Last time, I had spoken about how I treat the “Stage” and the surrounds as part of the sound scape and how they are used to create and make realism or focus based on what is important in the film at that given moment or flow. I also mentioned that I would write about how I handle the LFE and about balancing a movie.
It is important for all audio guys (especially beginners) to understand the difference between Subwoofer and LFE. The subwoofer is the speaker that reproduces the sound while LFE stands for Low Frequency Effects, and ranges from 5Hz to 120Hz. While it is easy to read and write about this, understanding this took me some time. It was because I was very confused with Bass-Management.
Why is bass management so important? Lets look at it. I mentioned earlier in Part 1 that the Stage speakers (the Left, Center and Right) are capable of reproducing from 20Hz to 20kHz. Now, it was (is) common practice to have an LFE signal sent from the main mix or stereo track to the Subwoofer. This is fine, but it is very important to realise one thing. The LFE has a crossover at 120 Hz. When you send a signal directly from the main channel to the sub, and you don’t have a filter, that means you are sending full bandwidth signal. This does not make the Speaker perform in the optimal way it is meant to because:
1) The Crossover has to filter
2) Signals are sent that are not necessary.
I have always maintained that the difference between Sound design and Mixing is that sound design is like making a painting, where the artist keeps adding layers on top of another on a canvas, while mixing is like sculpting where the artist removes layers from stone, yet both produce amazing art and both the tool welders are artists. So, in mixing it is a fundamental rule to remove unwanted elements before you end up struggling to make something cut through. The simple logic is that to cut though, it has to be sharp (obvious!) , and that would mean more high frequency and more high frequency is quick ear fatigue and unnatural mixes. So, coming back to the LFE, making the signal clean enough to the LFE is the first step. How do we do that? Usually 2 ways:
Filtering signals for the LFE is a good method to get a clean LFE, but there is something to be aware of. Phase reversal. It is a dangerous thing that can break the entire low spectrum by cancelling the signal from the main channel. How does this happen? Having really steep filters introduce something called Phase Distortion, in the lower spectrum. Studies in medicinal mapping have shown that this is enough to cloud reading and data. Now, imagine if you are sending a signal from the kick track to the LFE and there is a step filter. When done, and if the filter brings a distortion, there is every reason for the kick to be thinner and weaker when sent to the LFE because the signal from the Subwoofer would cancel the spectrum from the main channel. Now, since the Subwoofer works at considerably more amplification, the effect of cancellation is more pronounced. How can this be handled? Well, my technique is really simple. Lets consider the song mix. I try and get a full sounding mix from just the 5.0 channels without the LFE first. Once done, I take liberty in adding something to the LFE. (It isn’t a necessity really). If that decreases the impact, I immediately know there is phase issues. If it increases the impact, I make sure that the signal feeds are clean. There is another really important thing to consider in the LFE channel.
The speakers are handling low frequency that we can’t hear sometimes. This also means that the speakers have to push front and pull back to create the air pressure. In other words, the speaker has to have enough time to breathe. Else, we would end up reproducing complex waveforms that interfere. It is simple logic. This is what helps me decide what element I am sending to the LFE. I avoid sending fast synths just because of this. Also, if there are double kicks or faster percussions, those are also carefully thought of. Sometimes, I don’t send the kick and instead send the bass. Sometimes, I have the kick and the bass into the LFE, with a side-chain compression on the bass so that the speaker can still reproduce it with clarity. (Thats what we are aiming for aren’t we). I don’t have any fixed rule, but just these guidelines. (There was a scene in Billa 2, which is a fight in a club that had huge thumping kick and bass, yet nothing sent to the LFE. Just the punches. This created an illusion of an extended bass yet had all the pump of a club.) In a song mix in a film called Natrang (Apsara, Music by Ajay-Atul), there was no LFE at all except in the chorus rhythm and that too for the bass track not percussion. But nothing while listening to the song will make me think that I missed the body.
One of my secret (!) trick for LFE is to have a pitch down by an octave on the master and having a compression on that. The thought behind this is simple. Having an octave down on the LFE will produce an extended bass, and frequencies that are not present within the main channel. This is something I do for thunders or punches (and sometimes on winds). I use the aural exciter or the dbx 120A instead of this. Sometimes when using an aural exciter, I filter off signals below 40-50Hz (depending on the music) and have those present as a crossover on the LFE. This produces a cleaner extended body. It is important to fine-tune the crossover and the phase by constantly listening. One really cool shortcut in Pro Tools 11 for this is the vertical bypass (Control+Click on plugin inserts). This is a huge timesaver because you have instant A-B comparison between the sounds. My compression varies depending on what I am sending to the LFE. Faster release and slow attack on the punches while slow release on thunders can do wonders. I also sometimes put a delay on the LFE, with feedback as 0. The reason is that this will allow the bulk of the attack signals to come out from the main speakers while following up with the lows as an extended hold. (This has to do with how we perceive sounds. Attack always is more important followed by body.)
I also automate or remove elements when there is clashing frequency in music and say effects. It is not necessary for both to be in the LFE. (No breathing remember?). This makes for a cleaner and more impactful mix.