To achieve a continuous distribution of spatial impressions, the system requires several speakers distributed evenly throughout the venue. Using the information about distances between speakers above, an even pattern of speaker placements should be created that covers the entire intended audience area. When someone plays a musical instrument in a room, the listener hears the direct sound and the physical reflections of that sound. Turning on the system, the listener hears the direct sound from the source, direct sound from the nearest AFC speaker, and physical reflections from the source, including the sound of other AFC speakers.
The energy of the direct sound from the nearest AFC speaker is shown in this formula (Formula.1):
On the other hand, when the system is turned off, the energy of physical reflections from the sound source is shown in this formula (Formula.2):
Here, to realize twice the reverberation time, the energy should be double because energy density increases with reverberation time.
So, assuming the value of the output of the AFC speaker times the number of speakers equals the value of the output of the source (Formula.3):
The number of speakers for AFC is shown in this formula (Formula.4):
Q: AFC speaker directivity including installation conditions (=2)
r: Distance from a listener to an nearest AFC speaker X: Correction Coefficient S: Room Surface a: Average sound absorption coefficient R: Room Constant c: Sound velocity Es: The energy density of the direct sound from the nearest AFC speaker E0: The energy density of physical reflections from the sound source N: The number of AFC speakers Ws: The output of the source
This figure shows the result of this formula; the correlation between reverberation time (room’s natural RT without any enhancement) and the number of speakers. Each line is categorized by the capacity of the venue.
The labelled data points correspond to actual venues. The symbols from A to D are located on the line of 2,200 seats. The symbols from E to H are located on the line of 700 seats. This method for estimating the quantity of AFC speakers has one assumption. When considering the localization of speakers, the value of the energy of the direct sound from the AFC speaker is -22 dB relative to the energy value of physical reflections from the source.