low frequency effect

11/29/2024

We all know that the LFE in a surround/immersive system stands for Low Frequency Effect ... you know, BOOM! In fact the first use of the LFE was in the movie Earthquake. When it was time tor the quake to happen, these huge-assed subs under the seats began to go nuts and the crowd went wild!

After getting my studio in it's best shape ever, I finally have a way to playback 5.1 media (I can only playback 7.1.4 audio through my DAW) in the room using windows. We now have a second 5.1.2 system in the bedroom that's going though tuning with amazing results, so< I have a send place to check levels.

So maybe I've been a bit slow to learn, but getting back to basics LFE stand for BOOM ... not bass. In music mixing, I've been making the mistake that the LFE chanel can be used to support the bass. In fact, I'm finding the extended bass notes in the LFE only adds to muddyness of the mix, especially is the listener has a badly-tuned immersive system (which probably is most of them). I'm finally changing my practices in immersive music mixing to route only transient bass to the LFE bus (such as kick and other short term bass notes ... not sustain notes like bass guitar. Any thoughts?

https://www2.grammy.com/pdfs/recording_academy/producers_and_engineers/5_1_rec.pdf

4.6 Use of the LFE Channel As noted in section 1, the LFE ("Low Frequency Effects") channel was originally introduced by the film industry because early theatrical speaker systems were unable to generate loud low frequency signals without clipping. Sometimes referred to as the "boom" channel, it is used in film applications to add dramatic effect, almost exclusively carrying the rumble of volcanic eruptions, spaceships thundering into view, and bombs and planets exploding. 

In terms of multichannel music production, however, there is some debate as to whether the LFE channel is necessary at all. One can argue that the home theater experience is heightened by having the walls shake whenever rocket launchers are fired, but is there really that much value in having the listener feel every bass drum hit in such a similarly dramatic fashion? 

There is no clear-cut answer, but the mere presence of the LFE channel almost dictates that it be used. The danger lies in overuse, because too much reliance on the LFE channel to carry bass information can result in the loss of low end altogether on incorrectly configured or poorly designed home theater systems. In addition, the LFE channel is discarded by most matrixed encoding systems (such as Dolby Pro Logic; see section 1.1) and downmixing algorithms (see section 5.5), including those used for HDTV broadcast. Therefore, the LFE channel should never be used to carry the bass content of the main speaker channels — that is the job of bass management. Because bass management is employed by almost every consumer home theater system, placing too much information in the LFE channel will effectively result in double management — total bass overload and probable distortion. 

Instead, the LFE channel is best approached with caution. Only modest amounts of signal from specific instruments with significant low frequency content — kick drum, tympani, bass guitar, acoustic bass, low organ or piano notes — should be routed to the LFE, and in all instances those instruments should also be printed full range to the desired main channels as well. An experienced mastering engineer can help in correctly assessing the relative level of the LFE channel as compared with the main channels.

3.6 Speaker Calibration Procedure (LFE)

The final step is to set the level of each subwoofer relative to that of the main speakers. Common practice is to calibrate the subwoofer approximately 4dB above the reference level of the main speakers. This procedure differs somewhat depending upon whether the subwoofer is receiving the LFE channel only or whether bass management is being utilized to route signal to it from some or all of the main channels.

The recommended subwoofer calibration procedure when no bass management is being used is as follows:

1. Turn off all five main speakers.
2. Route band-limited pink noise (low-pass filtered at 80 - 120Hz) at 0 vu via the LFE channel bus to the subwoofer and raise its amplifier level until the RTA or SPL meter reads +4 dB over the selected reference level (i.e. 89dB if the selected reference level for the main speakers is 85dB).
3. Turn on the front left and right speakers.
4. Route full frequency pink noise at 0 vu to the front left and right speakers as well as to the sub. Adjust the subwoofer amplifier so that the gain boost when adding the subs to the mix does not exceed 4 - 6db, as measured by the RTA or SPL meter. 

The recommended subwoofer calibration procedure when bass management is being used is as follows: 

1. Route band-limited pink noise (low-pass filtered at 80 – 120Hz) at -10 vu via the LFE channel bus to the subwoofer. (The 10dB of attenuation compensates for approximately 10dB of “in-band gain” in the LFE channel as compared with a main channel.) 

Note: “In-band gain” refers to the fact that the level in each 1/3-octave band within the frequency range of the subwoofer is 10dB above the level in each 1/3 octave band in each of the main channels, averaged across the main frequency range. This does not mean that the LFE channel is 10dB higher in SPL than the main channels, however, due to the broader bandwidth (and correspondingly greater energy) in the main channels.