Stereophonic sound

Stereophonic sound, often called stereo, is a way of playing sound that makes it feel like it’s coming from many directions, just like how we hear things in real life. This is done using two separate audio channels, usually through two loudspeakers or stereo headphones. These channels work together to give the impression that sound is coming from different places.

The idea behind stereophonic sound is to create a three-dimensional listening experience. This means that whether you’re listening to music, watching a movie, or hearing something on the radio, it can sound more realistic and full. The term stereophonic can also include systems with more than two channels, like quadraphonic and surround sound, as well as binaural sound systems.

Stereo sound became very common in the 1970s and is now used in many types of entertainment, including broadcast radio, recorded music, television, video cameras, cinema, computer audio, and online music and videos. It has changed the way we enjoy audio by making it more immersive and lifelike.

Etymology

The word stereophonic comes from two ancient Greek words. The first part, stereós, means "firm" or "solid." The second part, phōnḗ, means "sound" or "voice." Together, they help explain how stereophonic sound works.

Description

Stereo sound systems come in two main types. The first type is called true or natural stereo. This method captures live sounds, including any natural echoes, using an array of microphones. These sounds are then played back through multiple loudspeakers to recreate the live experience as closely as possible.

The second type is called artificial or pan stereo. Here, a single-channel sound (called mono) is played through multiple loudspeakers. By adjusting how loud the sound is on each speaker, it can seem like the sound is coming from different directions. This adjustment is done using a control called a pan-pot. By combining several mono sounds with different pan-pots, a full sound field can be created, even though it isn’t a real live sound.

History

Early sound systems used only one channel, making it hard to tell where sounds were coming from. In 1881, Clémént Ader created a system in Paris that used two channels to make sound feel more real. This idea led to services like the Théâtrophone in France and the Electrophone in England.

Modern stereophonic sound began in the 1930s with British engineer Alan Blumlein. He invented ways to record and play back sound using two channels, making it seem like sound came from different directions. His work included special microphones and recording methods that are still used today. In the United States, Harvey Fletcher also worked on stereophonic sound, creating early test recordings with orchestras.

Stereophonic sound became popular in movies in the 1950s and 1960s. Films like Fantasia and This is Cinerama used advanced sound systems to create a more immersive experience. Over time, technologies like Dolby Stereo made stereophonic sound more common in theaters, allowing audiences to hear sound coming from different directions, just like in real life.

Modern home audio and video

The development of stereophonic sound, or stereo, moved slowly because of technical challenges. It was hard to record and play back two or more audio channels together perfectly. Also, stereo systems cost more—about twice as much as regular systems—because they needed extra parts like extra amplifiers and speakers. Early on, people weren’t sure if the better sound was worth the higher price.

Stereo experiments on disc

Thomas Edison started recording sound in a special way on cylinders and discs in 1877, and Emile Berliner began using a different method shortly after. By the late 1920s, new materials like acetate and vinyl were developed to make better records, especially for radio broadcasts. These new materials were quieter, but they had issues with low-frequency rumble when played.

In the 1920s, some people tried splitting sound into two parts—bass and treble—and recording them separately on the disc. Another experiment tried putting the left and right sound channels on opposite sides of the disc, but this did not work well. Later, Bell Labs tried a system where one channel was recorded in a lateral way and the other vertically, but this also had problems. Eventually, they tilted the recording head to share the problems equally between both channels, creating the 45/45 system we use today.

In 1952, Emory Cook created a special type of record with two separate sound channels cut into two groups of grooves on the disc. Each groove needed its own needle and speaker. Cook’s company began selling these records, which included many different sounds like trains and thunderstorms. By 1953, he had about 25 of these stereo records available for audio fans.

Magnetic tape recording

The first stereo recordings using magnetic tape were made in Germany in the early 1940s using Magnetophon recorders. About 300 recordings of symphonies were made, but most were taken after World War II. Some important recordings from that time still exist today.

In the US, stereo magnetic tape recording was shown for the first time in 1952. A few years later, more recordings were made, and by the mid-1950s, stereo recordings started to appear in some homes.

Stereo on disc

In November 1957, a small record company called Audio Fidelity Records released the first stereophonic disc made for many people to enjoy. Sidney Frey, the leader of the company, worked with engineers to create this special disc before bigger companies could. The disc featured music from the Dukes of Dixieland and sounds like trains to make listeners feel surrounded by audio. It was shown to the public in New York City on December 13, 1957. Only 500 copies of this first disc were made, and Frey offered free copies to industry people who asked for them, which created a lot of excitement.

Later in December 1957, another small label named Bel Canto Records made its own colorful stereophonic disc so that stores had more options to show off. Their discs, made with special blue vinyl, were popular for their jazz and easy-listening music. As stereo technology became more affordable, more people could enjoy these rich, surround-like sounds at home. By March 1958, four stereophonic discs were available for purchase, and more were added soon after.

Broadcasting

Radio

Early experimentation

The earliest way to create stereo radio used two separate broadcasts for left and right audio. Listeners needed two radios to hear the stereo effect. In 1924, Franklin M. Doolittle got a patent for this idea and tested it using a radio station in New Haven, Connecticut. The tests stopped because there weren’t enough radio frequencies and it was hard for people to use two radios.

More tests happened in Berlin in 1925 and later that year, a British broadcast used two stations for stereo sound. In 1946, a similar test happened in Holland.

In the 1950s, people tried again using both AM and FM stations together. But these tests were limited because they still needed two radios, and AM sound quality was much worse than FM.

FM standards

A system called the pilot-tone method became the global standard for FM stereo broadcasting. In the United States, tests led to the FCC adopting this standard in April 1961. Regular stereo FM broadcasting started on June 1, 1961, with three stations beginning to use the new standards.

The BBC started regular FM stereo broadcasts in 1962. Sweden used a different system but later switched to the pilot-tone system in 1977.

AM standards

Very few AM stations broadcast in stereo because of lower sound quality and few stereo receivers. One known method is C-QUAM, used in many countries. There have been tests with digital HD Radio for AM stereo, but issues have limited its use.

Television

In 1952, a performance of Carmen was shown in theaters with stereophonic sound. In the late 1950s, some TV shows used stereo by broadcasting one audio channel over TV and the other over radio.

After FM stereo became common in 1962, some TV music shows used a method called simulcasting, where the audio was also broadcast on an FM station. This was later made easier with satellites.

The BBC used simulcasting for many music programs from 1974 to around 1990. Cable TV systems also used stereo simulcasting, with MTV being very popular for this.

Japanese TV started stereo broadcasts in 1978, with regular stereo programming by 1982. Germany’s ZDF network began stereo programs in 1984.

Different systems are used around the world for TV stereo sound. In the Americas, Multichannel Television Sound is common. In Europe, NICAM is used, except in Germany where Zweikanalton is used. Japan uses the EIAJ FM/FM subcarrier system. Digital TV uses MP2 or Dolby Digital for audio.

MTS, or Multichannel Television Sound, is the U.S. standard for adding extra audio channels to TV broadcasts. NBC began sporadic stereo broadcasts in 1984, with regular stereo programming starting in 1985. ABC and CBS followed in the late 1980s.

Recording methods

See also: Microphone practice § Stereo recording techniques

A-B technique: time-of-arrival stereophony

The A-B technique uses two microphones placed some distance apart from each other and equally far from the sound source. This method captures how sound arrives at each microphone at slightly different times, as well as some differences in loudness — especially when the microphones are close to the source. When the microphones are about 60 cm apart, the time difference for a sound coming from the side is about 1.75 milliseconds. Making the microphones farther apart reduces the angle at which sounds can be picked up. At 70 cm apart, it’s similar to another setup called the ORTF setup.

This method can sometimes cause problems when the stereo sound is mixed back into a single channel.

X-Y technique: intensity stereophony

In the X-Y technique, two directional microphones are placed right next to each other, usually pointing at angles between 90° and 135° apart. The stereo effect comes from differences in how loud the sound is for each microphone. Because there’s no time difference between the microphones, recordings made this way feel less spacious and deep compared to A-B recordings. When two special microphones called figure-eight microphones are used at ±45° to the sound source, the X-Y setup is known as a Blumlein pair.

M/S technique: mid/side stereophony

The M/S technique uses two microphones placed together. One microphone faces sideways and is bidirectional, while the other faces the sound source at a 90° angle. The second microphone is usually shaped to pick up sound mostly from the front. Alan Blumlein described using a regular microphone in his original idea.

This method creates left and right sound channels by combining the signals from the two microphones in a specific way. It can still be played back as a single channel, and the width of the stereo sound can be adjusted after recording by changing how much of the sideways signal is used. This makes it very useful for movies.

If the M/S technique is used in a special stereo microphone that only outputs the final left and right signals, the original midpoint and sideways signals can be recovered. The midpoint signal is found by adding the left and right signals together, and the sideways signal is found by subtracting the right signal from the left.

Near-coincident technique: mixed stereophony

Other methods mix ideas from both A-B and X-Y techniques. For example, the ORTF stereo technique created by the Office de Radiodiffusion Télévision Française uses two microphones placed 17 cm apart at a 110° angle, giving a stereophonic pickup angle of 96°. In the NOS stereo technique of the Nederlandse Omroep Stichting, the microphones are 30 cm apart at a 90° angle. All these methods use at least 17 cm between microphones, which is about the distance between human ears, helping to mimic how we naturally hear sound. While these recordings are meant for speakers, they can also sound good on headphones.

The Stereo Ambient Sampling System (SASS) is another near-coincident method developed by Crown International in the late 20th century. It uses two closely spaced microphones inside a housing that blocks sound from the rear. The microphones are angled apart (usually between 50–90°), similar to other near-coincident methods like ORTF. In addition to the housing, SASS includes a material between the microphones that absorbs higher frequencies. This helps create differences in loudness between the ears, while the spacing creates time differences, together mimicking how humans hear in space. SASS works well with speakers and is often used for recording ambient sounds and environments. It’s sometimes called quasi-binaural because it uses both time and loudness cues shaped by the housing.

Pseudo-stereo

When restoring or updating old monophonic recordings, some methods create a pseudo-stereo effect to make it sound like the sound was originally recorded in stereo. These methods can use special hardware (see Duophonic) or software. For example, Multitrack Studio uses filters that send low frequencies to the left channel and high frequencies to the right channel. It also adds a tiny delay between the channels, which widens the sound just enough to be noticed without being obvious.

These artificial stereo methods are used to improve old monophonic recordings or to make them more appealing since many people expect stereo sound. Some critics worry about using these methods.

Binaural recording

Main article: Binaural recording

Engineers see a difference between binaural and stereophonic recording. Binaural recording is like stereoscopic photography. To make a binaural recording, engineers place two microphones inside a model of a human head, with each microphone where an eardrum would be. When played back through headphones, each ear hears the sound separately, creating a realistic feeling of being in the place where the recording was made. This gives an accurate copy of the auditory spatiality you would have felt there. Though it can be hard to listen to because it needs headphones, there is also a way to enjoy this with loudspeakers using Ambiophonics.

Some early stereo tapes and disc formats in the 1950s called themselves binaural, but they were just regular stereo methods, like putting a lead vocal on one channel and the orchestra on the other.

Playback

Stereophonic sound, or stereo, tries to make you feel like sounds are coming from different places, like voices or instruments, in the original recording. Engineers aim to create a special sound setup that helps you tell where each sound is coming from. When you listen through speakers instead of headphones, both of your ears hear sounds from both speakers. Engineers often use more than two microphones and mix them into two tracks to make sounds seem farther apart.

Stereo sound is described as helping you know where each instrument is, but this only works well if the speakers and room are set up just right. Many simple devices, like small portable players, can't make stereo sound feel real. Long ago, in the late 1950s and 1960s, stereo was advertised as sounding "richer" than regular mono sound, but this depended a lot on the equipment used. Some records were made just to show off stereo sound and often came with tips for setting up a stereo system.

For the best stereo experience, use two identical speakers placed in front of and equally distant from you, with you sitting right between them. This creates a triangle shape, with the speakers about 60 degrees apart from your view. Many good speaker systems come with instructions on the best angles and distances to get the most out of stereo sound.

Vinyl records

In 1958, the first mass-produced stereo vinyl records were released in the US and Britain. These records used a special system called the Westrex 45/45 single-groove system. Unlike old records that only had one sound channel, stereo records had two. This let the stylus move both side to side and up and down, creating a richer sound.

Stereo records could still be played on older record players, but it wasn’t recommended because it could damage the record. Stereo players, however, could play both old and new records, giving a more balanced sound. Over time, making true stereo recordings became easier, improving the listening experience by making sounds feel more natural and spread out in space.

Compact disc

The Red Book CD standard usually uses two channels for sound. This means that if a recording is just one channel, or mono, one channel might be empty or the same sound might be played on both channels.

Common usage

A stereo is a system that uses two channels to play sound. When people talk about a stereo recording, they usually mean a recording made with two channels. This can be confusing because systems with more than two channels, like those used in home theaters, are called surround systems instead of stereos.

Most modern music recordings start with many separate tracks. These tracks are then combined into a final two-channel recording. Sound engineers decide where each track sounds like it comes from in the left or right speaker using simple and more complex methods. The final recording might not match exactly where the musicians were standing when they first played the music. Sometimes, different parts of a song are even recorded at different times or places before being mixed together.

Classical music recordings are often different. They are more likely to capture the musicians as they were arranged during the actual performance, without adding extra tracks later.

Balance

See also: Panning (audio)

Balance refers to how much sound comes from each side in stereo audio recording. When the balance control is in the middle, both sides play equally. Turning the control changes the volume of one side while keeping the other side the same.