All About 10dB Sub Boost

Ultimately the boost is merely a gain setting. Whether or not it’s a problem for your subs depends on how much EQ you’ve applied, and how hard you’re driving them already.

BlueRockinLou said:

[*]If generating REW PEQ's do you crank the sub level 10dB after measurements (doesn't make sense to me)?

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Good for you, for figuring that out. :T The process has always been to EQ as needed, then adjust the sub gain afterwards, as it typically needs to have the level re-set after equalization. I typically use two-channel as the reference for sub levels, but admittedly with a lively movie that has all channels generating at once, a two-channel reference might leave the subs too low.

[*]The +10dB is not part of the house-curve concept, correct?

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Generally, it is. I’ve never been a fan of rigid numbers like this, because the amount of gain in sub level needed (if any) is very room dependant, and also on user preference. That said, a boost in the 4-8 dB range is common, with the 25-30 Hz shelf in sub response being about that much hotter than what is measured at the crossover frequency.

Regards,
Wayne

ellisr63 said:

10 db boost for subs is a lot of power. You need to make sure that your sub can handle the boost, and that you have clean power to do it.

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tonyvdb said:

Agree^^ non of the room correction systems do the boost automatically however after running it simply turn up the trim level on the AVR by a couple db. 10db is huge and very few systems can handle it unless you have Sonnies :hsd:

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Good answers! They weren't what I expected at first, but then I found the article I was thinking of. Of course, it turned out to be 15 years old, so may no longer be valid. It explains differences between subwoofer (bass management?) and LFE tracks.

INTRO:
"To understand the LFE track, we must first touch on the subject of calibrated loudness levels. In a production facility, pink noise is generated at 20 dB (Figure1) lower than the loudest sound the digital system permits. When this sound is played through each channel, the actual volume heard is adjusted to be 85 dB on a Real Time Analyzer (RTA). In this way, the loudest possible sound in the soundtrack, the peak, will be 105 dB for each channel. When a movie theater sets its playback volume by the same rules, the audience hears the soundtrack at the level and with the impact that the sound engineer intended. Of course, that doesn't mean the audience likes it that way. If you turn around during a crash boom sequence and you can see the teeth of the people sitting behind you, it is probably "too loud" for their tastes. In other words, maybe we don't really want to hear a 357 Magnum as loud as it is in real life, but that may be the engineer's design, and can easily be achieved with today's digital technology."



WHERE I GOT THE +10dB IDEA:


THEIR EXPLANATION:
"Dolby Digital's LFE channel carries additional bass information from 120 Hz on down. This is not a roll-off but a digital brick wall (i.e., no 121 Hz info), so the content is usually rolled off by the sound engineer starting around 80 Hz for a smoother blend. During both soundtrack production and in the movie theaters, the LFE channel, with that same level (-20dB) pink noise (but band limited to the subwoofers range), is calibrated to 95 dB on the RTA within the subÕs bandwidth (Figures 2 and 3). This is done so that it can play 10 dB higher than any one of the screen channels. Because of this 10 dB offset, the LFE channel can achieve a balanced output of bass as compared with the total output of bass from the three screen channels (in other words it can single handedly compete with the screen channels in terms of level). The only down side is that we lose a little S/N (signal to noise) performance on that track. Because our hearing is less sensitive to bass to begin with, the system gets away with it just fine."



HT SYSTEM REQUIREMENTS TO ACHIEVE THAT PERFORMANCE:
In an all out assault, with deep bass in all channels, the LFE channel makes possible an extra 6 dB of headroom over and above what the three screen channels can deliver as a group. If you are doing the math right now, you may have already concluded that in a text-book Dolby Digital theater, one of a subwoofer system is called for! Let me explain.

In a movie theater on the island of Utopia, with ideal hardware, any single screen channel should be capable of a clean 105 dB peak with it's own respectable bass. The LFE channel should be capable of a 115 dB peak. Drive all channels to the max and the system should be able to slam you with approximately 120 dB of bass information. Thank you Dolby.

Doing this right means having some powerful hardware. Deep bass at high output necessitates considerable air displacement. This in turn calls for large speaker drivers with appreciable excursion, which further calls for considerable amplifier power. For a modest cinema, two 18Ó drivers could be considered a practical minimum, with some road-show-sized theaters using as many as eight drivers, backed up by some 4,000 watts of amplifier power to deliver the goods."

LFE IN THE HOME ...or... MY LESSON:



"We home users donÕt use an RTA to set-up our systems, but if we did, it could look something like the figure above Bass which is redirected from a main channel would be balanced with the output of that channel, and the LFE data automatically playing 10 dB higher. Most of us don't have an RTA so an SLP meter (sound pressure level) has to do. There are numerous sources of test tones for setting the subwoofer level (different from LFE level), all optimized for a simple SPL measurement (AVIA, Video Essentials, and Delos Surround Spectacular, to name a few). It is very important to realize that if the subwoofer level has been set correctly using one of these tools to match a screen channel, the LFE data will be at the appropriate setting without any further adjustment. If you were to raise or lower the subwoofer level, the LFE level would rise or fall with it, tracking it at +10 dB. Processors that let you manipulate the LFE level independent of the subwoofer level are few, and most of these only let you reduce the offset to protect a less capable subwoofer from undue stress."

So do you know what they're talking about? I don't think you can separate LFE from sub channels in today's world.

Wayne A. Pflughaupt said:

Good for you, for figuring that out. :T

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Sincere thanks! That built my confidence.

Wayne A. Pflughaupt said:

The process has always been to EQ as needed, then adjust the sub gain afterwards, as it typically needs to have the level re-set after equalization...

...I’ve never been a fan of rigid numbers like this, because the amount of gain in sub level needed (if any) is very room dependant, and also on user preference. That said, a boost in the 4-8 dB range is common, with the 25-30 Hz shelf in sub response being about that much hotter than what is measured at the crossover frequency.

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Well said, and thank you! Helps launch me into creating house curves. I found the excellent guide you wrote. Here's an excerpt from the article I was thinking of when starting this thread.



"Once you have a smooth response you can then apply a room target curve to you’re your taste and source material. It is best to separate the EQ filters for taming modes from those used for creating the target curve. This allows the boost level to be altered as required quickly, as you would with tone controls. There is no single curve that will suit all music and movies.

You can determine the filters required using the REW EQ filters tool as shown previously. Since there are so many different ways to apply this target curve, detailed instructions are beyond the scope of this guide. In a two channel system, shelving filters may be added to the inputs. In an AV system it may be a simple matter of adjusting the subwoofer levels. In some cases it may be necessary to use a combination of both. Once the process and filters available are understood, it is not difficult to apply a target curve. It’s likely that you will need to adjust it and ideally have the ability to tweak the bass to suit different music at times. You may also find that you prefer running the bass more than 8 dB louder than the mains."

tonyvdb said:

Reference level is 75db when running one speaker at a time when you combine all channels minus the sub it is 85db with peaks of 105db
Adding the sub those peaks go up to 115db but average is 85db with the sub average will be around 95db all channels

105db sustained is really loud and will cause hearing loss.

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There seems to be quite a bit of confusion (and therefore mis-information) in this thread about theatrical sound. The quote above is incorrect. At a reference level of -20dBFS the output of EACH of the front (3) speakers is individually calibrated to 85dBSPL(C) at the datum point, usually around 2/3 to the back of the auditorium, not to 75dB. Peak output for each of the fronts is therefore 105dBSPL, although obviously that figure can be significantly higher at the front of the auditorium.

The front 3 speakers are all full range, they are capable of producing a balanced signal on their own and there is no bass management or no sub-channel and need for one. The reason we have a separate LFE channel is not for creating a balanced signal but for adding to it in special circumstances. Say in the case of an on screen explosion where we want to move a great deal of additional air to simulate an actual physical sensation of an explosion. Hence why it's called the LFE, Low Frequency Effects, channel. Because this all started with optical tracks (on 35mm film), where printing large bass amplitudes causes problems, Dolby's solution was to write the LFE channel 10dB lower. In other words, dubbing theatres are setup with their LFE b-chain (monitoring output) channel at +10dB, so when creating a mix with the LFE at +10dB the underlying audio channel feeding the LFE is effectively 10dB lower. Likewise the cinemas themselves always have the output to the LFE channel set at +10dB, thereby correcting for the -10dB LFE channel in the print. Incidentally, there is also a correction applied to the surround channels, which are effectively printed at +3dB.

Today, there is no longer any optical tracks (or any Dolby theatrical 5.1) but nevertheless, the -10dB printing of the LFE channel is still always observed. However, unless you actually have access to an original theatrical DCP (and the means to play it of course), the chances are that you are NOT listening to the theatrical mix anyway. What you're probably listening to on your DVD or BluRay is a mix specifically "re-versioned" for home consumer use! IE. Designed for consumer bass managed systems, which don't use -3dB in the surrounds or +10dB in the LFE.

Please note: You cannot setup up a home cinema to the specs used by commercial cinemas. For example, 85dBSPL(C) in a cinema does NOT equate to 85dBSPL(C) in a home environment. Also, to calibrate the LFE at +10dB we use filtered pink noise files, specifically designed for LFE calibration, you can't calibrate an LFE accurately with standard pink noise.

One last thing: Peak levels of 105dBSPL will not cause hearing loss/damage, although it could easily be uncomfortable. A continuous level of 105dBSPL, sustained over a period of time certainly can cause hearing damage but sustained, continuous 105dBSPL is never encountered in cinemas.

Hope this helps, G

All,

Need some quick education and advise. I have a Marantz 8012. A dedicated HT. 2 SVS PC 12 plus subs. I also have 2 subs in my DefTech 8060 fronts towers.

Q1. What does the Subwoofer Level in the Receiver do? How is this different from the Trim that is set by Audyssey?

Q2. Since I have 4 subs, should I set each one to 75db individually to gain match with my fronts?

Thanks!

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