Stereo requires the reproduction of signals from 20 Hz to 20 kHz. This is done with multi-way speaker systems, which use a combination of woofers and tweeters to achieve full-range response. These speakers are connected via a crossover network to route the appropriate frequencies to the various speakers in the system. This may be a two-way, three-way, four-way, or even-five way system, but in each case, the goal is to reproduce 20 Hz to 20 kHz evenly.
Today’s Dolby® Digital consumer decoders include a bass management system to do just that. Just as with the old stereo and Dolby Surround systems, the goal is to be able to reproduce all frequencies within the system. The five main channels and extra LFE channel provide more possible combinations of speakers, including five full-range main speakers and a subwoofer for the LFE; five small speakers for the main channels and a subwoofer for both the LFE and bass redirected from all five main channels; and various combinations of the above examples.
Studios must be able to reproduce all reasonable frequencies from each full bandwidth channel. Crossovers, subwoofers, and main speakers should work together to give flat response for each of the five main channels.
Many manufacturers of near-field monitors make complementary subwoofers to complete the system. Larger rooms may dictate the need for more than one subwoofer to achieve adequate bass response.
When using the LFE channel in a mixing situation, it is important to band-limit the information for this channel. During the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. This is true for both professional Dolby Digital encoders, as well as the interactive encoders found in game consoles such as the Playstation 4 and Xbox One.
Consumer decoders take the LFE signal and add any channels in need of bass management, as determined either by product design or user selection. The five main channels are then highpass filtered at either a fixed frequency of 80 Hz or a selectable frequency of 80, 100, or 120 Hz. The summation of the LFE and any other channels is lowpass filtered at the same frequency.
While the Dolby Digital encoder and decoder together will handle bass management in decoding, it is often not feasible to use them in this way when mixing, due to the delay through the encoding and decoding process. Therefore, it is necessary to have a separate crossover system in place to handle the bass management. Many manufacturers now offer such devices for this purpose.
To replicate what the consumer will hear, a third-order (minimum) 80 Hz filter in the LFE audio signal path to the recorder is recommended. It is advisable to include this filter in the console output before the monitor such that both the recorded information and the heard information are band-limited. Failure to include this filter will result in hearing substantial bass information above 80 Hz in the mix that will not be present in the Dolby Digital encoded version.