If your mix sounds very different from system to system…
(car, phone, stereo…) you probably have inconsistencies in your low end. This can include everything from 20hz to nearly 300hz. Now (300hz you say???), that’s low mids for most people, but I classify it as low end for this example. It has to do with the bottom density of the MAIN part of your music–the fundamental notes of the instruments and vocals.
Home/project studios frequently have problems in this range because of room characteristics and/or lack of a good subwoofer. Unless you have the budget or inclination to fix your mix environment, you may want to monitor this range in over-the-ear headphones. They have a far more accurate low end than most mix rooms. Headphones are the simplest way to listen carefully for “bottom of the mix” issues (though they may still have issues with certain single frequencies).
The problem areas can include the kick drum, bass, and the lows of instruments such as guitar/synth/keys. The low-midrange fundamentals of lead instruments and vocals can also have an impact. You may have masking frequencies or specific notes where these elements are too loud or quiet. There are several ways to get more consistent playback across multiple systems. Here is a good article by iZotope about ways to address masking.
Another trick is to use a little bit of compression and/or add harmonically relative distortion to just the low-end-focused monophonic elements of your mix. Using distortion plug-ins or outboard tube units, hitting analog tape rather hard or going through overdrive pedals/guitar amps are common for this purpose. Be careful with this, however! Too much can take it into a less dynamic place than intended. Read about adding distortion to the low end of a mix for deeper knowledge on this subject.
Lastly, remember that unfortunately, a vast majority of popular listening systems abruptly drop off below 100-150 Hz. (I’m talking to you, Bluetooth speakers!). It’s in your best interest to pay attention on sounds above that range (more on this later).
Here’s an example of low end inconsistency. This is a bass line in a rock song.
Look closely at the balance changes in all the frequencies below 300hz.
As you can see, in this single run there are large differences in the balance of each note’s overtones (the level of each frequency).
These differences can cause difficulty when balancing the bassline with a kick drum of similar fundamentals, especially when the bass is deficient or dominates certain notes.
Compression and distortion are commonly used to help balance these disparities.
This bass track was recorded through a pedal board where the player already used compression and distortion pedals. You will notice on raw bass tracks and certain synths that the differences from note to note are greater. They can even be + or -12dB in the fundamental or overtones!