Run a search for MS or Mid-Side.

Peace,
Nate

In many cases now, it's done with software on the final stereo bus. I use Sequioa/Samplitude and use the "stereo enhance" feature on the mix bus, which allows up to three bands of variable stereo separation and/or sum to mono, depending on the settings.

There are several presets, and it's simple enough to open the "Below 250 mono" setting and change the parameters to whatever freq you want, in your case, 50 HZ.

Everything above 50 hz remains untouched, but any freqs below 50 are now mono. Simple and sweet.

I'm sure all of the major software brands have this feature, or something similar. you could probalby roll your own if you got clever enough with designing a plugin.

This is relatively easy to accomplish with a simple multitrack software program. You need to take your stereo mixed recording and roll off everything below 60 hertz and save that as a new stereo sound file. You can actually do this with something akin to a graphic equalizer in software. Then you take that same recording again, rolling off everything above 60 hertz and convert that to Mono utilizing a channel mixer in software, along with doing the opposite with the graphic equalizer program. Now you will have 2 sound files. You slide those both into the multitrack program and mix that down again to another stereo sound file.

Full of base
Ms. Remy Ann David

In the old days when you cut a record on a lathe you had a very nice Elliptical Filter on your mastering console that mono'd up everything below a certain frequency. It was used so that the playback stylus would not jump out of the groves from an out of phase bass note. There are a couple of commercial pieces of equipment that say they have an Elliptical Filters built in but I have never tried one. I am surprised that no one has come out with a software version (or at least I have never seen one on the web). Elliptical Filters were not great from a phase linearity standpoint and in one publication the author describes the
phase response of the elliptical filter as "drunk fly on cross-country skis
in tornado" so I guess you should not use elliptical filters if phase linearity is important to you.

There is a lot of information on the web about Elliptical Filters but some of it can get very math intensive very quickly.

FWIW

Amazing responses, thanks very much guys... especially to RemyRAD

Run a search on this forum for "MS" or "Mid-Side"!!!
Unless you have sequoia or another program with multi-band stereo enhancement features.

One word of caution, you may want to be careful splitting the tracks as Remy described, as unpredictable phase cancellations near the "crossover" point may occur when re-blending them. Try using a phase-linear filter if you have one. Or try the MS technique - slightly more complicated, but without the phase issue...
I have actually done extensive experimentation trying to split frequencies, and it usually is hard to restore the same balance to the mix.

Peace,
Nate

Thanks

CrackBuddha,
I don't have sequoia or another program with multi-band stereo enhancement features.

Nevertheless, I can encode Mid-side, but how do you process the mid after this for tightening a mix under 60 hZ for example?
Sorry for my amateurism, I'd appreciate it if you want to walk me through this procedure. I'm trying this technique for the first time.