In the past, recreating concert hall performances in small listening areas has been accomplished through the use of reverberation generated by electronic recirculation of sound. This is a technique of simulating the ambient sound field made up of reflections of the original sound, with auxiliary loudspeakers used in conjunction with a primary loudspeaker. This simulates the "ambience" or "fullness" which occurs in a large concert hall. Reverberation techniques assume the original sounds to be pure and then add echo by electronic recirculation of the original sound. This produces a sound quality which approximates the concert as heard by a listener in a large concert hall.
However, due to the constant recirculation of the original signal with delays, phase cancellation in the audio signal can occur at various audio frequencies. This can give the listener the impression of booming or other resonances, which sound artificial and may detract from the enjoyment of the piece. Moreover, tonal coloration is changed by phase cancellation. This means that particular frequencies become much louder and others much softer in the auxiliary loudspeaker to which the recirculated signal is fed than they do in the primary loudspeaker. Normal listening is not disturbed by 3 dB of up and down frequency response, but long before the effect is noticeable, a situation may occur where at particular frequencies the auxiliary loudspeaker is ten times as loud as the primary loudspeaker. The result is that the auxiliary loudspeaker becomes a noticeable sound source. Moreover, as the reverberation time is increased by increasing any particular delay in the recirculation loop, a point is reached where a distinct echo is audible. This usually occurs when a delay of 50 milliseconds is reached.
To a lesser extent, phase cancellation also occurs in systems where the electronic signal is delayed and electronically summed prior to audio reproduction by one or more auxiliary loudspeakers. In such cases, the result sounds "tinny" or "twangy" when trying to create large concert hall ambience.
It is therefore desirable to eliminate phase cancellation in order to achieve a flat frequency response so that added sound gives the small listening room the appearance of being large without introducing extraneous effects. With a smooth or flat frequency response, the auxiliary loudspeakers are much less obvious as they create the large hall ambience.
By way of further background, reproducing the "ambience" of a large concert hall through the use of a "single" short delay on the order of 10 milliseconds is described in an article by E. Roerback Madsen entitled "Extraction of Ambiance Information from Ordinary Recordings," Journal of the Audio Engineering Society, October 1979, Vol. 18, No. 5, in which the Helmut Haas single delay system is described. In the Haas system, a primary speaker is fed directly from a signal source and an auxiliary loudspeaker is placed in the listening room spaced from the primary speaker. The auxiliary loudspeaker is driven by a replica of the original signal singly delayed by the aforementioned short time delay. The effect is adding "dimension" to the small listening room. Adding dimension refers to giving the small listening room the appearance of having longer dimensions than it actually has. The use of a single delay takes ambience in the original material and effectively distributes it around the small listening room to achieve a large concert hall sound. A certain "fullness" of sound is achieved by this technique. This "fullness" is not achievable by merely placing additional speakers around a listening room and driving them with undelayed signals.
In the system just described, for a single channel there is no attempt to enhance any acoustic effect, but merely to reproduce the original performance conditions. More fullness cannot be added by this system, and therefore, ambience enhancement does not occur with a single delay and a single auxiliary loudspeaker.