Over the last 50 years sound reproduction has moved from monophonic recordings with only one audio channel through to surround sound formats with upwards of 7 discrete channels. This AudioCourses article will give you a brief guide on the differences in the 'sound stage' between different formats.
Before we delve into the formats we need to quickly look at how we create space and depth in recordings.
The Sound Stage
For the purposes of this explanation we will talking about a stereo 'sound stage'.
If we imagine 3 axis, horizontal, vertical and depth, that give us 3 dimensions in which to place sounds, left to right, top to bottom and front to back.
The horizontal axis is where we place sounds with the pan control on a mixer. Panning a sound hard left makes the sound appear to come from the left, whilst leaving the pan control central places the sound equally in both speakers giving the impression it is coming from between the speakers - this is sometimes called a 'phantom channel'.
The vertical axis is related to frequency - high frequency content appears to be positioned 'above' low frequency material. A hi-hat for instance will appear to be 'above' a kick drum.
The depth axis is determined by volume. A loud signal, such as a lead vocal, will appear to be very close whilst quieter audio will seem to come from further back. Reverb can also be used to aid the positioning of a signal. Audio with no reverb will appear closer than audio with lots of reverb applied - it is common for backing vocals to have more reverb applied than the lead vocal for this reason.
For further insight into the creation of space and depth in a mix have a look at The Mixing Engineers Handbook.
Alternatively The Art of Mixing: A Visual Guide to Recording, Engineering and Production is a good book for absolute beginners and explains the sound stage with a lot of diagrams to help visualize positioning.
Mono
Mono is the most basic of all audio formats carrying only one 'channel' of audio. With only one channel there is obviously no scope for using pan so no space can be created on the horizontal axis. A mono recording can be reproduced with any number of speakers although most commonly it is either one or two.
With just one speaker the entire sound stage will appear to emanate from the individual speaker. Add another speaker and although the source is still monophonic, the sound will appear to come from between the 2 speakers. However the imaging will still be entirely static due to the lack of any panorama.
Stereo
This is the format that the vast majority of us are used to. Stereo contains 2 discrete channels, Left and Right, and as mentioned earlier, can give the impression of a 'phantom' central channel. With the stereo format we can position sounds on all 3 axis but we are reliant on the listener being positioned in the 'sweet spot' (the listener should be the 'tip' of an equilateral triangle with the speakers providing the other 2 'tips') When the listener is not in the sweet spot the audio can sound very different to how the engineer mixed it in the control room.
The phantom channel with stereo systems is usually occupied by sounds that are panned central such as lead vocals, certain drums etc. However, with some material the phantom channel may appear to move slightly off centre which is undesirable as it 'feels' unnatural.
Surround Sound
Surround Sound is often used to refer to many different formats such as Pro-Logic, 5.1, DTS etc Surround Sound systems will usually have at least 4 speakers and quite often a 'subwoofer' which deals with low frequency signals.
The vast majority of 4 speaker systems do not have 'discrete' audio channels but rather decode the audio from the stereo signal therefore meaning that the sound stage is not clearly defined. The most common 4 speaker setups are Left, Centre, Right and Rear Centre or Left, Right, Rear Left and Rear Right.
5 speaker systems are the most common at present although again there are several different configurations. Dolby Surround offers Left, Right, Centre, Rear Left and Rear Right although the rear channels are actually both producing the same mono signal.
Of most interest to music engineers are the advents of audio formats such as SACD and DVD-Audio which will allow engineers to mix for 5.1 systems which are the most prevalent in the consumer environment.
5.1 consists of Left, Centre, Right, Rear Left, Rear Right, Subwoofer. Obviously this opens up a lot more possibilities for the mixing engineer to create a sound stage. Also with more speakers the theoretical 'sweet spot' is larger than it is with stereo so speaker positioning is not as critical as it is with stereo systems.
With a 5.1 system the 3 axis we detailed at the start of this piece are extended back behind the listener to a certain extent. The lack of a Rear Centre speaker (a phantom channel may exist in it's place - 6.1 systems however do include a Rear Centre) means that the image is not quite as solid as at the front but the extra speakers allow a further degree of sound placement.
The Front Central channel ensures that the sound stage is always centred (unlike with a phantom channel which can move) so the Left and Right channels can be devoted more to the wider panorama. As the rear speakers are 2 separate channels then sounds can be rotated around the listeners position.
One of the obvious applications, especially when mixing live recordings, is to put the listener 'on stage' with the performers. Other applications may include mixing the music to the front 3 channels and then using the rear channels for ambience. The classical community has been quicker to embrace the SACD format than other genres although this situation may soon change.
As far as music is concerned, we are still in the early days of surround mixing - the early days of stereo mixing were characterized by some pretty wild panning decisions. At present there is much debate on how best to use the speakers available in a 5.1 system to best reproduce music. As the format matures it is likely that 'standard' methods of mixing for multi-speaker environments will emerge.
