Maybe you've heard from a mixing engineer, or more likely a mastering engineer, that you should keep your low-end as mono as possible.
Why should I obsess over that particular detail and how do I go about it?
Here's a couple of reasons "why" followed by a "how" section:
Mono sounds tighter
Because longer wavelengths and spatial perception don't really go along well.
Our brains have a really hard time determining the spatial position in the lowest part of the spectrum. In other words, it seems to be coming from everywhere at once. And our brain perceives a sound, coming from everywhere at once at roughly the same time and amplitude, as mono.
Mixing all your low-end in mono in that way helps with that perception. The brain doesn't struggle to find minute differences in amplitude and phase in the lowest frequency range to decide where it's coming from. The actual sound coming from the speakers perfectly corresponds with the easiest way for our hearing to interpret it. So to human ears and brains, it sounds precise and stable.
It survives lossy encoding better
No psychoacoustics here. Just looking at the dirty secrets of lossy encoders that still roam free throughout the internet, be it music streaming services or audio-for-picture.
I'm talking about MP3, but the principle started as early as FM radio transmission:
What should we throw away first when the signal gets weaker?
The majority of engineers concluded that the side or stereo information is a better candidate than either the left or the right channel. As the majority of useful information is usually in the mid channel (it's RMS level is almost always far greater than the side's), it makes more sense to sacrifice the side channel in an event of bandwidth loss. Compared to throwing away the right channel, we lose far less than half of the audio.
Today's lossy encoders do much of the same, amongst other techniques for throwing stuff away. If something has to go to meet the bitrate, the side channel is the first on the list.
We mixing engineers tend to obsess over the low-end presentation of the mix. We would love to have it untouched when it traverses different codecs and mediums of delivery. Making sure it ends up in the centre - mid channel - in mono can have a profound effect on that.
It's a must for vinyl
Vinyl records have become increasingly popular in the last couple of years. Making sure your mix sounds good on them is a good skill to have in your arsenal.
The sound on the vinyl record is encoded in a similar way like the FM radio or the lossy codecs. Again, it's the mid/side mode, or lateral/vertical if referring to the actual movement of the needle.
If a lot of your low-end ends up in the side or vertical channel, it can cause the needle to bounce slightly and cause unwanted distortion. It's much easier for the needle to track low-frequency grooves in lateral movement, so that's why the mid channel is encoded in this way.
How to make sure your bass is mono?
Basically, it's either completely manual or a one-knob operation. For the latter, you have to own or be subscribed to the Brainworx' bx_* plugins. If you do/are, you can find this technique as the "Mono-Maker" knob, with the adjustable frequency of the mono-collapse.
But you don't need anything fancy or expensive regarding gear or plugins to pull this off manually. All you need is a way to split the signal into mid and side channels and a high-pass or low-shelf filter!
When listening to the side channel in solo, sweep the high-pass filter in the 200Hz area. Somewhere between 100Hz and 200Hz, you will find a spot where weird, out of phase rumble disappears. Leave the filter there and listen to the whole mix - if done right, nothing important disappeared, but the centre-panned elements are now better defined and the stereo image got clearer!
Even though it's mono, it helps with the stereo image.
If the high-pass filter is not available, we can use the low-shelf instead in much the same manner. Engage mid/side processing, lower the gain of the low-shelf on the side channel and find the corner frequency where it cleans the stereo image up without taking away the body of the hard-panned instruments.
This is not a "night and day difference" kind of technique, but it can be very helpful when you're going for those last 5%.
But as with any other rule, this one too is meant to be broken... from time to time. If used to achieve an interesting effect, especially on headphones, out-of-phase stereo instruments can be pretty "funky", but it's good to keep in mind what it entails for different mediums.