Ever since 5.1 surround sound came on the scene, there has been a discussion about what the ‘.1’ is for. With the introduction of Dolby Atmos, this is even more important. In this article, we discuss whether LFE stands for Low Frequency Effects or Low Frequency Extension.

Some of the most challenging sounds to reproduce from a sound engineering standpoint are considered to be the extremely low-pitched sound effects in the 20 Hz range, such as those used to simulate the sound of an explosion, earthquake, 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 energy for the human ear to hear them. What’s more, sounds at these frequencies are more felt, rather than heard. When 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.

The History Of The LFE Channel

Dolby explains that the LFE channel was originally devised for 70mm movie productions to deliver a separate bass signal to one or more additional subwoofers placed behind the movie screen.

• This allowed deep bass effects, such as large explosions and other LF content, to be added to movie soundtracks without having to upgrade the existing speakers and amplifiers in the three main screen channels.

• 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.

• Finally, no additional frequency crossovers would need to be retrofitted into existing cinema processors to redirect the bass from the main left, centre, and right channels to the subwoofer(s).

Taking advantage of the available channel capacity on 70mm 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. To maintain full compatibility with existing theatres, the Dolby Digital film format includes a separate LFE channel. Hence the name LFE: Low Frequency Effects.

In the 1990s surround sound home theatre systems became available, designed to enhance the experience of viewing DVD and Blu-ray films at home. With home theatre 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 consisted 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 theatre systems, especially home theatre in a box systems, were incapable of accurately reproducing LFE in the 20 Hz range.

LFE Stands For Low Frequency Effects

If you consider that LFE stands for Low Frequency Effects then the view is that you should only put effects related content in the LFE channel.

The LFE channel was designed to deliver bass-only information to supplement the overall bass content. The LFE channel content is not the same as the content of a subwoofer output found on many home theatre systems. The LFE channel is intended to be used to carry additional bass information in the surround sound programming, 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. Beginners often get confused between the role of the LFE channel and bass management, in which monitor systems use the subwoofer speaker to achieve two roles.

It is important to understand that the LFE is a specific content channel and that it’s actually a 120Hz bandwidth-limited channel (hence the 0.1 descriptor, highlighting that it isn't a full-bandwidth channel) that also has an additional 10 dB of electrical headroom compared to the other main channels. Its purpose is a way of adding content specific low-frequency energy, and because it is an effects channel it should only be used for effects so that in systems that have LFE reproduction the consumers will enjoy it, but that the mix will still hold together if its not there..

The argument goes that If you put other content through the LFE channel like music elements then you run the risk that when it is played back through a domestic system with bass management that the LFE channel and the bass management path could be out of phase and so your bass elements will be significantly reduced, or going the other way, the bass management and the additional material added to the LFE channel could ‘double-up’ and overload the consumer’s system.

Also because the LFE channel is calibrated differently and the crossover frequency may be different it is very difficult to make sure your mix will translate across if you use the LFE channel to enhance other content like music. 

Finally, it is recommended not to put anything critical to the mix exclusively in the LFE channel. This is because when the audio is folded down, to sat stereo, the LFE channel is usually dropped completely and so any content routed to the LFE channel won't be reproduced when the fold-down mix is used. This way, as long as the LFE channel is only used to enhance very low-frequency effects then their removal from fold down mix is not catastrophic to the mix. 

LFE Stands For Low Frequency Extension

The argument for using the LFE channel as a Low Frequency Extension channel starts off with that old saying, "There's more than one way to skin a cat".

Dolby Pro Logic decoders employed Low Frequency Extension by routing anything below 80Hz in the front channels to the subwoofer, effectively providing bass management, whether the consumer’s system needed it or not. Opening up the history books, in the theatrical delivery workflow, back when 5.1 digital audio was optically encoded into the spaces between 35mm film sprockets, the analogue Dolby LtRt "SVA" optical track was used as a "safety fail-over" in case the fragile digital optical track got damaged. So, the LtRt Pro Logic track had to have a comparable spectral response to the digital 5.1.

It was for this reason also, that the Dolby Fold-Down (LtRt) settings for making this safety track, reflected the relative gain settings of the speakers in Dolby licenced dubbing theatres. The Left and Right were at unity, the Centre at -3dB, the surrounds at -3dB and the LFE included, at +6dB. 

Compatibility Vs Consistency

This raises the issue of compatibility versus consistency, which brings us to the problems faced when trying to deliver a mix that sounds consistent in a domestic uncalibrated environment.

Basically, with a budget home cinema kit set up by whoever, it's virtually impossible to ensure that the playback lineup is correct in a domestic environment compared to a theatrical cinema workflow where it is much easier to have a fully calibrated system end-to-end. As 5.1 and more recently 7.1 and Dolby Atmos has become more available in domestic situations, certain problem areas have been identified. These are usually incorrect centre and surround speaker placement and subwoofer level.

The ITU fold-down specification does not use the LFE track. Delivery specifications for 5.1 TV mixes often have the dialogue diverged out across the Left, Centre and Right channels, to mitigate problems with dialog levels because the centre speaker may not exist or be in completely the wrong place, but unfortunately, these are standards that have been put in place in response to reported playback issues.

Any decent cinema mix or playback room has twin subwoofers. Surely, it would be such a waste not to use these channels for LF Extension when they're available and so capable. 

Those lucky enough to have heard (and felt) a live orchestra in a good venue will know just how powerful it is. Like a rock concert, you feel as much as you hear. Low Frequency Extension in music is as much of an ‘Effect’ as say the sonic pulse of an explosive going off or a dinosaur's footsteps. Imagine the power of the iconic "Jaws" score, shaking you in your seat. In concert mixes for TV using the Low Frequency Extension idealogy, the LFE is used for kick drums, bass guitars, bass drops, anything that can recreate that ‘wall of air’ you would feel if you were at the concert.

Finally, if we go back to the history books, the first surround system with LFE tracks was the original Disney "Fantasia". They had multitrack surround sound and ‘vibrator’ channels on optical film playback, running in sync with three projectors, giving super widescreen and a fully immersive audio experience. They pushed the technology to be able to bring an immersive audio experience and shouldn’t we take inspiration from their innovation and stop trying to just accommodate the lowest common denominator. 

What Do You Think?

Are there different answers, one for theatrical mixes and one for mixes for home entertainment and mobile consumption? What are your views? Which type of LFE do you use? Low Frequency Effects or Low Frequency Extension? Do share your experience in the comments below…