Audio formats capable of carrying 5.1 channels of sound include a so-called ".1" channel that is used to convey Low Frequency Effects, or LFE information below approximately 120Hz. The history of the LFE channel harkens back to the days of 70mm discrete magnetic film (mag), and as its name implies, it was used to carry additional low frequency effects. Upon playback in theaters, a subwoofer was used to reproduce the LFE information. In an effort to ensure that enough headroom existed on the very narrow magnetic track that carried this LFE information, the subwoofer in both the mixing stage and the movie theater were adjusted to reproduce 10dB higher than the other full-range channels. This increased output level caused the LFE tracks to be mixed 10dB lower, thereby preserving additional headroom.
Modern systems such as Dolby Digital (AC-3) have carried on the tradition in the modern digital theaters, and on DVDs and digital television. While no longer necessary for headroom reasons, the 10dB of increased sensitivity of the subwoofer in the mix stage and also at the reproduction side (commercial or home theater) has been maintained for compatibility reasons. It is important to note that the Dolby Digital (AC-3) system allows the LFE channel to be shut off, thereby creating a 3/2 (or 5.0) program for example. In many consumer AV receivers, reproducing a bitstream encoded with the LFE channel shut off will cause the indication to change, sometimes displaying only five speakers instead of the full 5.1. This can cause phone calls, but it is unlikely that the consumer has heard a difference; rather they have likely just noticed that the program is not indicated as being 5.1 channels.
In summary, the LFE channel is used to carry low frequency information below 120Hz, and a subwoofer reproduces this information. However, the subwoofer and LFE are not one in the same as a subwoofer can reproduce more than just LFE information and simply shutting off the LFE channel may not change the audio.
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