It is particularly in adjusting the tone and dynamics of each sound that the mix engineer controls the loudness of the mix. As you already know, sounds with a lot of upper midrange energy and with relatively flat dynamics have the most loudness. But unlike the composer’s freedom of choosing which notes actually make up the piece of music, the mix engineer’s tools can only modify the sound of the notes that have already been recorded. Fortunately, those tools are varied and powerful.
The most powerful sound-shaping tool available to mix engineers is EQ. This tool alone can make any sound bright or dull, strident or subdued, thin or heavy. And with such a powerful tool comes great responsibility. A mix engineer, like a composer, could quite easily make a loud mix by making every sound brash and strident. Of course, this wouldn’t be very pleasant to listen to.
A more appropriate use of EQ is to make sure each sound has a distinctive character and role in the mix. EQ can make for a louder mix by making sure that each area of the mix – the bottom, the low mids, the upper mids and the top – is clear and focused. Think about which sounds will dominate in those areas and make sure other sounds aren’t competing. This will make it much easier to make a mix loud. On the other hand, a mix that is muddy and indistinct will fight every step of the way to loudness.
You’ll probably also find that the higher the frequencies, the more room there is in the mix. The upper mids in a mix can often accommodate a few distinct prominent sounds. The very top of a mix often needs almost no carving at all. By contrast, the bass region can often only fit one or two different sounds, and the subs can barely fit one. This is why common mixing advice includes high pass filters and lower-mid cuts to increase clarify and space in the lower ranges.