How to recognize good candidates for Bass EQ and what to fix
Any movie where sound effects and bass is lacking in weight are potential candidates.
Looking at the Peak-Average graph from the The Low Frequency Content Thread (films, games, music, etc) thread can tell a lot about how a movie sounds.
Kon-Tiki is a film with a good sound track utilizing the whole frequency spectrum, observe that the curve is a slightly tilted line extending all the way down to the very lowest frequencies:
The storm scene has content all the way down:
This film does not need any Bass EQ.
Battleship has met with an accident somewhere in the sound-making process:
There is an obvious drop right below 35Hz, looks like a steep high-pass filter has been applied.
This is something to work on for Bass EQ.
A good target response is likely an approximately tilted straight line, that extends as low in frequency as possible.
Start with applying equalization that bring the Peak-Average curve closer to this straight tilted response.
Evaluation by listening is necessary to determine appropriate equalization, and care must be taken to ensure not to clip the signal anywhere in the chain.
How low in frequency should you try to equalize to flat.
Depends on the original sound design, and whether there is any content to dig out, it may be buried in noise at the very lowest frequencies.
Fixing something destroyed with a 30Hz filter and mangaging to retrieve flat down to 20Hz can make a huge difference.
If you can extend it down to around 15Hz, even better.
Below around 15Hz is questionable, certainly depends on your playback system, as tactile feedback from floor or moving house structure may be required to notice it at all.
The lower you go the greater the risc of amplifying noise instead of usable content, and if you can bring a sound track with virtually no low bass back to life with significant content down to 15Hz, I would say that is a very good achievement.
If the filtering was done using a shelve filter it is more likely that there is content to retrieve.
A shelve filter can be recognized by looking for a flat level floor below the first cut-off.
In the Battleship graph you can see the level flats out below 20Hz at around -50dB, this indicates that a shelve filter has been used.
Then you can use an opposite shelve filter to retrieve that lost content.
Gravity, opening scenes (RTA from playback):
This film has a droning, constant tone around 20Hz, as can be seen in the curve.
This is part of the sound design.
The level below 20Hz drops off very quickly, but it is not likely that equalization can bring improvements, because likely there is no content that has been removed.
The relationship between frequency and time dictates that continuous tones, which have a long time span, are narrow in frequency distribution.
Impulse like transients, like cannons and gunshots, have short time span and a wide frequency distribution.
If there are no impulses, there may not be low frequency content to retrieve.
When the Explorer is hit by the debris, there are sound effects added that adds to the sensation of the incident, these sounds are more dynamic and impulse-like:
The spectrum reveals that there is content below the 20Hz drone, and that this content may have been filtered using a steep high pass around 20Hz.
Gravity is another candidate for Bass EQ.
Avatar, flight scene (RTA from playback):
This scene sounds good, the feeling of realism and athmosphere is good.
Right at the end of this scene, where the plane drops down the cliff, there is a sense of weightlessness, making it feel like you are on the plane.
The spectrum reveals why this effect is so good - there is significant content at the very lowest frequencies, peaking around 13Hz.
Avatar, monster stomp-stomp (RTA from playback):
Here the stomps sounds rather boomy and does not have the appropriate feeling of weight and impact.
The spectrum reveals why - the low frequency content below 25Hz is removed.
Considering the previous flight scene, the conclusion is that finding the best Bass EQ for Avatar will be a compromise.
Some scenes could benefit from a huge low-frequency boost, but then other scenes would sound horrible with too much in the low end.