Habituation is the name for our tendency to respond less to something the more we’re exposed to it. While the concept is academically important to psychologists and biologists, it also has enormous significance for anyone serious about mixing or mastering music.

We likely come by this tendency through evolution. Our hunter-gatherer ancestors relied on habituation to “tune out” uninteresting elements in their surroundings. They depended on seeing movements among unchanging trees and grasses, and hearing small sounds amid constant background noise, both to seek out food and to avoid becoming food! But even today, this dulling of our senses to repeated information is important. We’d quickly succumb to information overload if we focused on all the stimuli fed to us by our senses, 11mbps, or 120Gb per day, by some estimations. Without the ability to filter this information for what’s important, we’d be swamped.

The mechanism by which we filter is known as sensory adaptation. In short, our brains place greater significance on changes in stimulus than on the absolute overall level of the stimulus. You’re likely most familiar with this effect as it relates to light sensitivity and temperature sensitivity. Step indoors on a sunny day, and you’ll find yourself blind for a few moments. Run your hands under cold water after handling snow, and you’ll realize that cold water only feels “cold” when we have warm water for contrast. Our sensory perception is relative.

As a mixing engineer, it’s important to realize that human hearing is also subject to sensory adaptation. Notice how you become aware of an air conditioner or fridge when the device stops making sound. Your brain keys in on the change rather than the overall level. In fact if we constantly focused on these background sounds, we’d have little attention left to notice the sounds that might really be important. We’re not listening for predators like our ancestors, but we still need to hear the phone ring, or the car approaching from around the corner!

The importance of this to people working with music is nicely summed up in a mixing tip from Sound On Sound magazine:

Don’t assume that your ears always tell you the truth. Rest them before mixing and constantly refer to commercial recordings played over your monitor system, so that you have some form of reference to aim for.

Our ears grow used to repeated sounds. And mixing is exactly that: Listening to the same sounds repeatedly. So mixing engineers need to be especially aware of sensory adaptation, and the dulling effect it has on our hearing.

By checking a reference CD periodically while you mix, you ensure that habituation and sensory adaptation don’t get in the way of a balanced sound. The reference CD gives your ears a change and keeps your brain honest.

As a practical example of sensory adaptation in action, contrast The Smiths’ track How Soon Is Now with my track Brand New Car.

If you haven’t listened critically to The Smiths song before, it’s probably immediately obvious how thin the low end is. However, as you listen your ears adapt to the mix, and by the end it sounds balanced. Follow this with the extro from Brand New Car, deliberately mixed to have a thick bottom end, and the contrast makes the bass in the 2nd track sound overpowering. But after 30 seconds or so, and your ears will again adapt.

Now alternate between the two tracks to hear sensory adaptation at work!

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