But our brains also use other cues to determine distance. For example, human hearing excels at matching a sound with the echoes and reflections it causes, to localize its source. And we can apply this principle to add realism when creating the sound stage in a mix.

The Speed of Sound

Consider this picture, and the accompanying audio samples below.

In the scenario illustrated above, sound from the guitar reaches the listener almost immediately, whereas the reflections off the rear wall make a 40-foot round trip, and therefore arrive 40ms later. (Sound travels approximately 1 foot per millisecond.) With the drum kit, on the other hand, the direct and reflected sounds arrive at almost the same time.

The series of events goes something like this:

Our ears and brain are sensitive to these differences in sound arrival time, and use the information (along with other cues, like volume) to judge where a sound source is located in the space around us. Our brains know that sounds and reflections arriving together at our ears must have originated close to a wall, where sounds that arrive much before their reflections must be close to our ears.

Hear it in practice

Here are two short instrumental samples, both mixed from the same raw tracks, to illustrate how this can apply in a mix.

In the first sample, I’ve placed the drums closer by adding a delay between the direct drum sound and the reverb, so the reflections arrive 40ms later than the direct sound – which tricks our ears into hearing a 20ft distance between the drums and rear wall, as with the guitar in the above diagram:

In the second sample, I’ve simulated moving the drums further back by having the direct sound and reverb occur together, both 40ms later than the guitar.

Note that the levels are the same in each clip. I changed the delay times only, to illustrate the effect.

Issues

Caveat: The illustration above is grossly over-simplified. Sounds in a real room reflect off all the walls and surfaces, not just the rear wall. And our ears depend on much more than just timing differences to determine distance. But for the technique at hand, those complications generally aren’t important. The idea here is to trick listeners’ brains by exploiting a property of their sense of hearing, and whether there’s one wall or 4, human ears and brains interpret reverberant sounds the same. (If your listeners are mostly non-human, then all bets are off.)

Implementation: In Sonar, I configure sends (i.e. busses) with delay plugins for each delay time that I need, and I route tracks accordingly. But any platform that allows bussing or routing the signal can accomplish the same end result. So long as you can independently control the delay on the direct sound and on the reverb, you can manipulate the relationship between the two as described above.

Other levels: In practice, you’ll also reduce the level of the drum kit somewhat to make it sound more distant, and adjust the reverb level as required to make the effect more obvious.

Pre-delay

As an addendum: Most reverb units and plugins have a pre-delay setting for controlling the delay between the input sound and the reflections it generates. Pre-delay serves exactly the same function as placing a delay between the direct sound and the reverb. In essence, it “moves” the sound further from the simulated reflecting surface. So if your reverb unit or plugin supports pre-delay, you can accomplish much of the above technique without a separate delay plugin.

And remember this simple guideline when using reverbs for realistic 3d sound stages: To bring a sound forward in the mix, increase the pre-delay.

See Also: Reverb on kick drum, Reverb possibilities

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There may be slightly more to it, however. Lifehacker recently featured a list of body hacks, and it included this great tip about using our ears:

If you’re stuck chatting up a mumbler at a cocktail party, lean in with your right ear. It’s better than your left at following the rapid rhythms of speech… If, on the other hand, you’re trying to identify that song playing softly in the elevator, turn your left ear toward the sound. The left ear is better at picking up music tones.

Neuroscientists have learned that because of the brain hemisphere to which each of our ears is connected, our left and right ears hear things differently. The right ear responds more to rhythm and speech, while the left ear is more attuned to music.

From a study published in the Canadian Journal of Experimental Psychology:

This finding provides support for claims of a right-hemisphere bias for the processing of melodic contour… In the present study, we sought to determine whether short tone sequences presented monaurally (to one ear at a time) are processed differentially by the two hemispheres. Studies of auditory processing often indicate that linguistic and musical stimuli are processed preferentially by the left and right hemispheres, respectively

This has an obvious implication for mix engineers: If you favour one of your ears, it could affect your mix decisions. Dye’s trick of checking a mix for mono compatibility could yield different results depending on which ear you stick your finger in!

Of course, you can use this to your advantage too. For example, when you need to make a decision about a mix’s rhythm elements, you might find it easier if you listen only with your right ear.

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!