To maintain full compatibility with existing theatres, the Dolby Digital film format includes a separate LFE channel. Taking advantage of the available channel capacity on 70 mm prints to deliver a separate bass effects signal proved to be the most direct, convenient, and economical way to supplement the low- frequency capability of movie soundtracks. Finally, no additional frequency crossovers would need to be retrofitted into existing cinema processors to redirect the bass from the main left, center, and right channels to the subwoofer(s). It also meant that the headroom of the 70 mm magnetic audio recordings would not be taxed at low frequencies, which would have detracted from their loudness capability at mid and high frequencies.

This allowed deep bass effects to be added to movie soundtracks without having to upgrade the existing speakers and amplifiers in the three main screen channels. The LFE channel was originally devised for 70 mm movie productions to deliver a separate bass signal to one or more additional subwoofers placed behind the movie screen. Its purpose is to supplement the overall bass content of the program or to ease the burden on the other channels. In contrast to the main channels, the LFE channel delivers bass-only information (<120 Hz) and has no direct effect on the perceived directionality of the reproduced soundtrack. 5.1-channel audio consists of five discrete, full range main channels (Left, Center, Right, Left Surround, and Right Surround) plus an optional band-limited Low- Frequency Effects (LFE) channel.

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