When producing sound on a stage in a hall the sound will propagate to the audience. For the listeners in the direct vicinity of the stage the so-called direct sound will be dominant. As the sound propagates in the hall the sound field will be reflected and absorbed by surfaces in the hall like the ceiling, walls and the floor. The reflected sound field is the reverberant sound field of the hall. The received sound field from a listener’s position is a combination of the direct sound and the reverberant sound field at his position. This blend of reflected and direct sound fields determines the quality of music from a listener’s perspective.
One of the key factors is the power of the reverberant field, expressed in the reverberation time of a hall. The reverberation time of a hall is the time a sound needs to drop 60 dB in level. Each auditorium has its own optimum reverberation time. Table 1 gives some examples.
When the volume of a hall is constant, the reverberation time is highly dependent on the amount of absorbency of the surfaces of the hall. In a more reflective room it will take longer for the sound to die away, this type of room is said to be 'live'. In a very absorbent room, the sound will fade away quickly and the room is described as acoustically 'dead'.
If we hold a microphone in the reverberant field and connect it to a loudspeaker in a wall or ceiling through an equalizer and an amplifier, we can invert the absorbent effect of the surfaces of the hall and increase the reverberation time by amplifying the microphone signal. This is the fundamental principle of a MCR system.
Click on the images to the right for an animation illustrating this.
The MCR system consists of a number of independent equalized, amplified microphone loudspeaker channels. The number of required channels is based on the needed amplification of the reverberant field.
By carefully tuning every separate channel each desired reverberation curve in hall can be created.
