Speaker timing and levels
Most people with undamaged hearing can 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 is 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 not 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 are a low rumble that is hard to place until the plane flies directly overhead. When it does, the 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.
Some sounds are easier to place:
Trucks, cars, and motorbikes generate a constant combination of low- and high-frequency sound, allowing you to track their movement.
Individual human voices are sounds that people are most familiar with, and contain a lot of high-frequency content.
In a surround playback system, you need to set different levels and different delay times for each speaker. This allows you to compensate for latency perceived at the listening position, which affects your ability to correctly “place” where sounds are coming from.
Level—in particular—can alter your perception of how close a sound is, so you should ideally set the same level for the front left and right speakers. These speakers are usually used for incidental music/effects tracks and the main score of a film, and also often carry an amount of the dialogue track.
The center speaker is typically used for dialogue and incidental music/effects tracks. Its level should be similar to the left and right speakers, but can be increased to enhance the intelligibility of dialogue.
You should aim to have the sound from all front speakers arriving at the listening position at the same time.
You should set the levels of surround speakers and the subwoofer (LFE) to be immersive, and part of the surround stage, rather than “additions” to the front speakers. In general, surround speakers (and the subwoofer) are used for surround effects, main score, and incidental music/effects tracks.
Also critical for the surround and subwoofer channels is the delay time. Assuming that the levels of all speakers are suitable, the timing of the surround (and LFE) speakers may seem slightly “out” in comparison to the front speakers. Most surround amplifiers allow you to negatively or positively adjust the delay of these speakers.
The surround encoding process—performed in Compressor, available in the Mac App Store—writes “surround encode flags” for the surround speakers, depending on the chosen format. These flags are understood by surround decoders (AV receivers, decoding software, or surround amplifiers).
There is no need to set slight delays between tracks when working in Logic Pro X. The surround encode flags are designed to handle this.