LFE
Encyclopedia
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 DVD
s and Blu-Rays.
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.
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.
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.
" 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".
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. This track is normally sent to a speaker that is specially designed for low-pitched sounds called the subwoofer
Subwoofer
A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...
. While LFE channels originated in Dolby Stereo
Dolby Stereo
Dolby Stereo, is the trade mark that Dolby Laboratories used for the various analogue stereo cinema sound formats that they produced.Two basic systems used this name. The first was the 'Dolby SVA' system used with optical soundtracks on 35mm film...
70 mm film prints, they became commonplace in the 1990s and 2000s in home theater systems used to reproduce film soundtracks for DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
s and Blu-Rays.
Low-pitched musical parts
LFEs include both low-pitched musical notes and low-pitched sound effects. The musical soundtrackSoundtrack
A soundtrack can be recorded music accompanying and synchronized to the images of a motion picture, book, television program or video game; a commercially released soundtrack album of music as featured in the soundtrack of a film or TV show; or the physical area of a film that contains the...
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
Double bass
The double bass, also called the string bass, upright bass, standup bass or contrabass, is the largest and lowest-pitched bowed string instrument in the modern symphony orchestra, with strings usually tuned to E1, A1, D2 and G2...
or the pipe organ
Pipe organ
The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...
's pedal keyboard. After the 1980s, film scores increasingly used synthesized instruments, including synth bass keyboards, which included very low-pitched notes.
Sound 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 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
Clipping (audio)
Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability...
(distortion), unwanted rattle or resonance in the wooden cabinet, or excessive "chuffing" sounds from the bass reflex
Bass reflex
A Bass reflex system is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.A reflex port is the distinctive feature of a...
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
U-571 (film)
U-571 is a 2000 film directed by Jonathan Mostow, and starring Matthew McConaughey, Bill Paxton, Harvey Keitel, Thomas Kretschmann, Jon Bon Jovi, Jack Noseworthy, Will Estes, and Tom Guiry...
(2000) to test the accuracy of subwoofer systems.
Development
The LFE channel originated in Dolby StereoDolby Stereo
Dolby Stereo, is the trade mark that Dolby Laboratories used for the various analogue stereo cinema sound formats that they produced.Two basic systems used this name. The first was the 'Dolby SVA' system used with optical soundtracks on 35mm film...
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
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
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
Dolby Digital
Dolby Digital is the name for audio compression technologies developed by Dolby Laboratories. It was originally called Dolby Stereo Digital until 1994. Except for Dolby TrueHD, the audio compression is lossy. The first use of Dolby Digital was to provide digital sound in cinemas from 35mm film prints...
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
70 mm film
70mm film is a wide high-resolution film gauge, with higher resolution than standard 35mm motion picture film format. As used in camera, the film is wide. For projection, the original 65mm film is printed on film. The additional 5mm are for magnetic strips holding four of the six tracks of sound...
prints, and modern sound processors have a bass management system to redirect bass from all channels to a subwoofer.
Home theater systems
In the 2000s "surround soundSurround sound
Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...
" 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
Home theater in a box
A "home theater in a box" is an integrated home theater package which "bundles" together a combination DVD player/ multi-channel amplifier , speaker wires, connection cables, a remote control, a set of five or more surround sound speakers and a low-frequency...
" 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
Bass management
The fundamental principle of 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, whether the latter are the main system loudspeakers or one or more special...
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".