Subwoofer 'reference level'

[Vaughan]

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main channel bass (105 dB peaks per ch) to a subwoofer that must also voice LFE content, surely that 115 dB figure is no longer valid.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.

[kenpeter]

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.

[Vaughan]

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 ?

[revboden]

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.

[bjorno]

Originally Posted by Vaughan

..I think you need to check your math..

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.

...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)...

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

[soho54]

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.

[Vaughan]

Originally Posted by revboden

You're thinking it's additive, like when streams group together to form a river.

It is additive. Have you heard of Pan's Law ? The rest of your post I didn't follow.

[Vaughan]

Originally Posted by soho54

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.

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.

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.

What are you talking about ?

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.

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'.

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.

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.

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.

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.

[soho54]

Originally Posted by Vaughan

It is additive. Have you heard of Pan's Law ? The rest of your post I didn't follow.

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.

[soho54]

Originally Posted by Vaughan

What are you talking about ?

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/z...Guidelines.pdf

Originally Posted by Vaughan

There shouldn't be any difference in peak level between BM engaged or disengaged.

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?

Originally Posted by Vaughan

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.

So the sub will need to play higher SPLs, but the peak level will remain the same? Think that through again.

Originally Posted by Vaughan

That isn't to say that you will require 120 dB's but that it's the maximum possible peak level requirement from the sub at reference level with BM engaged (assuming a flat calibration).

Now you are saying the peak level will be higher?

Originally Posted by Vaughan

Well I don't know about that. If the sub requires 123 dB's at reference level that's a pretty big pill to swallow.

It doesn't, and never will.

Originally Posted by Vaughan

I'm not talking about HP filters.

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.