What is the LFE Channel? - Tip 7Many people call the LFE channel the “subwoofer channel.” In fact, the LFE channel is the space on the storage media for the .1 encoded (and usually band limited) audio channel. The “subwoofer channel” is not a “channel” as such, as the subwoofer loudspeaker, together with the bass management system, replays a specific low frequency bandwidth. The subwoofer feed may consist of the LFE channel and/ or the low frequencies of the main channels depending on whether bass management is used and how the system is set-up.
In a 5.1 audio production, the five main channels (Left, Centre, Right, Surround Left and Surround Right) are all full bandwidth, i.e. 20 Hz to 20 kHz. Before encoding, the LFE channel is not band limited, i.e. it is just another full bandwidth channel.
After the LFE channel is being encoded it usually has a limited bandwidth (hence the label ‘.1’, roughly the 10th of the full bandwidth). This encoded bandwidth will range from 20 Hz up to various upper cut-off frequencies depending on the encoding formats – see the table below. Note that some recent multichannel formats deviate from this principle as the LFE channel stays at full bandwidth even after the encoding stage. In these cases, the naming ‘.1’ is consequently not technically correct.
||LFE Upper Cut-off Specifications
|Dolby Digital (AC-3)
Dolby Surround EX (6.1)
Dolby TrueHD (7.1)
Full bandwidth channel
|DTS Digital Surround
Full bandwidth channel (1)
||Full bandwidth channel
|DSD / DXD (SACD)
||Full bandwidth channel
||Variable (up to 1 kHz)
||Most likely variable…
||Variable up to 330 Hz
Table – LFE channel upper bandwidth encoding cut-off specifications
Table note (1): in all DTS ‘Coherent Acoustic’ codec the LFE channel is band limited at the encoding stage at 120 Hz. In the DTS-HD codec only [DTS-HD Master Audio™ (lossless) and DTS-HD High Resolution Audio™ (lossy)] the DTS-HD decoder applies a low-pass filter at 100 Hz (-3 dB) with a 60 dB/octave roll-off onto the LFE channel.
The .1 channel was originally designed for use in theatres where the main channels could not reproduce the lower frequencies and additional headroom was required at these low frequencies to reproduce high SPL. It is often given different names depending on its use:
• Low Frequency Enhancement
This is a commonly used name as the LFE channel is often used to ‘enhance’ the bass in some way.
• Low Frequency Effects
This name is often used as the LFE channel is commonly used for explosions and other special effects requiring high levels of low bass energy.
• Low Frequency Extension
This is not an accurate name as the LFE channel’s frequency response extends down to the same frequency as the main channels, i.e. 20 Hz.
The use of the LFE channel is not consistent throughout the audio industry, as the needs of Movie Theatres (Cinemas), Home Theatres, Digital TVs and Music only production are all very different. Also, sound engineers new to multichannel surround sound mixing are experimenting with new ideas and techniques all the time. Various multichannel encoding/decoding processes exist, but one consistent feature for all formats is that all main channels remain full bandwidth and the LFE channel has variable upper cut-offs (or even a full bandwidth) as already detailed.