The low-frequency effects (LFE) channel is the name of an audio track specifically intended for deep, low-pitched sounds ranging from 3-120 Hz. This track is normally sent to a speaker that is specially designed for low-pitched sounds called the subwoofer. While LFE channels originated in Dolby Stereo 70 mm film prints, they became commonplace in the 1990s and 2000s in home theater systems used to reproduce film soundtracks for DVDs and Blu-Rays.
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LFEs include both low-pitched musical notes and low-pitched sound effects. The musical soundtrack for many films includes bass instruments that produce very low notes. Until the 1970s, most of the low-pitched instruments were natural, acoustic instruments, such as the double bass or the pipe organ's pedal keyboard. After the 1980s, film scores increasingly used synthesized instruments, including synth bass keyboards, which included very low-pitched notes.
The most challenging sounds to reproduce from a sound engineering soundpoint are usually the extremely low-pitched sound effects in the 20 Hz range, such as those used to simulate the sound of an earthquake, a rocket launch, or submarine depth charges. The human ear is not very sensitive to sounds at these low frequencies, so it takes a tremendous amount of amplification for the human ear to hear them. As well, since they are sound effects, they may have a longer duration or sustain than many low-pitched musical notes, which makes them harder to reproduce accurately.
It is a formidable challenge for an amplifier, subwoofer speakers, and cabinet to reproduce these sound effects at a high volume without problems such as power amplifier clipping (distortion), unwanted rattle or resonance in the wooden cabinet, or excessive "chuffing" sounds from the bass reflex vent (if a vent or port is used in the cabinet). Sound recording magazines sometimes use the loud, rumbling sound effects simulating the sound of the submarine depth charges which were used in the World War II film U-571 (2000) to test the accuracy of subwoofer systems.
The LFE channel originated in Dolby Stereo 70 mm Six Track film prints, as a way of providing louder bass and sub-bass effects, without detracting from the quality of the standard audio channels. The LFE channel is conventionally played back 10 dB louder than the main channels, giving significantly more recording headroom. Also, the separate recording allowed a straightforward installation of extra dedicated subwoofers, and removed the need to upgrade the main speakers.
Later formats such as Dolby Digital retained the LFE channel, although this is more through convention and backwards compatibility than necessity, as digital formats have greater dynamic range than the magnetic analogue recordings on 70 mm prints, and modern sound processors have a bass management system to redirect bass from all channels to a subwoofer.
In the 2000s "surround sound" home theater systems became available to enhance the experience of viewing DVD and Blu-Ray films. Prior to the advent of home theater systems, when VCRs were used, the enhanced sound option was stereo high-fidelity sound. With home theater systems, a multichannel audio system was used to deliver different sounds to six or more different speakers. The widely-used 5.1-channel audio system consists of five full range main (Left, Center, Right, Left rear Surround, and Right rear Surround) plus a Low-Frequency Effects (LFE) channel. Many typical home theater systems, especially "home theater in a box" systems, are incapable of accurately reproducing LFE effects in the 20 Hertz range.
The LFE channel delivers bass-only information to supplement the overall bass content. The LFE channel content is not the same as the content of a subwoofer-out jack. The LFE channel is used to carry additional bass information in the Dolby Digital program, while the subwoofer output is bass information from up to all six channels that has been selected to be reproduced by the subwoofer via a bass management system.
The bass management in surround sound replay systems is that bass content in the incoming signal, irrespective of channel, should be directed only to loudspeakers capable of handling it. The bass management system may direct bass to one or more subwoofers (if present) from any channel, not simply the content of the LFE channel. As such, it is incorrect to call the LFE the "subwoofer channel".