Matching Human Perception
Limitations Of 5.1 Surround Sound
However, 5.1 surround sound systems are limited by some tradeoffs. The first has to do with rendering the front soundstage. Research into imaging showed that spreading the front channels wider than the standard ±30° results in a greater difference at the two ears and produces more envelopment. In concert hall acoustics this is called Auditory Source Width and, increasing it, is a desirable feature. But increasing the spread beyond ±30° for the front speakers was found to cause many imaging problems. This fundamental tradeoff in 5.1 surround sound systems between better frontal imaging and improved envelopment is a limitation.
The second tradeoff is related to the ITU recommended placement of the two surround channels at ±110°. Surrounds at ±90° were found to produce the most envelopment compared to other angles, while surrounds at ±135° produce better rear phantom imaging capability. Thus ±110° was selected as the best tradeoff between envelopment and imaging. This is another limitation of 5.1 surround systems because one must choose between better rear imaging or better envelopment.
More Channels Are Needed
Experiments have shown that human localization is better in front than to the sides or behind. This means that for front-weighted content such as movies and most music, good engineering dictates that we employ more channels in the front hemisphere than the back. Imaging is also better horizontally than vertically and so good engineering also dictates that channels must first be added in the same plane as our ears before going to higher elevations.
Perception is not the only factor. The physics of room acoustics for music have been well studied, and their correlation with subjective impression increasingly understood over the last 30 years. This research has shown that we have strong built-in preferences for the direction, frequency response, and time of arrival of reflected sound. Additional channels and surround sound processing are needed to properly render these components.
Where Should The New Channels Be Placed?
The next most important acoustical and perceptual cues come from reflections above the front stage. Audyssey DSX provides a pair of Height channels (LH and RH) that should be ideally positioned at a 45° elevation angle.
Improved Surround Envelopment
Audyssey Dynamic Surround Expansion is a scalable system that can adapt to the practical needs of the user. The first level of surround stage enhancement comes from adding the Wide channels. The second level comes from adding the Height channels. If it is practical to have both Wides and Heights then the surround performance will be further enhanced. Audyssey DSX automatically reconfigures its processing to optimize surround rendering over any number of available speakers beyond 5.1.