Apple Logic Pro 9 - Manual (Page 1184)

The reason, put simply, is due to the way human beings “hear.” Most people with
undamaged hearing can easily identify where a sound is coming from: to the left, right,
in front, or behind them.
Certain sounds, however, are very difficult to “position,” in relation to the listening position.
For example:
• A gunshot or car backfiring: These are hard to place because the sound is both loud
and quick. You may initially be able to tell that it came from your left or right, but your
brain will become confused as to where, specifically, to the left or right it came from.
This is because early reflections (reverberations) rapidly build up and diffuse, making
them hard to discern, directionally, from the initial sound peak.
• Aircraft jet engines: In general, this is just a low rumble that is hard to place, until the
plane flies directly overhead. When it does, the sheer volume of the sound, and the
higher frequencies of the jet engines enable you to hear it moving from left to right
or front to back.
Sounds that are easier to place include:
• Trucks, cars and motorbikes: As these move, a constant combination of low- and
high-frequency sound is heard, allowing you to track their movement.
• Human voices: This is the sound that human beings are most familiar with, and contain
a lot of high-frequency content. Interestingly, grouped human voices, such as a crowd
in a sporting stadium become very nondirectional. This is due to the reflections and
reverberation of the arena.
As an individual creating a sound mix, you can try to approximate the “real-world”
characteristics of certain sounds, or artificially enhance them, as heard in many Hollywood
blockbuster films.
Whichever approach is taken, a certain amount of “lag time” (latency) is perceived between
speakers, in relation to the listening position.
To compensate for this perceived lag time, most surround amplifiers offer a calibration
routine that allows you to set different levels and different delay times for each speaker.
Care must be taken with this, as level—in particular—can alter your perception of how
“close” a sound is, so you should ideally set the same level for the left and right speakers.
The front left and right speakers are usually used for “incidental music/effects” tracks and
the “main score,” and also often carry an amount of the “dialogue” track (at a slightly
lower level than the center speaker). They may also be used for “surround effects,” such
as crowd noise or ambience in a scene where the main actors are in the center of a group
of people.
1184 Chapter 39 Working with Surround