All About Stereo Audio

Stereo audio is a term that refers to sound that is divided into two (or more) distinct channels (but, usually left and right channels). Stereo audio is typically played back simultaneously from two stereo speakers. Stereo audio creates a full sound and also allows for the ability to mix sounds between the two channels. Because human beings have two ears to hear sound, stereo sound usually provides a more realistic sound setting than does mono (single channel) sound.

Stereo Audio Basics

Stereo audio is recorded using two or more independent microphones in order to create a symmetrical sound configuration. This symmetrical configuration is then played from two loudspeakers on separate channels to create the impression that the sound is coming from different directions - as is the case with natural hearing.

Stereo audio is in direct contrast to mono audio, where the sound is centered in only one channel. With monophonic audio, the sound is often centered in the sound field. For this reason, stereo audio sounds much more natural than does mono sound, which can often sound muted or like it is coming from a tin can.

Recording Techniques

In most cases, stereo audio is recorded by using one of two methods: X-Y (intensity stereophony technique) and A-B (time of arrival technique)

The A-B technique involves using two parallel omnidirectional microphones that are placed a distance apart. This recording method captures time of arrival stereo information and some level or amplitude difference information. This technique is especially effective when the microphones are placed in close proximity to the source of the sound. When microphones are placed at distances of about 60 cm from the sound source, the time delay between signals reaching the microphones is about 1.5 ms, which results in very realistic sound.

The X-Y technique uses two directional microphones at the same location. Typically, these microphones are pointed at angles of 90 and 135 degrees in relation to each other. The stereo audio effect is achieved by natural differences in the sound pressure level between each directional microphone.

When listening to loudspeakers, a difference in sound levels of about 18 DB is needed to determine from which direction the sound is coming. With the X-Y technique, there is no substantial difference in the time of arrival or sound phase; therefore, this technique usually results in a lower sense of space and depth when compared to recordings made with the A-B technique. However, placing two figure-eight microphones that are at facing 45° angles, with respect to the sound source, creates what is called a Blumlein Pair, which is much more realistic and is almost holographic in sound dispersal.

The History of Stereo Audio

The words "Stereophonic" is derived from the ancient Greek word "Stereos", which means "solid and phone", and was first coined by Western electric.

The very first stereo transmission was created by a Clement Ader in 1881 and was produced telephonically. Later, the world's first stereo broadcast was made by the BBC in December 1925. However, it was not until American record company, EMI, patented stereo records and films that stereo audio became the standard in high fidelity sound in music and films.

Although stereo audio has taken a backseat to multichannel, or surround, sound in recent years, stereo audio is still the format of choice for serious audiophiles or audio enthusiasts. While surround sound offers a more realistic sound setting for films and video, stereo audio is still much more representative of a live music experience.

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