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Making It Move: Psychoacoustic Manipulation Of The Stereophonic Listening Experience

Techniques to add another dimension of creativity to mixes as well as to present a wider and deeper spatial soundscape.

Digital mixing consoles offer engineers access to an incredible range of techniques we can utilize to create great mixes. For now, with few exceptions, nearly all those mixes are created for stereo reproduction, a phenomenon which exploits the brain’s ability to discern direction and relative distance of sounds arriving at our ears at slightly different times.

Clement Ader is credited with the first use of stereophonic reproduction in 1881. The French engineer used two telephone lines to broadcast live performances from the Paris Opera to special rooms at the Paris Electrical Exhibition. Listeners were amazed by the realism stereo provided.

Meanwhile, modern stereophonic sound was invented in 1931 by Alan Blumlein at EMI (Electric and Musical Industries) in London in an attempt to impress his wife. Blumlein initially wanted to improve audio for the young cinema industry, but his work produced over 70 patents related to stereophonic broadcast and reproduction.

Earlier in my career, I settled into a long-term position mixing in-ear monitors (IEMs) for a band with a high channel count. I was able to delve deep into stereophonic mixing as I worked to clarify their mixes while benefiting from the extra information that width and ambiance provides.

With IEMs, the engineer can control the entire soundscape simply by panning mono signals in the stereo field but that only scratches the surface of what’s possible. By panning two instruments to noticeably different positions, they can be easily discerned from one another while existing at the same relative volume.

After a few years of constant refinement in monitor world, I moved back to the other end of the snake, utilizing what I’d learned to improve both my live mixes and the recordings they generated. Technically, IEMs and headphones are totally binaural, with almost complete isolation between the left and right signals. Extreme stereo effects sound huge through both of them, but this often can’t translate to PA systems because of comb filtering and other modes of cancelation – those interference patterns can easily ruin a mix.

Further, not all stereo effects will translate through a stereo PA, so with that in mind, here are a few of the techniques I use to give my mixes more width and depth in the stereo field.

Arranging The Space

The first tool is the panning of mono and stereo instruments in the stereo field. I prefer to pan things as they are ordered on the stage as viewed from front of house, but with as much symmetry as can be arranged. That’s just one way to pan; no firm rules dictate where you turn the knob.

The purpose of panning is to provide separation by direction. My drumkit is panned very wide, with kick and snare in the middle, overheads panned hard to each side and the toms arranged in order, lowest on the mid-left, highest on the mid-right.

Another creative use of panning employs multiple signals to create a unique stereo image of a typically mono instrument. For example, using two sonically different sources, like a microphone and a DI, on an electric guitar amp. Pan the microphone partially left and the DI partially right (with delay to compensate for the mic’s distance). When they combine, the resulting image will be balanced between the two pan positions, but since those two signals will have slightly different frequency responses, the summation will seem to drift slightly in the stereo field.

It’s a subtle effect but one that contains much more spatial information than a simple mono signal. With my stage layout panning technique, resulting recordings convey a sense of how the stage was arranged, allowing for a more immersive re-listening experience.

Forward & Back

The second tool is changing the low-pass frequency of my reverb returns while also varying the pre-delay. Acoustically in most environments, air resistance over distance tends to absorb more of the higher frequencies. Therefore in nature, when we hear reverberant reflections of distant sounds, the reflections have less high frequency content than the reverberations from closer sources.

When I build my mix, I like to hear the lead vocals and some lead instruments closer to the listener, with the drums and percussion sounding farther away, and rhythm instruments and backing vocals occupying the space between. I achieve this effect by using a low-pass filter on the drum reverb returns.

I start with a stereo plate decay time around 2.5 seconds, low passed around 6 kHz. The vocal plate, with a decay time of 2.3 seconds, is low passed around 12 kHz. The rhythm instruments and backing vocals sit somewhere between those values.

Choosing brighter or darker reverbs allows me to move an instrument forward or back in the mix. The variation in pre-delay values simulates the distance from a sound source to the first large reflective surface, such as the distance from the drum kit to the ceiling and walls of a hall. Longer pre-delays on the drum reverb help reinforce the distance effect caused by the difference in reverb low pass. My pre-delay values range from 5 to 10 milliseconds (ms) for near sources and from 40 to 60 ms for far sources.

Going Wider

My final tools are the subtle (and sometimes not-so-subtle!) psychoacoustic effects that serve to artificially widen single instruments, sections or even the whole mix. These represent the “special sauce” which can make a mix exciting and bigger than life, but they must be used with discretion, or the recipe ends up muddied and ruined.

There are hundreds of different hardware and software effects capable of manipulating perception of time and direction from the original source. I love putting vocal doublers on both vocals and horns. These are usually two or more pitch shifters (+/- 5 cents), followed by a very short delay (5 to 25 ms), panned hard left and right. This adds width to the original source in three dimensions: location, time and pitch.

Mono keyboards can benefit from using short delays, with different delay times for left and right, to impart an artificial sense of stereo to a mono source. Delay times of less than 40 ms work well here.

Adding motion to a delayed or reverberated source can also sound exciting. Ping pong panning or even a simulated rotating speaker effect can work well to create movement around the delayed source.

This just scratches the surface of what’s possible. When used sparingly, these techniques result in another dimension of creativity to explore in my mixes as well as a wider and deeper spatial experience for the listener.

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