In this article, with the extraordinary support of Fernando Cubillos, we will explain the masking principle and some tips to avoid this problem that makes your mixes sound like a war of sounds fighting each other, getting an amateur and muddy results.

Introduction to Frequency Masking

A higher-amplitude frequency masks a similar lower-amplitude frequency. This issue is known as “Frequency-Masking”. We can perceive it more obvious when the sources are coming from the same place, or they are close to each other.

A classic example is the kick and bass balance. They are living in the same frequency range and it may occur that the kick loses definition because of the low end of the bass.

In the real world, frequency masking does not occur when the sound sources come from totally different locations. Just like a stereo mix, where the sound comes from 3 different locations (Left, Center, and Right), there are three options for sounds to get masked. 
In a mono mix, all the sounds come from the same location, so frequency-masking is highly likely to occur.

How Frequency Masking Can Affect Your Mix?

Imagine you are mixing a song where the elements are: Electric Guitar, Lead guitar, Lead vox, and some lead keys. All of these sounds live in the mid-high frequency range, so when it comes to mixing, the elements will “fight” for a room and presence.

How to Fix That?

There are many ways to reduce Frequency-masking issues:

Making a good panning strategy: With panning, we can avoid frequency masking. If you have two or more elements living in the same frequency range, you can distribute them on the stereo field, at different locations. This method doesn’t applies if you want to maintain mono compatibility.

Through Equalization: If panning is not an option, you can eq the elements of your mix. The trick consists in identifying the element that you want to place at the front of the mix, so that element has to get the higher amount of important frequencies. You can try boosting these. 
On the other hand, if all of the elements of the mix already have a higher amount of these frequencies, you can cut these from the other elements you want to place at the back of the mix.

Other techniques: There are too many other techniques like side-chain compression, volume automation, or a combination of all of them.

Some mixing tips:

• Use equalization as the last option: If an element is masking another, you can try panning it. If panning is not working, you can eq it. To use too much Eq in a mix may cause phase issues resulting in a lazy mix.
• If you are mixing a song with a LOT of elements that are living in the same frequency range, you can try keeping only the important ones of the arrangement. Sometimes, masking occurs because the arrangement contains too much information.
• Use volume and pan automation: You can make room for an important element in the mix, just move the other element to a different place in the stereo field.
• Use many high pass filters as tracks you have: Keep the low end clean. It helps to make room for the kick and bass.

Also read: Audio Filtering Explained

• Reverberation may mask dry signal too: Reverb return tracks are just like another audio track, it is a source of information. Try the FX return tracks with panning/volume automation and eq to avoid masking.