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Bass Management
in 5.1 and other playback systems
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September 2011 Audio Reviews HDTV All Reviews
See also Stereo Subwoofers for Music Reproduction.
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Introduction
Bass management in 5.1 and other multichannel audio playback systems is a well thought-out process by which low-frequency information in the numerous playback channels is distributed to the loudspeakers best able, and most appropriate, to reproduce it.
Bass management is part of a playback system; it is not part of the mixing process.
Bass management is much more than simply sending all the bass to a single subwoofer.
Motion picture sound is completely different from music reproduction. This article addresses the reproduction of motion picture soundtracks at home.
The 5.1 System
Some time around 1990, Tom Holman, inventor of 5.1 and THX sound, was in a SMPTE subgroup meeting where various proposals for multi-channel motion picture sound were being discussed. Everyone was proposing various discrete channel counts and speaker positions, when Tom stood up and proposed a totally ridiculous "5.1 channels." Everyone stopped and stared. "What's a 'point one' channel?"
Tom explained a proposed Low-Frequency Effects, or LFE channel, the "0.1," as part of the movie sound systems we all take for granted today.
The LFE channel is a special low-frequency channel with extra headroom (+10 dB) used most often for reproducing loud booms. In fact, it's actually called the "boom" channel in Hollywood. (Only if you're curious, we get 10 dB more headroom by reducing the audio levels in the LFE channel by 10 dB and increasing the playback level of the LFE channel by 10 dB. Since these are digital as brought home, what's going on is that -20 dBFS in LFE should play at 95 dBC, while the other channels are set to 85 dBC at -20 dBFS if you're following THX reference levels. Yes, a lot of $30 Radio Shack sound level meters are all over big Hollywood studios for setting this!)
Most consumers improperly call the LFE (0.1) channel the "subwoofer channel," but that's not strictly correct outside of the home.
The five other channels are full-bandwidth. They all contain low bass. The 5.1 system (and other "x.1" or "x.2" systems) is at least five full-bandwdth channels, as well as one or two extra LFE (0.1 or 0.2) channel(s). 5.1 is not five little speakers and one subwoofer as are most home systems; that would only be (5x0.9) .1.
Movies are mixed with low bass running all over. There is a lot of very low bass happening in movies, with all the crazy effects which go far deeper than most music. If a helicopter pans over your head from left front to right rear, the bass pans along with it. It's obvious even listening to a movie on your iPad through headphones that the bass is in stereo, although with only one subwoofer, you can't hear the bass move.
In a pro system with five fullrange channels the bass will follow the helicopter, while in most home systems the bass doesn't move and only the higher-frequency information associated with the helicopter does. This is fine for watching a movie; unless you're a mixer you're probably not closing your eyes and only listening to the sound; your brain knows what's going on and the story is told, so all is well.
Professionally however, to get everything that's in a 5.1 soundtrack, if you don't have five big full-range speakers, you need at least six subwoofers: at least five for each of the main channels, and the last for the LFE (0.1) channel.
Bass Management
If you don't have at least five huge full-range speakers (or extra subwoofers) at home, the "bass management" function of your receiver, preamp or player reroutes the bass from the five or more fullrange and LFE (0.1) channels to wherever it can be reproduced best. You may have seen these settings labeled as something else, like "speaker size," in your equipment setup menus.
In most home systems with little speakers and only one subwoofer, this means the bass management system reroutes all the bass from the five fullrange channels into one mono subwoofer, along with the LFE information that was there in the first place.
If some of your speakers are big full-range speakers, the bass management system should be able to be programmed to send bass for those channels to those speakers instead of being hijacked into the mono LFE ("subwoofer") channel.
For instance, if you've got one big pair of left and right speakers, but the usual puny center and surround speakers, setting the bass management properly will route the bass to your front speakers and your subwoofer. Music will now be reproduced properly with stereo bass going to your main speakers, while the boom (LFE) track still comes through the mono subwoofer.
If you have no subwoofer, setting your bass management correctly will route the LFE information to your other speakers.
And so it goes. All you should have to do is set your bass management (or speaker size) menus to tell your processor what you've got, and it should do the rest for you — but you do have to find and program it if you have anything other than 5 small speakers and one subwoofer.
Professional Studio Applications
Even though your home and local theater probably only have have 5 smaller speakers and one subwoofer, Hollywood studios need to create all this sound, and need a way to monitor it properly so they know how it's going to sound on all possible systems.
In a professional (studio monitoring) application, ideally you'd want to be able to send as much low bass as possible to your main speakers to hear what's in what channel, and send only the bass that these speakers can't reproduce to your subwoofer(s).
Home receivers and movie theater decoders usually only offer the choice of BIG, SMALL or NONE for each speaker position, so you have no real control of the frequencies above or below which bass gets sent to one speaker or another.
TMH Labs (Tom Holman's company) has offered a professional bass manager that allows one to select these frequencies individually for each speaker, and more.
You can learn more about bass management by reading the white paper about the TMH Bass Manager, as well as its users manual. Once you understand the concept that bass management is simply routing the bass from the various signal input channels to the best speakers to reproduce it, this gets much simpler.
Music reproduction is very different from movie sound, and for serious stereo music listening, you should have at least two subwoofers for stereo.
That's it, that's bass management.
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