Getting extremely loud, low frequencies are the meat and potatoes of movie soundtracks. Movie directors and sound supervisors want to deliver the sonic impact of explosions, volcanoes, and earthquakes while you’re sitting in a dark room with a bunch of other aural thrill seekers. The need to reproduce low frequency sounds that impact an audience requires a sound system that can deliver very high signal levels at low frequencies.
It’s not so difficult to do at a commercial movie theater but it does present a real challenge at home. The specification of the Dolby Digital (AC-3) compression scheme used to power the soundtracks of DVD-Video discs goes down to 0 Hz. The signal level needed to drive extreme levels is likely to cause digital clipping. We’ve already talked about how a 10 dB gain is provided to the LFE channel in the analog domain to achieve the desired amplitude levels for the bombs, car crashes, and firepower of the US Navy. The diagram below shows the LFE signal can be combined with the main left and right signals and sent to the main loudspeakers.
Figure 1 – Two main channels and the LFE channel added to the main speakers (from the ITU document).
If the main loudspeakers are equipped to handle the energy required for the low end, the LFE channel with the 10 dB of additional gain provided in the analog domain by the AVR can be added to them. The diagram shows two main channels but the same principal applies to a monophonic setup. In that case the 6 dB attenuation would not be required because there are half as many channels. In reality, adding the LFE channel to all of the loudspeakers of a multichannel setup would require more than 6 dB attenuation.
How should the LFE channel be connected to the subwoofer speaker? The diagram below (also from the ITU-R BS. 775-3 document) shows the wrong way to make that connection. The problem in this arrangement is that the main speakers are small and unable to handle the low frequencies delivered by the left and right channels. The subwoofer is only getting the “low frequency effects or enhancements” that would drive the small mains into distortion. The frequencies at the very low end would be reproduced but the low end of the main program would not.
Figure 2 – Bass frequencies being sent to the small main loudspeakers.
The right way to get the entire range of frequencies required by the program is to use bass management, which we’ve seen is a method that extracts frequencies lower than 120 Hz and adds them to the LFE channel to be reproduced by the subwoofer speaker. Take a look at the diagram below. It shows how properly filtered left and right main signals can be combined with the LFE (with the analog boost of 10 dB) to ensure the full program is reproduced.
FIGURE 3 – A properly configure 2.1 reproduction system with bass management.
The LFE channel and subwoofer are not the same thing. The LFE channel is the “enhancement” of the low-end and the subwoofer is the loudspeaker that delivers is AND/OR a bass managed combination of the low-end.