LFEs and Subwoofers In Perspective

I've gotten involved in surround sound as a music producer, mixer, and user consultant, all at once. This month, I'd like to share what I've been finding about how we are using the subwoofer and the LFE (low frequency effects) channel.
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Alert readers will recall that I've gotten involved in surround sound as a music producer, mixer, and user consultant, all at once. This month, I'd like to share what I've been finding about how we are using the subwoofer and the LFE (low frequency effects) channel. In subsequent months, I'll take a look at other elements of surround-sound production.

WHY HAVE SUBWOOFERS?

Subwoofers evolved to convey serious low frequency and infrasonic (below low) sounds to movie audiences, to enhance the thrill factor of earthquakes, howitzers, car crashes and other indicators of imminent doom.

It is unreasonably expensive to simply make all the speakers in a theater array capable of generating those frequencies and sound-pressure levels.

Low frequencies at high levels require large enclosures with large drivers and lots of power. Further, because in reverberant spaces, low frequencies are difficult to localize, it makes sense to have a separate channel and speaker to generate those effects, distinct from the more conventional effects, music and dialogue.

As home theater evolved, this led to the use of partial-range satellite speakers for all normal channels and a single low-frequency channel, for significant cost savings. I've written about this before, and I'm optimistic that it is generally understood.

What isn't so clear is how we actually use subwoofers and the LFE channel in production, as well as how they affect the end-user playback. Because I've recently been working on both ends of the transmission chain, I thought I'd share with you some of what I've found.

I've already described the origin of LFE channels and subwoofers. That treatment and practice seems to hold generally true for DVDs sold for domestic use.

The LFE channel carries the occasional low-frequency punctuation to on-screen action. Sometimes, it is a low-frequency extension of a broadband effect, such as an explosion. Sometimes it is simply a low-frequency noise perfume, added to underscore the gravity of an on-screen situation. Very little broadband music is included in the LFE channel. I ran across one low-pitched fundamental of a string bass note included in the LFE channel in "Das Boot," for instance, during a transition from a LFE to the musical score.


(click thumbnail)Fig 1. shows the left channel spectrum for approximately one minute of "Das Boot" titles, taken directly from the output of a DVD player, referenced to dBv.

(click thumbnail)Fig. 2 shows the left surround channel for same time segment.

(click thumbnail)Fig 3 shows the LFE channel for same time segments.LFE signal levels are quite conservative--typically 10 dB below the broadband channels. Interestingly, a great deal of low-frequency material (effects and music) is present in the main channels and occasionally in the surrounds.

As result, the LFE/subwoofer contribution to the soundtrack is typically modest.

Take a look at some real-time spectra I took from a short segment of the title sequence of "Das Boot," where sonar impressions of a ship passing overhead climax and the orchestral score begins.

Fig. 1 shows the left channel spectrum for approximately one minute of "Das Boot" titles, taken directly from the output of the DVD player, referenced to dBV. Note the low frequency peaks around 50 Hz, at -30 dBV and at energy as low as 20 Hz.

Fig. 2 shows the left-surround channel for same time segment. Note the low-frequency peak is at -25 dBV, 5 dB louder than the left-front channel. This is a function of the effects pan from front to rear as the signal grows louder. Notice also that there is significant energy down to 20 Hz.

Fig. 3 shows the LFE channel for the same time segment. Note the low-frequency peak has a different spectrum (and different sonic material), is 6 dB softer than the left channel; and 11 dB softer than left surround, with less energy at 20 Hz than either of those channels.

Interesting, eh?

(click thumbnail)Fig 4. shows the left channel in 12 seconds in the bridge of the "Theme From Shaft."

(click thumbnail)Fig. 5 shows the left-surround channel for the same time period.

(click thumbnail)Fig 6 shows the LFE channel of the same segment.
IN MUSIC

In music production, things are quite simple. The LFE generally is not used. We've learned to drop a summation of all the channels, rolled off above 100 Hz and attenuated by 10 dB, into the LFE channel just so end users will know the channel is on and working.

If the system is well-calibrated, the LFE channel will not be audible.

Again, take a look at three spectra of music from the remake of "Shaft." This is 12 seconds in the bridge of "Theme From Shaft," where a little low-frequency foley is dropped in for atmosphere.

Fig. 4 shows the left channel. Note a typical broad musical spectrum with plenty of bass extending all the way down to 32 Hz.

Fig. 5 shows the left-surround channel for the same time period. This is reverb wash from the front channels.

Fig. 6 shows the LFE channel of the same segment. This is a little low-frequency foley related to a fantasy car drive-by in the soundtrack. The point here is that the LFE material is completely submerged by the music and of minor importance.

IN THE HOME

Now that I've set up some dedicated home theater systems, I have a little more feel for how this all plays out. In a good system, a great deal of the low-frequency material is played back through the main channels. LFE means what it says.

If we set all the channels at unity gain, the LFE channel will be inaudible almost all the time, except for mild extensions during moments of high drama.

I've experimented and satisfied myself that dialing in 10 dB of gain (relative to the other channels) in the subwoofer channel is a reasonable thing to do. At that level, we can hear the effect of the subwoofer, and it sounds both dramatic and reasonable.

One final problem for the home user has to do with cheap systems where the sub is supposed to fill in up to fairly high frequencies, say 200 Hz, because the satellites are so small. Under this circumstance, we are depending on the subwoofer to carry the low-frequency portion of all the main channels, via a downmix that is invoked in the receiver by selecting "small" speakers.

The result is a crude mix of LFE, main channel and low-frequency material going to the subwoofer. Not particularly pleasing. Not under our control. Not something we should mix for, as far as I'm concerned. It is one of the economic compromises that consumers sometimes make, but it's beyond our ability to correct.

The subwoofer is probably the least critical element in surround audio production and playback. It is dedicated to effects and those moments when the end is nigh. Mixing for it can be treated quite straightforward and conservatively.

No need to obsess here. Just get enough audio in it so that the end user knows the sub is working, and so that when something particularly exciting happens, maybe there's a little LF adrenal hit. Thanks for listening.