must also voice LFE content, surely that 115 dB figure is no longer valid. He said that it shouldn't make much difference.
To ME that makes zero sense. Surely the required output and overall burden placed on the sub would increase if handling LFE + directed bass rather than LFE alone ? "
The answer here is that you are both right, as I tried to explain in my first post. The LFE "reference" level does not change because BM is activated. It is a reference point it, as such it does not change. In that sense it doesn't make much difference, in fact it makes zero difference.
The amount of SPL the subwoofer is asked to reproduce does change though. If your sub can't handle the new load you won't be able to play "reference" levels. It will be the lower frequencies that will determine the subs abilities to keep up, as all subs are program SPL limited by their low end capabilities, and using BM tends to alter the low end signal makeup more so than the higher frequencies.
You will never have a full strength exact copy of a bass signal present on all channels on a commercial disk. You won't find it across the front three, and LFE either. If you want to play hypothetically, you would find that with BM turned on with this full-on signal you would just clip the LFE/BM channel out, as it is limited the LFE dynamic range plus 5dB. See the link in my last post to a dolby paper explaining their DSP engine throughput.
I actually have a disk I made myself with such a signal on it, and the reason it isn't on my Test DVD at AVS is because it was useless for most people, unless you wanted to see how much hard clipping your sub could take before melting.
I'm seriously trying to help here, but you keep zigging to my zag. 😉
The FP contained this, "I asked the instructor what if BM was utilized. If you redirect all the main channel bass (105 dB peaks per ch) to a subwoofer that must also voice LFE content, surely that 115 dB figure is no longer valid. He said that it shouldn't make much difference.
To ME that makes zero sense. Surely the required output and overall burden placed on the sub would increase if handling LFE + directed bass rather than LFE alone ? "
The answer here is that you are both right, as I tried to explain in my first post. The LFE "reference" level does not change because BM is activated. It is a reference point it, as such it does not change. In that sense it doesn't make much difference, in fact it makes zero difference.
The amount of SPL the subwoofer is asked to reproduce does change though. If your sub can't handle the new load you won't be able to play "reference" levels. It will be the lower frequencies that will determine the subs abilities to keep up, as all subs are program SPL limited by their low end capabilities, and using BM tends to alter the low end signal makeup more so than the higher frequencies.
You will never have a full strength exact copy of a bass signal present on all channels on a commercial disk. You won't find it across the front three, and LFE either. If you want to play hypothetically, you would find that with BM turned on with this full-on signal you would just clip the LFE/BM channel out, as it is limited the LFE dynamic range plus 5dB. See the link in my last post to a dolby paper explaining their DSP engine throughput.
I actually have a disk I made myself with such a signal on it, and the reason it isn't on my Test DVD at AVS is because it was useless for most people, unless you wanted to see how much hard clipping your sub could take before melting.
I'm seriously trying to help here, but you keep zigging to my zag. 😉
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Instead of quoting line by line (😉), I'll agree 110% with you. The requirements from the sub change, but the total peak output remains the same in room.
There are movies with significant bass in all channels. I don't know what level the peaks were recorded at, but movies like Pulse have super low bass in each channel. I don't think its impossible for a film to have all channels peaking, but it is improbable, I would agree with you there.
Then, I suppose, if we try to ascertain a more probable scenario for the expected output from the sub in the vast majority of cases, I would assume (since I don't know) the level requirement from the sub with BM engaged would be closer to 118-119 dB's. This is all hypothetical, but it's a figure I would imagine. I know members on AVS with flat room response calibrated flat and still achieve movie peaks of over 120 dB peaks in movie sountracks (with BM engaged). Sometimes much higher (123-124 dB peaks but this is uncommon ).
Thank you for helping. Not my intention to zig and zag. 🙂
There are movies with significant bass in all channels. I don't know what level the peaks were recorded at, but movies like Pulse have super low bass in each channel. I don't think its impossible for a film to have all channels peaking, but it is improbable, I would agree with you there.
Then, I suppose, if we try to ascertain a more probable scenario for the expected output from the sub in the vast majority of cases, I would assume (since I don't know) the level requirement from the sub with BM engaged would be closer to 118-119 dB's. This is all hypothetical, but it's a figure I would imagine. I know members on AVS with flat room response calibrated flat and still achieve movie peaks of over 120 dB peaks in movie sountracks (with BM engaged). Sometimes much higher (123-124 dB peaks but this is uncommon ).
Thank you for helping. Not my intention to zig and zag. 🙂
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conditionals make all the difference 😛 Yes, if you are playing the same tone through five separate speakers which would give you additive summing in room, then if you asked the sub to reproduce that effect it would need to produce a spl that would simulate the in room summing.
Thing is... there are compressor/limiters on the LFE+BM circuits to protect the sub from such demands. When you sum LFE and BM you get less low frequency sound in room because of the compressor/limiters than if you kept them separate, ie. pan law.
I've heard an optomized HT that had truly full range (20hz-30khz) speakers on all seven channels and it was truly moving (pun intended) when a coherent low frequency effect was put through all 7.1
Thing is... there are compressor/limiters on the LFE+BM circuits to protect the sub from such demands. When you sum LFE and BM you get less low frequency sound in room because of the compressor/limiters than if you kept them separate, ie. pan law.
I've heard an optomized HT that had truly full range (20hz-30khz) speakers on all seven channels and it was truly moving (pun intended) when a coherent low frequency effect was put through all 7.1
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The LFE ch and bass in the other main channels are separate. We can all agree that the LFE on it's own will never exceed 115 dB's no matter what. But I'm talking abouit redirected bass + LFE. If there is coherent bass mixed in a film and that scene requires 118 dB's from the sub + redirecteed bass, then that will be the requirement. If a film like War of the Worlds requires 122 dB's at reference (using BM) in a few scenes then that will be the requirement.
Remember, there should no difference in peak output in the room between BM or no BM. So if the total in room peak is 120 dB's the same requirement should be asked of the sub + redirected bass (ie no limiting of that and no protection otherwise you've just messed up the mix).
Remember, there should no difference in peak output in the room between BM or no BM. So if the total in room peak is 120 dB's the same requirement should be asked of the sub + redirected bass (ie no limiting of that and no protection otherwise you've just messed up the mix).
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revboden said:
That doesn't make sense. There should no difference whatsoever in peak output between the two configurations. You've just shifted all the burden to the sub. That's the only difference so the requirements are that much higher.
And everyone should have active tri-amped mains and surrounds... but they don't.
If a person has a system that needs BM they probably don't have full range speakers (20hz-20khz) on all 5-7 channels so they would not notice a loss of low frequency SPL when switching between LFE only and LFE+BM to the sub because they didn't have it in the first place. Actually, they would notice an increase in low frequency SPL using LFE+BM.
if anyone is following this academic 🙄 vs real world discussion it continues at: Reference level - Audioholics Home Theater Forums
Yes, and many of the people on Audioholics seemingly don't understand what they are saying. It's funny what happens when you just bury your head in the sand and accept what a few people say without investigating the matter.
In any case, Soho54 at least grasps the situation a lot better than most of the guys there. In a world where one assumes that LFE + redirected bass is the same as LFE....then you have a problem. Also, the math was all over the place there.
Fortunately there are people who do agree with my views, guys like Ed Mullen. I recently followed an AVS thread on this very subject and lo and behold, the same issues cropped up (same inaccurate math too) which was then addressed. So my math is correct.
Fact is, it seems unlikely that the sub would require 123 dB peaks at all times with all movies using directed bass + LFE. However the expected peak levels will most certainly be higher than 115 dB's in the LFE alone (assuming flat calibration and a flat response in-room) assuming the mixer maxed out the LFE and included deep bass in the main channels. That seems to be the trend....to just add louder and deeper bass in movie soundtracks.
I know plenty of people with full range speakers that still set their speakers to 'small' because whichever way you cut it the scenario always works out better (ie same output levels sound cleaner and less stressed, more linear playback).
Also, the bass response in a room with 5 full range speakers playing back similar bass info in each (assuming the mix was created that way) would be a disaster in-room. Positioning of 5 full range speakers for best bass response would most likely yield crappy imaging.
In any case, Soho54 at least grasps the situation a lot better than most of the guys there. In a world where one assumes that LFE + redirected bass is the same as LFE....then you have a problem. Also, the math was all over the place there.
Fortunately there are people who do agree with my views, guys like Ed Mullen. I recently followed an AVS thread on this very subject and lo and behold, the same issues cropped up (same inaccurate math too) which was then addressed. So my math is correct.
Fact is, it seems unlikely that the sub would require 123 dB peaks at all times with all movies using directed bass + LFE. However the expected peak levels will most certainly be higher than 115 dB's in the LFE alone (assuming flat calibration and a flat response in-room) assuming the mixer maxed out the LFE and included deep bass in the main channels. That seems to be the trend....to just add louder and deeper bass in movie soundtracks.
I know plenty of people with full range speakers that still set their speakers to 'small' because whichever way you cut it the scenario always works out better (ie same output levels sound cleaner and less stressed, more linear playback).
Also, the bass response in a room with 5 full range speakers playing back similar bass info in each (assuming the mix was created that way) would be a disaster in-room. Positioning of 5 full range speakers for best bass response would most likely yield crappy imaging.
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