Have you ever wondered why the music you listen to everyday, sounds realistic? It is as if you were sitting in a room, listening to your favourite band play live, right in front of your eyes. This aspect of sound reproduction, is something that every audio engineer should understand. Hence, if you aspire to be one yourself, ask this – What is Stereo Sound?
The subject for today’s article is something that resonates with many audio enthusiasts and practitioners. But not too many of them are aware of the guiding principles behind the concept of stereo audio. You will learn more about the various methods of achieving the stereo effect, and how listeners are affected by them. Are you curious to know? Me too!
Introduction – Stereo? How?
Otherwise known as “stereophonic sound”, stereo is a sound reproduction method that produces the illusion of sound coming from multiple directions. This is done by using two or more discrete audio channels through a configuration of two or more loudspeakers (or stereo headphones) to re-create “natural hearing” (the impression of sound heard from various directions).
The term “stereophonic” does not only apply to the common two-channel, two-speaker systems, but also to “surround-sound” systems as well. It is different from monophonic (mono) sound, where audio is perceived to come from one position, often centered in the sound field (similar to a visual field). Stereo sound is widely used in broadcast radio and TV, recorded music and the cinema.
Here are the topics that we will be covering today:
- Types of stereo sound
- Recording applications
- Common usage
Types of Stereo Sound
There are two forms of stereo system and the first one is called “natural” or true stereo. This is where a live sound performance is recorded by an array of microphones. Thus, any natural reverberation or ambience present, will also be captured by the mics. The signal will then be played back through multiple loudspeakers to recreate the live sound experience (as accurately as possible).
The second one is called “pan-pot” or artificial stereo. This is where a sound that is recorded on a single-channel (mono) is played back through multiple loudspeakers. When the relative amplitude of the signal sent to each speaker is being attenuated, an artificial sense of direction (based on the listener’s perspective) can be created.
The relative amplitude of the signal can be adjusted by using a control known as a “pan-pot” (panoramic potentiometer). When multiple “pan-potted” mono signals are combined together, a complete, yet entirely artificial, sound field can be produced.
Two-channel Stereo Recording
When recording for two-channel stereo, two microphones (strategically positioned) will record the sound source simultaneously. Both recordings will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. These subtle differences in timing and sound level will be used by the listener’s brain to define the positions of the recorded sounds.
Playing stereo recordings on mono sound systems will typically incur significant losses of fidelity. Since the sound is captured by each microphone at a slightly different time, the sound waves will be out of phase. This causes constructive and destructive interference when both tracks are played back on the same speaker (also known as phase cancellation).
Stereo sound is about creating a perception of location for sound sources (voices, instruments, etc.) within the original recording. Audio engineers typically aim to create a stereo “image” with localization information. When a stereophonic recording is played through stereo speaker systems (rather than headphones), each ear will of course, hear sounds from both speakers.
In most recording situations, the audio engineer will use more than two microphones (in some cases, many more mics are used) and may mix them down to two tracks in ways that highlight the separation of the instruments. This is intentionally done in order to compensate for the mixture that occurs when listening through speakers.
Stereo sound may be hyped to be all about localizing the position of each sound source in space, but this would only truly happen in a carefully engineered system and environment, where speaker placement and room acoustics are seriously considered. Many sound systems, such as home theatres and the like, are not capable of creating a realistic stereo image.
To get the best results when listening to stereo recordings, two identical loudspeakers have to be used, placed in front of and equidistant from the listener, with the listener positioned on a center line between the two speakers. This essentially forms an equilateral triangle, with the angle between the two speakers around 60 degrees as seen from the listener’s point of view.
It is important to take note of the common usage of terms related to stereophonic sound. Essentially, a “stereo” is a two-channel sound reproduction system, and a “stereo recording” is a two-channel recording of any sound source. This can cause some confusion among consumers, since five (or more)-channel home theater systems are not widely advertised as “stereo”.
Keep in mind that not all audio recordings that are mixed down to two-channels, uses stereo recording techniques in them. Take for example pop music, where recordings are usually done using close miking techniques, which artificially separate signals into discrete tracks (mono). The engineer will then position these individual tracks within the stereo field.
Simple techniques such as “left-right” panning controls, to more sophisticated ones (extensively based on psychoacoustics) such as channel equalization and mid-side processing, are used to adjust the position of the mono tracks in the stereo mix. This process produces little to no resemblance of the actual spatial relationship of the musicians (during the performance).
In reality, it is very common for different tracks of the same song to be recorded at different times and in different studios, before finally being mixed down to a two-channel stereo recording for commercial release. There are exceptions such as classical music, where it is typically recorded in one complete take, thus preserving the realistic spatial relationship of the performers.
We have come to the end of the article. Hopefully by now, you have a decent understanding of stereophonic sound, and how it basically works.
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