First, regarding no mixing houses have response below 25hz. Here is a quote from FilmMixer about the studio at ToddAO:
Originally Posted by FilmMixer
There is no bass management used on the stages when mixing films..
We do, however, add LF extension subs on the main channels... usually 2 18" Bag End subwoofers with no filters.. they go flat well into the teens...
If a film is mixed on a "smaller" stage, or a facility that hasn't spent the kind of money for such a setup, their room is still certified to dub in if it matches the x-curve (along with other requirements outside of frequency response.)
Originally Posted by filmmixer
My stage is 2700 sq ft (about 2 stories high.)
We had 18 18" subs (12 for LFE, 2 each LCR for low frequency extension)..
We removed the 12 LFE and went to a single sealed 4 x 18" box.
In terms of extension and dynamics, performance was improved.
I couldn't find a direct quote from someone in the industry whether <25hz content is intentional or accidental but lets look at the facts.
-Numerous large budget films have content below 25hz
-Vast majority of this content matches with what is happening in the movie
If all of this content is accidental, then the studios must be very lucky that all of it matches with what is happening in the movie and is at levels that add to the experience and do not detract from it. If it was all accidental and they did not have any concern with <25hz content, shouldn't there be more posts about ULF in movies that doesn't belong or seems out of place?
When any enthusiast can look at the frequency spectrogram of each channel of a movie with free software, I find it hard to believe that the industry pros don't also have a way to do the same, with much better software.