When producers use the term \u201cstereo image,\u201d they’re specifically referring to the difference between the signals in the two channels of stereo audio. This difference can allow the listener to perceive the \u201clocation,\u201d or \u201cwidth,\u201d of a sound source in a recording. But in a more technical sense, the term \u201cstereographic\u201d refers to a technique by which a three-dimensional object is mapped onto a two-dimensional plane. In this post, we\u2019ll use this analogy to understand the various flavors of multi-channel spatial audio techniques used by audio professionals to trick our brains into \u201clocating\u201d sounds.<\/p>\n
Monaural sound, usually referred to as \u201cmono,\u201d is the oldest and most basic category of sound reproduction, using only one channel of audio. When speakers reproduce audio signals, our brains perceive \u201cwidth\u201d based on differences in the signals. Like a single point on a geometric plane, mono sound only has one source, so when a mono sound is reproduced on a stereo speaker system (like headphones or most speaker setups), the mono signal is reproduced in exactly the same way in both audio channels. No difference in the signals means that no psychoacoustic localization occurs, so the sound source has no \u201cwidth.\u201d<\/p>\n
While most audio today is recorded, produced, and played back in stereo, mono audio is still used for some applications like radio broadcasting. Since it requires half the channel count of stereo audio, mono signals are simpler and therefore a better choice than stereo signals for certain applications.<\/p>\n
The vast majority of modern music is released in stereo, and stereo output is the default in most, if not all, DAWs. But what does it really mean? Stereophonic sound has two channels, left and right.<\/p>\n
This might not sound like a big difference from mono, but this means that stereo recordings can give engineers the ability to imbue audio with a sense of \u201cwidth\u201d – in other words, having the ability to mix the left and right channel separately means having the ability to create two different \u201csides.\u201d In stereo, listeners\u2019 brains can locate audio based on amplitude in each channel; if a sound is louder in the left channel than it is in the right, the listener\u2019s brain localizes the audio on the left. Stereo audio allows our signal to be measured across one spatial dimension, just like a straight line on a geometric plane.<\/p>\n
As an aside, there is a specific case of stereo audio known as binaural audio<\/strong><\/a> that uses psychoacoustics to simulate spatial effects that do not exist in traditional stereo recordings. For more on this case, check out this blog post that explains more about binaural recordings.<\/a><\/p>\n Surround sound protocols allow engineers to mix even more channels together: in a standard 5.1 surround setup, six channels of sound are mixed together: five channels with full bandwidth and one for something called LFE, or Low Frequency Effect. These channels\u2019 signals are then sent to speakers set up around the listening position, including behind the listener. This way, the villain in a horror movie creeping up behind the hero really sounds like he\u2019s creaking the floorboard in the hallway behind you.<\/p>\n If stereo is like a straight line, you can imagine surround sound as a square. Two channels of audio allowed us to place sounds to the left and right in the stereo field. Surround sound allows us to use that stereo field as a flat plane instead of simply a line: it has stereo \u201clength.\u201d Although it probably doesn\u2019t make sense for most musical artists to mix their tracks in surround, the power of another stereo dimension historically has been a great choice for film, especially in the context of home theater experiences where a full professional audio installation is overkill.<\/p>\n Where do we go from here in terms of situating audio in a spatial field? This is a question being answered by companies, researchers, and developers in the realm of ambisonics, the next dimension up from surround sound. Through ambisonics, users are able to position sounds in three dimensions – like a sphere around the listening position. Just as a single point (mono) has no length and a single line (stereo) has no width, a single plane (surround) has no height.<\/p>\n Ambisonic audio adds the dimension of height in audio to make sounds seem as though they are coming from above<\/em> or below<\/em> the listener. In the case of Dolby\u2019s Atmos systems, for example, this is achieved by placing extra speakers on the ceiling, but one of the coolest features of ambisonic audio is its ability to be automatically downmixed when reproduced on a system without this type of speaker installation. This means that engineers who mix in ambisonic typically aren\u2019t required to generate a separate deliverable for stereo and surround sound formats.<\/p>\nSurround sound<\/h2>\n
Ambisonics<\/h2>\n