How Low Can You Go?

THE FINER POINTS OF SURROUND BASS MANAGEMENT
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One of the most-commonly misunderstood aspects of surround monitoring is the relationship between the low-frequency effects (LFE) channel and the subwoofer. Despite the fact that in surround-sound mixing formats the term point one, as in “5.1” and “7.1,” is used to refer to both, and despite the fact that the two are directly related, they also differ in important ways. The real-world difference between them is the result of a process called bass management.

The LFE provides a dedicated channel for the sort of low bass that is used in theaters and theme parks to add impact to star destroyers, explosions, and really big dinosaurs. The LFE is band-limited to frequencies below 80 Hz to 120 Hz.

In playback systems, the term point one refers to a subwoofer, a speaker dedicated to carrying only low frequencies. Subwoofers have been used for many years to extend the response of speaker systems. Here, I'm referring to subwoofers that are designed to extend accurate reproduction down to 20 Hz (or lower), and not to the sort of bazookas that are made to rattle car hoods and passersby.

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FIG. 1: Bass management redirects the low-frequency content of each directional channel to the subwoofer, where it is combined with the LFE channel

Building full-range speakers is neither easy or inexpensive, largely because re-creating low frequencies cleanly and effectively requires large drivers and a great deal of power. A subwoofer takes over the low-frequency duties from multiple channels so the directional speakers can be built more easily and economically. It could even be argued that the explosion in affordable home-theater-in-a-box-style surround audio systems has occurred because of the use of subwoofers.

Though the LFE and the subwoofer may seem similar, the LFE is a production channel, whereas the subwoofer is a playback channel. The mix engineer determines what goes in the LFE, and the designer of the home-theater system determines what ends up coming out of the subwoofer.

Making the Point

Bass management takes the low frequency from the satellites, or the directional speakers, and combines it with the LFE to be played through the subwoofer. Because bass is less directional than treble, our ears localize sounds according to their placement in the directional speakers regardless of where the subwoofer is placed. We have little trouble synthesizing a sound's easily localized overtones with its disembodied fundamental. The net effect is ample bass response with good directionality at reasonable cost.

The details of a bass-management system are seen in Fig. 1. The signal from each of the five directional channels is sent through a highpass filter so that only frequencies above 80 Hz are sent to the corresponding satellite speaker. At the same time, a lowpass filter redirects frequencies below 80 Hz from each directional channel to the subwoofer. That is why bass management is also referred to as bass redirection. Meanwhile, the LFE passes through only a lowpass filter to ensure that just the frequencies below 80 Hz are passed to the subwoofer. The low frequencies of all five (or more) directional channels are summed together with the LFE channel and sent to the subwoofer.

As with speaker placement (see the Square One column “Covering All the Angles” in EM's November 2005 issue), bass-management cutoff frequencies are subject to varying standards. It makes sense, for example, that an 80 Hz cutoff is inappropriate if your directional speakers don't extend that far down. That wouldn't be an issue with studio monitors, but consumer playback systems will sometimes use (or provide the option of using) a higher cutoff to accommodate very small satellite speakers. The LFE's cutoff is sometimes cited as being at 120 Hz.

The Monitoring Question

Whether one should mix with a bass-managed system is debatable. After all, you've been crafting great stereo mixes on your “real-world” near-fields for years, so why should surround be any different? And if you can afford five directional speakers that extend to 20 Hz, why shouldn't you use them? Certainly those audiophiles who spend as much on their speakers as most folks do on automobiles will be listening to your mixes that way.

If, like most of us, you work with monitors that roll off in the 40-to-50-Hz range, you would be wise to add a bass-managed subwoofer to your system. If you don't, you could find out the hard way that bass-managed consumer playback systems easily reveal low-frequency problems that you never heard in the control room. In the past, you could be forgiven for missing mic-handling noise, airplanes passing overhead, air-conditioning noise, and other problems as you monitored on your NS-10s, because most stereo bookshelf speakers were incapable of reproducing those frequencies. But with the increasing use of subwoofers in people's homes, those days are gone. Because many people even listen to their CDs on bass-managed systems, you should at least check your stereo mixes with the subwoofer engaged.

Even if you monitor with true full-range speakers, you must check your mixes on a bass-managed system to guard against phase cancellation. With six channels of bass frequencies being summed, the potential for such interference is high. Your full-range speakers might hint at the problem, but acoustic phase cancellation depends highly on the position of your head and is subject to mitigation by reflections. When out-of-phase signals cancel each other “in the wire,” however, their cancellation is direct and irreversible. The only way to be sure those problems don't appear in the consumer's playback system is to do a critical listen through bass-managed speakers.

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FIG. 2: Cakewalk Sonar 4 Producer Edition''s surround options include software-based bass management. It can be turned on or off at will, and it supports several different cutoff frequencies

Defeatable bass management is commonly built into surround monitor controllers so you can mix without it, and then engage it to check for phase problems. Some facilities keep a consumer-level playback system on hand, either in the control room or in a separate room, for quality-control checks. Another option is software-based bass management (see Fig. 2).

A further argument in favor of bass management is that even with full-range speakers, it may be impossible for their real-world bass response to be identical at the mix position. Innumerable variables, including reflections from console surfaces and the speakers' proximity to walls and ceilings, conspire to prevent uniform frequency response from all five directional speakers. Bass management puts the low end of all channels on an equal footing.

Get Down

Subwoofers are sometimes frowned upon because it can be difficult to integrate their response smoothly with that of the satellites. Nevertheless, as an enabling technology for the explosion of home-theater surround systems, they are a fact of life. Understanding how bass management affects the playback of your surround mixes is a critical step in ensuring that your work translates to varied playback systems. After all, you always check your stereo mixes for mono compatibility, don't you?

Brian Smithers is Course Director of Audio Workstations at Full Sail Real World Education in Winter Park, Florida.