Low-frequency effect

Low-Frequency Effects (LFE) is the name of an audio track specifically intended for deep, low-pitched sounds ranging from 3-200 Hz. This track is normally sent to a speaker that is specially designed for low-pitched sounds called the subwoofer or Low Frequency Emitter. While LFE channels originated in Dolby Stereo 70 mm film prints, in the 1990s and 2000s they became commonplace in home theater systems used to reproduce film soundtracks for DVDs.

LFE is sometimes expanded as Low Frequency Enhancement [Holman, T.,(2000) "5.1 Surround Sound: Up and Running"] .

Types

Low-pitched musical parts

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.

ound effects

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. These sound effects are at the threshold of human hearing, 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 accurately reproduce.

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 WW II film "U-571" (2000) to test the accuracy of subwoofer systems.Fact|date=February 2008

Development

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 straightforward installation of extra dedicated subwoofers, removing the need to upgrade the main speakers, or install an extra LFE Crossover.

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 bass management functions to redirect bass from any channel to a subwoofer (LFE Crossover).

Home theater systems

In the 2000s, many westernFact|date=June 2008 consumers began purchasing "surround sound" home theater systems to enhance the experience of viewing DVD 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.

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 bassinformation 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 a subwoofer, either by a simple crossover network, which filters out all but the low frequencies, or with a more sophisticated digital 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. As such, it is incorrect to call the LFE the "subwoofer channel".

See also

* Surround sound
* LFE Crossover

References

External links

* [http://www.dolby.com/uploadedFiles/English_(US)/Professional/Technical_Library/Technologies/Dolby_Digital_(AC-3)/38_LFE.pdf Dolby Laboratories - What is the LFE channel?] (PDF)