The Bottom Line

• The 0.1 channel, called LFE, is provided for more headroom below 120 Hz, where the ear is less sensitive and "needs" more SPL to sound equally loud as mid-range sound.

• LFE is a monophonic channel. For stereo bass, use the five main channels.

• Bass management in monitoring can be used to reproduce both the very low-frequency content of the main channels, as well as the LFE channel, over one or more subwoofers.

• The LFE channel recorded reference level is -30dBFS for masters using -20dBFS reference on the main channels.

• When both are measured in 1/3-octave bands using pink noise at the same electrical level, the LFE channel 1/3-octave band SPL reference is 10dB above the level of one main channel; the pink noise for the LFE channel is band limited to 120Hz, and is wideband for the main channels. (See Figs. 2-11 and 2-12, comparing the in-band level in the subwoofer operating region.)Typically, LFE will measure about 4dB above the SPL of one main channel playing

Fig. 2-14 Block diagram of a bass management system, with typical characteristics for pro-audio shown. High-pass filters in each of the main channels are complemented by a low-pass filter set in the subwoofer feed, considering the effects of the loudspeaker responses, so that each of the channels is extended downwards in frequency with a flat acoustical response. Note that this requires five matched bandwidth loudspeakers.The LFE channel is low-pass filtered with an anti-aliasing filter, which may be part of the encoding process or be simulated in monitoring, with its level summed into the bass extension of the main channels at +10dB relative to one main channel.

pink noise on a C-weighted sound level meter. The LFE level is not +10dB in overall sound pressure level compared to a main channel because its bandwidth is narrower.

Calibrating the Monitor System: Frequency Response

Equalizing monitor systems to a standard frequency response is key to making mixes that avoid defects. This is because the producer/engineer equalizes the program material to what sounds good to them (and even if not doing deliberate equalization, still chooses a microphone and position relative to the source that implies a particular frequency response), and a bass-shy monitor, for instance, will cause them to turn

up the bass in the mix.This is all right only if all the listeners are listening to the same monitor system; if not, then monitor errors lessen the universality of the mix. Although room equalization has a bad name among some practitioners because their experience with it has been bad, that is because there has been bad equalization done in the past.

I did an experiment comparing three different equalization methods with an unequalized monitor. I used a high-quality contemporary monitor loudspeaker in a well-qualified listening room, multiple professional listeners, multiple pieces of program material, and double-blind, level-matched experimental conditions. The result was that all three equalization methods beat the unequalized condition for all the listeners on virtually all the program material. Among the three methods of equalizing, the differences were much smaller than between equalizing and not equalizing. You will not hear this from loudspeaker manufacturers typically, but it is nonetheless true.

Considerations in choosing a set of equalizers and method of setting them are:

• The equalizer should have sufficient resources to equalize the effects of rooms acoustics; this generally means it has many bands of equalization.

• The method of equalization should employ spatial consolidation. Measuring at just one point does not well represent even human listeners with two ears. Averaging or clustering the responses and nominating one best response based at several points generally leads to less severe, and better sounding, equalization.

• Equalizers that fix just the direct field, such as those digital equalizers that operate only in the first few milliseconds, seem to be less useful in practical situations than those that fix the longer-term or steady-state response.

• If a noise-like signal is the test source for equalizing, temporal (time) averaging is necessary to produce good results. For 1/3-octave band analysis, averaging for 20 seconds generally produces small enough deviations. Trying to average the bouncing digits of a real-time analyzer by eye produces large errors.

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