To me, the 11.1 db boost is part of one possible corrective EQ technique. The original dialog isn’t great audio quality, (I find it ‘brittle’ to the ear) but that’s probably because our friend hosting the video wants to share this way of dealing with this type of audio problem. The problem being that the audio is not in ‘balance’ across the frequency spectrum and lacks some low frequency content. To help counteract its overall ‘brightness’ in this case, carefully add some ‘low/low mid’ back into the audio track. The correction will help rebalance the audio spectrum to sound fuller/richer.
So what happens is this… the Low Shelf is boosted by what can appear to be an excessive amount. However what we are really interested in is the curve of the filter above the frequency that has been selected (116 Hz) at the moment.
Normally for this type of problem I would start with a frequency way below where the lowest part of the voice/instrument/whatever could be found, then give the Low Shelf a large gain boost. Sweeping the frequency upwards slowly until I hear the upper-edge of the filter curve start to lift the lowest parts of the audio spectrum. (This will normally be the fundamental and the first set of harmonics/overtones/partials). Now the curve or slope of the Low Shelf furthest away from the selected frequency will still be 0 db in gain. As you work your way back towards the highest point of the filter, the gain value will change and boost the harmonics at different amounts allowing us to ‘shape’ the sound more.
Once I’m happy with my frequency range that is being boosted, I would then adjust the gain amount and modify the curve shape to fine tune the response of the audio further. Those last two parameters are sort of ‘juggled’ back and forth to find the best results so you need to play with the values here and assess the results. (By the way, not all ‘Low Shelf Filters’ allow you to change/modify their response shape, so you might need to use a third party EQ that does if your DAW’s EQ isn’t equipped with this feature).
With what happens below the selected frequency here really depends on whats found in that audio region after the first part of this technique has been applied. I most likely would then use a HPF (High Pass Filter) as you would, to roll off any unwanted low/sub content and possibly a tiny portion of the fundamental as well to keep it more under control. It all depends on the frequency range of the audio file in the recording as to how successful this can be. Having a HPF which has an adjustable cutoff slope (6, 12, 18 db, etc) and ‘Q’ or resonance control is a bonus here as well.
Onto the -11.1 db at 568 Hz…
I’m thinking and hearing that this is to help reduce the ‘boxiness’ from the audio recording after the initial sweeting of the lows and highs. Unfortunately I find the audio still has some unwanted resonance issues and boxiness when our friend has apparently finished dealing with the EQ… That throws up the question of what is he monitoring on, but that’s for another thread.
The big lesson here is always let your ears decide how much of a boost or cut should be used and not your eyes… Values are misleading!