Reference level

[Vaughan Odendaa]

As per the article, the maximum bass attained can be up to 120 dB's with no bass management (without taking dialnorm into account that figure may jump much higher than that). This should be no different with bass management. The only difference is that each channels low bass has been redirected so the subwoofer will be required to handle that burden.

Obviously 115 dB's is not going to cut it at reference level if you use bass management. Whether you want to achieve reference level or not is up for debate but it's not part of this topic. I'm discussing what the absolute highest peaks will be in a home theater environment at reference level using bass management (calibrating to reference standard).

 

[Vaughan Odendaa]

If you're considering just pure signals, then you are correct that the sum will be greater than 115. From what I gather, you want to combine one 115 dB tone with 5 105 dB tones (all at say, 40 Hz). Adding those together will yield an spl of ~117 dB. You can use this calculator

The conversion from dB to Pascals is required in order to sum SPL :

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 (not 117 dB's)

In fact, if you summed only three main channels + 115 peaks that would be around 120 dB's. Add in the additional channels and you bump right up to 123+ dB's. Assuming this is all in phase (no reason to assume otherwise).

If you want to get nitpicky, that would be the absolute peak (if calibrated correctly).

 

[Vaughan Odendaa]

Here is another bit of interesting information from Dolby themselves :

http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/38_LFE.pdf

"The LFE channel carries additional bass information to supplement the bass information in the main channels. The signal in the LFE channel is calibrated during soundtrack production to be able to contribute 10 dB higher SPL than the same bass signal from any one of the screen (front) channels. Even if all three screen channels are active, enough bass could be delivered by the LFE channel
alone to bring the theatre’s subwoofer into acoustic balance with the screen channels. This allows filmmakers to unburden the main channels by diverting the strongest bass to the separate LFE channel, as needed. Under the most demanding program conditions, where the bass is fully loading the left, center, and right channels, the LFE channel could increase the bass intensity by up to 6 dB.

Have no idea what you guys are on about.

 

[skers_54]

The conversion from dB to Pascals is required in order to sum SPL :

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 (not 117 dB's)

In fact, if you summed only three main channels + 115 peaks that would be around 120 dB's. Add in the additional channels and you bump right up to 123+ dB's. Assuming this is all in phase (no reason to assume otherwise).

If you want to get nitpicky, that would be the absolute peak (if calibrated correctly).

You don't have to convert units. You just have to know how to manipulate logs. Or, you can use your method but you have to add the square of the pressure from the channels and take that sum over the square of the reference pressure because the sound sources are not co-located/incoherent. You used a formula for co-located/coherent sound sources. See below

http://www.sengpielaudio.com/calculator-leveladding.htm

The quote from Dolby is describing how the LFE channel increases the output over the MAINS. Three 105 dB signals combines to 109 dB, which is 6 dB less than the max output of the LFE channel. It is not describing a 6 dB increase over the LFE output.

 

[Vaughan Odendaa]

The quote from Dolby is describing how the LFE channel increases the output over the MAINS. Three 105 dB signals combines to 109 dB, which is 6 dB less than the max output of the LFE channel. It is not describing a 6 dB increase over the LFE output.

Your math is off. If you combine three 105 dB signals you get 114 dB's (assuming in-phase) + 115 dB's for the LFE and what do you get ? You get 120 dB's of bass information ! What did that article say I referenced earlier ? The article said that all three channels fully loaded....the LFE could add an additional 6 dB's. That adds up to 120 dB's ! This does NOT take the surround channels into account.

You could get more gain in the bass if you have "the same" bass in the surround channels, but that assumes a few things.

 

[digicidal]

I don't know and couldn't give a tiny rat's about whose math is off, but as I read through your numerous posts I do have a question about your premise. Even if the summed output is 2 million db... that's the summed output. You would still only be requiring the same 105db per distinct channel and 115db for LFE in order to achieve it. Say you have 100 channels all adding to the same 40Hz tone... with each having a peak level (independent of each other) of 105db... your requirements would simply be equipment capable of delivering peaks of 105db per channel.

Now on the other hand if you are saying that in order for the LFE channel to be audibly 10db louder than the total summed output of all the discreet channels at a given frequency... then I can see the issue you are raising. However, this is not a problem from an install or equipment specification standpoint. Provided that the amp and speakers are capable of delivering the 105db peak levels, and the amp/sub is capable of driving a 115db peak level - even if what you can measure in-room is a 120db level... there is no greater demand being placed on the system.

I'm not now nor do I ever intend to become an expert on the technical requirements of system certification at a given referrence level - I'm simply someone that enjoys good sound and doesn't ever listen to any of those levels anyway.

However, your arguments concerning the math seem to be avoiding something much more basic... at least to my (admittedly limited) understanding. Take it for what it is - a simple observation.

 

[WaynePflughaupt]

Your math is off. If you combine three 105 dB signals you get 114 dB's (assuming in-phase) + 115 dB's for the LFE and what do you get ? You get 120 dB's of bass information !

No, his math is correct: If you combine three acoustical sources generating 105 dB, the total SPL is 109.7 dB. Check it yourself and see: Generate a pink noise signal from one of your speakers and take an SPL measurement. Then add a second and you’ll see a ~3 dB increase in the reading.

What you’re talking about here is something entirely different, the combined electrical signal from the various channels being summed as a single output. How that plays out from an acoustical output standpoint in a home theater is wholly unpredictable from one system to the next. For instance, in one system that “summed signal” could be dumped into a single subwoofer. In another it might be split out to multiple subwoofers, each one capable of generating an additional 3 dB SPL over what the single sub might with that “summed signal.”

As digicidal notes (as did I previously), ultimately none of that matters, as there are accepted guidelines that dictate the proper calibration of a home theater system. Of course, people are free to tweak settings to their own taste.

Regards,
Wayne A. Pflughaupt

 

[Vaughan Odendaa]

Even if the summed output is 2 million db... that's the summed output. You would still only be requiring the same 105db per distinct channel and 115db for LFE in order to achieve it. Say you have 100 channels all adding to the same 40Hz tone... with each having a peak level (independent of each other) of 105db... your requirements would simply be equipment capable of delivering peaks of 105db per channel.

I'm talking the sub here. Not the speakers...only the sub requirements. With speakers set to 'large' and handling all the bass within those channels, the requirement is 105 dB's (with bass information). The second you redirect that information to the subwoofer is the second you have just changed the level of burden placed on the subwoofer ! It can't get any more simpler than that. Redirected bass + LFE does not equal same burden as LFE on it's own. It can't. Makes no sense at all.

 

[Vaughan Odendaa]

Check it yourself and see: Generate a pink noise signal from one of your speakers and take an SPL measurement. Then add a second and you’ll see a ~3 dB increase in the reading.

The math tells me I should get a 6 db gain when adding two equal signals (coherent) You might get only 3 db with random frequency random phase pink noise, but I have seen a real world, measured at my seat 6 db gain when I measure sine sweeps on 2 channels verses 1.

In any case, we are discussing the requirements from the sub if handling LFE and redirected bass. Total in room SPL levels should be about the same with or without BM being activated but when bass is redirected to a subwoofer, the subwoofer needs extra capability as compared with a straight LFE channel with no redirected bass. There is no free lunch.

Now I'm being told that the sub won't be required to have any extra capability if handling LFE + redirected bass instead of just LFE. Doesn't make any sense at all.

 

[Matt34]

None of this makes much sense to me.

I think the entire paragraph needs quoted:

The signal in the LFE channel is calibrated during soundtrack production to be able to contribute 10 dB higher SPL than the same bass signal from any one of the screen (front) channels. Even if all three screen channels are active, enough bass could be delivered by the LFE channel alone to bring the theatre’s subwoofer into acoustic balance with the screen channels. This allows filmmakers to unburden the main channels by diverting the strongest bass to the separate LFE channel, as needed. Under the most demanding program conditions, where the bass is fully loading the left, center, and right channels, the LFE channel could increase the bass intensity by up to 6 dB.

 

[Vaughan Odendaa]

Are you saying that I don't make any sense ?

 

[Matt34]

Are you saying that I don't make any sense ?

Not you, just the information available. I've looked a a few Dolby pdf files I've Googled and none really address this specific issue you have brought up.

I do know that 115db+ is freaking loud and the only time I've been able to achieve those kind of numbers are when the family is out of the house.

 

[AcuDefTechGuy]

The sub is what's getting the LFE chanel through the RCA cable while the actual main channel speaker still gets its normal wiring.

Corner loading a sub in the left & right is usually a good place, but conversely, the same is not true for the main L/R.

1) I hook up the BP7000 using a single simple speaker wire directly from the amp without using any kind of RCA cable. The internal Linkwitz crossover automatically directs the bass (LFE) to the sub. There is absolutely no need to even use the RCA/LFE hookup. The BP7000 is not a modual speaker like the RBH T2, which is a bookshelf sitting on top of a subwoofer.

2) Corner loading the sub may be good in an extra large room, but I don't think it is good for a 15' x 16' room because I think it makes the bass a little "boomy". Moving the subs out from the walls makes the bass "tighter" IMO.

As far as the 115dB LFE Max Reference, I think we are forgetting that a movie soundtrack (and music) is very dynamic and every movie soundtrack is different. I think the 115dB is more like the "AVERAGE" or "nominal" max reference, not something absolute set in stone. It may be 115dB max most of the time, but sometimes it may get to 120dB or only 110db throughout the entire movie, depending on the movie soundtrack itself.

 

[AcuDefTechGuy]

I do know that 115db+ is freaking loud and the only time I've been able to achieve those kind of numbers are when the family is out of the house.

when I had the SC Trinity, the only time I've achieve 115dB is when I'm outside the room with the doors closed and with earplugs on.

 

[skers_54]

The math tells me I should get a 6 db gain when adding two equal signals (coherent) You might get only 3 db with random frequency random phase pink noise, but I have seen a real world, measured at my seat 6 db gain when I measure sine sweeps on 2 channels verses 1.

In any case, we are discussing the requirements from the sub if handling LFE and redirected bass. Total in room SPL levels should be about the same with or without BM being activated but when bass is redirected to a subwoofer, the subwoofer needs extra capability as compared with a straight LFE channel with no redirected bass. There is no free lunch.

Now I'm being told that the sub won't be required to have any extra capability if handling LFE + redirected bass instead of just LFE. Doesn't make any sense at all.

You don't have coherent signals. Real speakers are separated in space which makes them by definition incoherent and gives a 3 dB SPL increase when combining identical sources. You'd need all the speakers sitting on top of each other to get the 6 dB increase you're describing, which isn't how HTs are set up. At least, my LCR and surrounds are not all sitting right on top of my subwoofer.

So, 117 dB would be the equivalent output if all 5.1 speakers were playing the same signal at their respective reference outputs. This is also what your subwoofer would theoretically have to provide to replicate this situation. It is NOT what's going on in the electrical signal chain before the amplifier.

Moving this into the electrical realm, you have to consider digital clipping. -0 dB is the absolute max and corresponds to 115 dB acoustical output at reference. Bass management occurs in the digital realm and will result in clipping if you exceed -0 dB, which is highly likely in your scenario. Gene ran into this with the Oppo BDP-83SE.

http://www.audioholics.com/reviews/transports/high-definition-dvd-players-hd-dvd-blu-ray/oppo-bdp-83se

Like Wayne said earlier, the electrical performance will be case-dependent meaning not every piece of equipment will exhibit this behavior. However, this issue would show at any volume because this type of clipping occurs before the preamp stage. Yet this isn't an issue on program material for anyone, even at volumes below reference. This shows good evidence that multiple full-scale signals are not routinely combined and redirected to the LFE channel.

 

[Vaughan Odendaa]

Whether the mixing engineer maxes out the LFE or the main channel's bass is up for debate. Obviously this depends on the movie. Some movies have very loud and deep bass recorded in the main channels together with LFE and this seems to be a popular trend nowadays.

To make matters worse, some movies are recorded hot. But setting that aside, if you talk about the absolute maximum possible peak you would expect in a home theater environment, calibrating to 75 dB's per ch and using bass management (and assuming the disk was mastered at the correct levels), can we all agree that the max dynamic peaks attainable would be 120 dB's from the sub (handling all redirected content from the three main channels, in phase with the LFE ?) Because I'm talking about the maximum peak level possible.

Something that is not really discussed is the dialnorm situation. Dialnorm is not used in theaters. Movie theaters do not use bass management either.

In addition, if you have a THX receiver THX "Reference Level" calibration assumes that a Dialnorm value of -27 was used at the encoding stage of the DD DVD. If a DD DVD was encoded with a Dialnorm value of -31, playing back at THX "Reference Level" will add 4 dB to all of the Dolby Maximum numbers.

How is that for a kick in the preverbial pants ? Have you guys watched M&C in both DD and DTS ? The soundtracks both net completely different levels. If calibrated to reference, the DTS soundtrack is considerably louder than the DD one ! Which means considerably more output from the sub !

Which means that this 115 dB peak level spec is kind of meaningless.

 

[Vaughan Odendaa]

You don't have coherent signals. Real speakers are separated in space which makes them by definition incoherent and gives a 3 dB SPL increase when combining identical sources.

Why not ? It's certainly possible. Have you seen waterfall charts for the movie 'Pulse' ? http://www.avsforum.com/avs-vb/showthread.php?t=1127163&highlight=pulse+waterfalls

You'd need all the speakers sitting on top of each other to get the 6 dB increase you're describing, which isn't how HTs are set up.

Not at all. If the signal amplitude and phase is equal then the net result is a 6 dB increase.

So, 117 dB would be the equivalent output if all 5.1 speakers were playing the same signal at their respective reference outputs.

It's 123 dB's. Not 117. If the 3 main channels + LFE were working full tilt, then it's 120 dB's. 114 dB's summation from all three channels + LFE (115 dBs) = 120 dB's. If it's 5 channels, 105 dB's + 115 dB peaks = 123. How did you get to 117 ?

 

[Vaughan Odendaa]

Perhaps Gene might chime in to offer his views.

 

[skers_54]

Whether the mixing engineer maxes out the LFE or the main channel's bass is up for debate. Obviously this depends on the movie. Some movies have very loud and deep bass recorded in the main channels together with LFE and this seems to be a popular trend nowadays.

To make matters worse, some movies are recorded hot. But setting that aside, if you talk about the absolute maximum possible peak you would expect in a home theater environment, calibrating to 75 dB's per ch and using bass management (and assuming the disk was mastered at the correct levels), can we all agree that the max dynamic peaks attainable would be 120 dB's from the sub (handling all redirected content from the three main channels, in phase with the LFE ?) Because I'm talking about the maximum peak level possible.

Something that is not really discussed is the dialnorm situation. Dialnorm is not used in theaters. Movie theaters do not use bass management either.

In addition, if you have a THX receiver THX "Reference Level" calibration assumes that a Dialnorm value of -27 was used at the encoding stage of the DD DVD. If a DD DVD was encoded with a Dialnorm value of -31, playing back at THX "Reference Level" will add 4 dB to all of the Dolby Maximum numbers.

How is that for a kick in the preverbial pants ? Have you guys watched M&C in both DD and DTS ? The soundtracks both net completely different levels. If calibrated to reference, the DTS soundtrack is considerably louder than the DD one ! Which means considerably more output from the sub !

Which means that this 115 dB peak level spec is kind of meaningless.

You haven't listened to a single thing anyone on this thread has told you. Speakers that are NOT IN THE EXACT SAME SPOT combine at 3 dB if they have the same phase and amplitude. They ONLY combine to 6 dB if they are stacked. Look at the links I provided (it has a breakdown of the formulas and a calculator) and re-read Wayne's post. Remember, we are talking about acoustical sources. You are using math for electrical sources.

Dialnorm is irrelevant because it adjusts the reference point. If you play a movie above reference, then you'll absolutely exceed 115 dB.

The waterfall graph is worthless because it doesn't quantify the encoded digital values. Sure, it's loud but that doesn't mean its running at 0dBFS on the disc.

Percieved loudness is NOT the same thing as peak SPL. They're related, sure, but psychoacoustical studies show that much more goes into it than what your spl meter reads.

 

[Vaughan Odendaa]

You haven't listened to a single thing anyone on this thread has told you. Speakers that are NOT IN THE EXACT SAME SPOT combine at 3 dB if they have the same phase and amplitude.

That assumes RANDOM PHASE ! I'm not talking about random phase. If phase and frequency are coherent it is 6 dB's -- not 3 dB's.

Dialnorm is irrelevant because it adjusts the reference point. If you play a movie above reference, then you'll absolutely exceed 115 dB.

If you calibrate your system to reference level the max peak level can vary considerably (ie M&C...WOTW etc) from DD to DTS soundtracks. Are you also going to dispute that as well ? I know first hand how much louder many of these soundtracks are......even after calibrating to the correct levels. It's not rocket science, anyone can verify this for themselves.

The waterfall graph is worthless because it doesn't quantify the encoded digital values. Sure, it's loud but that doesn't mean its running at 0dBFS on the disc.

But each of those main channels have considerable low bass recorded as well as the LFE. The requirements if redirected would tax the subwoofer more than simply the LFE on its own. Unless in your world LFE+redirected bass is the same as just reproducing LFE.