An echo chamber is a hollow enclosure used to produce reverberation, usually for recording purposes. For example, the producers of a television or radio program might wish to produce the aural illusion that a conversation is taking place in a large room or a cave; these effects can be accomplished by playing the recording of the conversation inside an echo chamber, with an accompanying microphone to catch the reverberation. Nowadays, effects units are more widely used to create such effects, but echo chambers are still used today, such as the famous echo chambers at Capitol Studios.
In music, the use of acoustic echo and reverberation effects has taken many forms and dates back many hundreds of years. Sacred music of the Medieval and Renaissance periods relied heavily on the composers' extensive understanding and use of the complex natural reverberation and echoes inside churches and cathedrals. This early acoustical knowledge informed the design of opera houses and concert halls in the 17th, 18th, and 19th centuries. Architects designed these to create internal reflections that would enhance and project sound from the stage in the days before electrical amplification. Sometimes echo effects are the unintentional side effect of the architectural or engineering design, such as for the Hamilton Mausoleum in Scotland, which has one of the longest reverberation times of any building.
Electro-acoustic
Developments in electronics in the early 20th century—specifically the invention of the amplifier and the microphone—led to the creation of the first artificial echo chambers, built for radio and recording studios. Until the 1950s, echo and reverberation were typically created by a combination of electrical and physical methods.
Acoustically speaking, the "classic novel" echo chamber creates echoes in the same way as they are created in churches or caves—they are all simply large, enclosed, empty spaces with floors and walls made of hard materials (such as polished stone or concrete) that reflect sound waves well. The basic purpose of such chambers is to add colour and depth to the original sound, and to simulate the rich natural reverberation that is a feature of large concert halls.
The development of artificial echo and reverberation chambers was important for sound recording because of the limitations of early recording systems. Except in the case of live performances, most commercially popular recordings are made in specially constructed studios. These rooms were both heavily insulated to exclude external noises and internally somewhat anechoic—that is, they were designed not to produce any internal echoes or sound reverberation.
Because virtually every sound in everyday life is a complex mixture of direct sound from the source and its echoes and reverberations, audiences naturally found the totally 'dry' and reverberation-free sound of early recordings unappealing. Consequently, record producers and engineers quickly came up with an effective method of adding "artificial" echo and reverberation that experts could control with a remarkable degree of accuracy.
Producing echo and reverberation in this form of echo chamber is simple. A signal from the studio mixing desk—such as a voice or instrument—is fed to a large high-fidelity loudspeaker located at one end of the chamber. One or more microphones are placed along the length of the room, and these pick up both the sound from the speaker and its reflections off the walls of the chamber. The farther away from the loudspeaker, the more echo and reverberation the microphone(s) picks up, and the louder the reverberation becomes in relation to the source. The signal from the microphone line is then fed back to the mixing desk, where the echo/reverberation-enhanced sound can be blended with the original 'dry' input.