Have you ever had the experience of working on a mix, and while the balance is generally working, a couple of instruments seem to be clashing or covering each other up? This could be related to a psychoacoustic phenomenon known as auditory masking. Masking occurs when one signal distracts the ear from another signal happening around the same time.
Mixing kick and bass
A classic example of this comes when trying to figure out the relationship between a kick drum and a bass part in a mix. You might listen to them individually and find they sound big, full, and loud on their own, but when you listen to them together, the kick drum seems to get lost. You might turn the bass down so you can hear the kick drum again, but then the bottom falls out of the mix and the bass doesn’t have the power it needs to anchor the rest of the harmony. Countless engineers have spent hours trying to solve problems like these, but a quick understanding of how masking works can help you identify some solutions and start creating better-sounding mixes right away!
EQing bass and kick to relieve masking
One of the reasons bass tracks especially mask kick drums is because they both are competing for the low end of the mix. A kick drum is going to have some substantially high-mid frequency attack that helps it sit on top of a mix, but how do you get that satisfying thump in your chest with the kick drum, without sacrificing the sound of the bass?
One answer may lie in complimentary equalization. This is a simple process of identifying which frequencies collide between the kick and bass, and then boosting those frequencies slightly in the kick, while making a complimentary cut to those same frequencies in the bass. In the illustration below, we see the EQ for the kick drum on the left, and for the bass on the right.
Notice that the kick drum has a boost around 85 Hz, and the bass has a cut at the same location. At 200 Hz, the kick drum has a cut and the bass gets an inverse curve. You’ll notice these are very slight adjustments, just a couple of dB up and down respectively, but that might be all it takes to help unmask the kick drum from the bass.
Using sidechain compression
Another complicating factor in the example above is that bass parts are generally sustained, whereas each kick drum impact is going to be quick and percussive. Because the bass is constant, our ear may naturally gravitate towards that, in turn letting the kick drum get masked. This same effect can happen when mixing vocals, where a lead vocal could be competing with big, double-tracked guitar parts.
While some complimentary EQ can certainly help, sidechain compression can also be used to clamp down on the guitars when the lead vocals are present, while opening up and letting them be loud during instrumental sections. This can be taken one step further by using a multiband compressor or dynamic EQ, and only applying sidechain compression to the area of the guitars where vocals are competing the most. Often, this is in the presence and intelligibility area between 1-4 kHz, so some sidechained compression or dynamic EQ focused in that area could do just the trick!
Both the Dynamic Equalizer and the Multiband Compressor in iZotope's Neutron can be keyed from a sidechain input, or internally from different frequency bands.
Other tips for relieving masking in your mixes
Prioritize and balance. Sometimes, it’s simply an issue of reassessing the balance of the mix to make sure everything is prioritized the way you want it to be. A great way to do this is to break down the mix to its most simple elements. For a rock mix, this might just be the kick, bass, hi-hat, and lead vocal. Getting that balance just right, and then layering in other elements around it without adjusting those four tracks again, can help you keep those core elements balanced, and force you to make decisions about how the other parts can fit in around them.
Use the stereo field. If a guitar and a piano are covering each other up, try panning them to opposite sides. Or get even crazier and try adding a stereo delay to the guitar so that the piano is allowed to mask the attack of the note, but the guitar gets to have an elongated sustain with lots of motion.