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Ears + Head

Stereo either works or doesn’t because of the way we hear.  So if we are going to record and mix for stereo, some basics of human hearing are worth investigating.

Humans have two ears on either side of a head.  When a sound originates from the left, the arrangement of our ears and head causes some specific things to happen:

These three differences -- time, volume and frequency – allow us to know where something is.  In fact, most of the time we can close our eyes, hear something, and point right to it by sound alone.  So the human Ears + Head system allows us to locate sound.  This is known as Localization. If our ears receive sound at exactly the same time, volume and frequency our brain localizes the source as directly in front of us.  Anything that isn't in front of us will produce sound at both ears, but with differences in time, volume and frequency.  The farther off center, the more significant the differences at each ear.

Space

There is more to what we hear than a direct sound.  If we are in a cave or a bathroom, or anywhere inside, the direct sound bounces off the boundaries and we hear that too.  We not only locate sounds directly, but we understand where we are and how big that space is when we listen in it.  Our Ears + Head get the same timing, volume and frequency cues that we use to locate a single sound to also give us a sense of Spaciousness.

In this context, let’s follow the sound as it travels from a source.  First, our near ear gets the direct sound.  Then shortly after that the far ear gets the direct sound (with some reduced volume that is frequency dependent).  Then our ears get the sound reflections bouncing off of walls, ceiling and floor.  These first reflections are also known as Early Reflections. 

After that, our ears get reflections of those reflections that continue to bounce around until the sound dissipates.  Reflections of reflections are called Reverberation, or just Reverb.  Sometimes Early Reflections and Reverb are grouped into the single idea that a space provides a complex set of reflections that together are known as Ambience.  Ambience can also refer to birds singing, crickets chirping, and other environmental noises outside, but for our purposes here it will mean the combination of Early Reflections and Reverb.

There tend to be fewer boundaries and fewer reflections outside, but there are some outdoor places with a lot of reflections.  In or out of doors, our hearing both Localizes and senses Spaciousness without effort.  It is possible to Localize the direct sound with no reflections at all.  It requires a special room designed to absorb every bit of sound, so there is no Ambience.  The complete lack of reflections is called Anechoic, which literally means “without echo”.  With no Ambience, the width of everything we hear is defined solely by those direct sounds.  But we normally hear in Echoic conditions, bathed in reflections.  The width we hear is no longer constrained to point sources.  The gaps between direct sounds are filled in by the Ambience and extend past the edges of direct sound.  Our Ears + Head exist in constant sound reflection from everywhere at once, like being underwater.

With all of that information arriving at our ears, why isn’t it just a huge jumble of sounds?  More precisely, how do we distinguish the direct sound from Ambience?  As we saw above, the direct sound arrives at our ears first.  The reflections follow.  The brain identifies the first sound as direct, the sounds that follow as Ambience.  This is known as the Precedence Effect, the law of the first wavefront, or the Haas Effect after German researcher Helmut Haas who identified the concept in his 1949 doctoral thesis.

Depth

Let’s consider a musical instrument playing directly in front of us, 3 feet away.  If that same instrument remains directly in front of us and moves to 6 feet away, we can tell by listening alone that the instrument moved back.  How?  Well, the volume of the instrument will be less.  Volume changes according to the inverse square law, meaning that a doubling of distance reduces the volume to a fourth its original strength.  But something else happened:  the Early Reflections and Reverb remained about the same volume.  Things that are further away are lower in direct sound volume but have the same amount of Ambience.  The ratio of direct sound to Ambience changed.

Now if we raised the volume on something that was further away, we would also increase the volume of Early Reflections and Reverb, proportionally.  The ratio between them remains basically the same for that distance no matter what the volume.  When the position changes, it causes the ratio to change.  So we not only locate sounds to the left or right, but the direct sound to Ambience ratio also allows us to hear front or back.  If two people are speaking to us in a room and the second person is further away, our Ears + Head combined with our brain notice the closer person is relatively louder.  We also notice the ratio of direct sound to Ambience is higher (softer relative reflections) for the closer person.  These perceptions can be used when we record and mix in stereo to help provide a sense of depth.

Human hearing is far more complex than we’ve examined here.  As Gareth Loy puts it:

“Incredibly, for each sound source in the environment, our hearing automatically and instantaneously creates a physiological image of the sound with its direction and distance encoded so that we register it subjectively as an object in space/time, together with the nature of the acoustical environment that it lies within.  Pretty Amazing.  But that’s not all.  We can also tell whether the sound is coming from above or below, its rate of relative motion, its rate of relative acceleration, and much, much more.”

So this investigation is just basics.  To record and mix to two channels, these basics give us some perspective on how it works before we get out the microphones, speakers, headphones, etc.  Human hearing helps us understand why stereo needs some things to be different in the two channels – because that’s the way we hear. Certainly all of this complexity is a challenge to record and mix in a way that seems Real.  This also gives us some tools for creating stereo sounds for Fantasy.

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