Stereo Perception

How do we know where sound is coming from? The answer lies in the fact that we have two ears separated by our head. Yes, blindingly obvious. But what happens when we hear sound from a given direction?

The sound reaches the two ears at different times, and…
The two ears receive sounds of different intensities.

Interaural Time Difference (ITD)

Our ears are separated by about 18cm.
If a sound is made directly in front or behind someone, or is made anywhere along the median plane, it will arrive at both ears simultaneously. If the sound is off to the left the left ear will receive the sound before the right ear. The time difference between the two ears depends on the difference in lengths that the two sounds have had to travel. The ear also uses the difference in phase to resolve the direction.

Interaural Intensity Difference (IID)

When a sound is made off to the left or right, each ear receives different intensities due to the shading effect of the head. As the source moves away from the median plane the level at one ear (furthest away from the source) progressively reduces and the level at the other ear progressively increases.
As the sound direction angle changes the intensity ratio between the two ears varies (roughly) between 0 dB to 20 dB, depending on frequency.

The interaural intensity difference is used as a cue for direction at frequencies above 2.8 kHz.
The interaural time difference is used as a cue for direction at frequencies below 700 Hz.
We're not as good at resolving direction in between these two frequencies.

Front to Back and Elevation

So how do we know whether a sound source is in front or behind us, or how high or low it is? Two ways…

Sound striking the pinnae is reflected into the ear canal by the ridges on the ear. These reflections introduce very small delays and therefore comb filtering phase cancellations. Such small delays only affect high frequencies (above 5 kHz). Also, we all have different shaped ears so we all have a different idea of what a sound 'behind' us actually sounds like.
We move our heads towards sounds we want to localise until all delays and intensities are the same. This is why headphones make sounds feel like they're 'in the head'; moving the head doesn't change the intensities or delays between the ears.

ITD and IID Trading

The same part of the brain deals with both ITD and IID perception. It is possible for one cue (e.g. intensity) can cancel out another (e.g. delay). This is known as interaural time difference versus interaural intensity difference trading, and is shown in the diagram below.

The Haas Effect

The ear will attend to the direction of the sound that arrives first and will not attend to the reflections providing they arrive within 30ms of the first sound.
The reflections arriving before 30ms are fused into the perception of the first arrival. However, if they arrive after 30ms they will be perceived as echoes.

A delay gap of less than 20ms makes a room sound 'intimate'.

Recording Links

An AB stereo pair uses ITD and is known as delay stereo.
An XY stereo pair uses IID and is known as intensity stereo.
Binaural stereo recording reproduces both ITD and IID very effectively, but the recordings must be played back over headphones as any cross-ear-coupling would destroy the effect.

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