All Bass Is Covered - Part 1

Bass Management is a subject that even the best music engineers don’t always seem to know a whole lot about, and is easily misunderstood – or simply missed. Richard Elen decided it was about time to lay down the law and take a fundamental, controversial and pragmatic approach to the subject – one that he hopes will annoy music engineers enough for them to think about what how their monitoring is set up – whether they are new to surround or have been doing it for years. In the first part of this two-part article, he looks where bass comes from in a 5.1 system – and where it goes.

PART I: 5.1 AND LOW FREQUENCY EFFECTS

DVD is apparently the most popular consumer electronics phenomenon there has ever been, but the home theater revolution has brought with it several problems. The way bass is handled in 5.1 systems is perhaps the most problematical, and whether you’re new to music (as opposed to film) surround mixing or have been doing it for years, I would ask you, as a result, to consider whether or not you are monitoring your surround music mixes in a way that will give your listeners at home the best experience, and one that is as close as possible to what you intended them to hear.

A studio consultant will no doubt have set up your monitoring correctly, but if you did it yourself, please read on.

Today’s 5.1 systems have developed from surround audio configurations originally designed for movie theater applications, and they bring with them a number of curiosities – some of which are not particularly appropriate either for modern digital audio distribution, nor for music mixing. None of these is more archaic than the LFE.

Low Frequency Effects

“LFE” stands for “Low Frequency Effects ”, and “Effects” means “sound effects”. To allow the reproduction of powerful sub-bass effects in analog movie soundtracks – asteroids crashing into the Earth and dinosaur footfalls for example – it was necessary to assign such signals their own channel to minimize intermodulation distortion and other unwanted artifacts, and maximize headroom.

The home theater revolution, however, has been founded on digital, and not analog, audio. Intermodulation of extreme bass with other channels does not happen. Not only that: all the channels have a complete low end – you can put almost any amount of bass on any channel you like without problems.

There is debate about whether there are still reasons for keeping a special channel for extreme bass effects, for reasons of headroom for example. But whatever the outcome of that discussion, you do not encounter extreme bass effects in music recording. At all. Asteroids do not fall into the average music studio very often, even when you’re recording heavy rock music (sorry, I couldn’t resist). The nearest you might get must be recording real cannons for Tchaikovsky’s 1812 Overture, as Telarc did a year or two ago when they recorded their new version for SACD and DVD-Audio release. But the new 1812 – like several Telarc recordings and some from other companies – uses the LFE to carry height information. “With Bass Management,” says the blurb in the booklet for Telarc's SACD-60541, “the LFE channel from the disk is redundant.” Aha!

So the single most likely piece in musical history to need, or benefit from, the use of a Low Frequency Effects channel doesn’t have one. Why? Because it isn’t necessary. It’s as simple as that. Even DSD cannons don’t qualify. If every channel in the system can carry all bass frequencies, then why do you need a special channel for bass? The answer: you don’t.

So here’s Rule #1:
If you are mixing music, don’t put anything in the LFE.

You might want to put a label on the box so that nobody at the record company thinks you forgot to put anything on the LFE (or that you’re using it for something more sensible, like height). Nobody at home will know if you didn’t put anything in the LFE: if they have a sub and put their ear to it, they will hear something (the bass from all the other channels: bass management in action) so they can’t tell.

Butbutbut… you say. Suppose I want to put some bass signal in the LFE, you know, to give the track a bit more oomph in the chorus? Well, you can easily give the track more oomph: just don’t do it like that.

LFE and Sub

As noted previously, all the channels in a modern digital distribution medium can handle the full audio range – with the possible exception of Dolby Digital, where the LFE may not allow audio information above about 120 Hz (so you can’t use it for height either). This means that any bass frequency you can imagine can go on any of the “main” channels with no problem, and there is no need to waste a channel on it.

What happens to these bass frequencies when they leave a consumer’s DVD player? Well, they run straight into the receiver or preamp’s bass management circuitry.
The purpose of bass management is to insure that whatever channel bass is on, it is fed to speakers that can handle it. There are many different speaker configurations that a consumer replay system might have, from five full range speakers and no sub to four tiny little boxes with a hefty powered subwoofer in the corner. Even a system with five full-range speakers needs bass management – to insure that any T-Rex footfalls that appear only in the LFE make it to the speakers that are actually present.

What, no sub? But surely the LFE is the “subwoofer channel”? Wrong. Here’s Rule#2:

The LFE is not the subwoofer channel. Don’t treat it as though it is.

If you do, you could be in big trouble. At the consumer’s home, the LFE is not connected directly to the subwoofer, and it should not be connected that way in the studio. There may not even be a subwoofer at home, if the listener is lucky enough to have five full-range speakers. Yes, bass in the LFE will end up in the speakers, but it will arrive along with any other bass on the appropriate channels. Similarly, in those horrible little satellite speaker systems with five tiny little boxes and a sub, all the bass from all six channels will come out of the subwoofer, (including anything in the LFE).

This means that not only is there no benefit in putting something in the LFE (bass will come out of the same place in the end anyway); there might even be a disadvantage, as in some cases your bass content might end up being boosted significantly – more than you planned, I mean – if your sub is connected direct to your LFE – as this is never the case elsewhere. To quote Dolby, discussing the role of the LFE in film sound (in What Is The LFE Channel?):

“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…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.”

The above refers, strictly speaking, to Dolby Digital encoding, but the principle can be applied more widely. There is a good deal of discussion about that extra 10 dB (or is it 6?) in surround circles, and as a result you will often encounter material that seems to have too much, or too little, bass. Listeners may have problems even if both you and they have bass management. If you’re mixing music in surround, you can avoid headaches here by following Rule #1 and not using the LFE at all. But check the final mix via a consumer-like system with bass management just to make sure. See also this Dolby 'Multichannel Music Mixing' document's section on Bass Management.

In Part II, Richard Elen looks at studio monitoring systems and how to get them right.