AFC3

One AFC system utilizes four microphones for operation. If multiple AFC systems are needed for a single venue, each additional system will also require four microphones. For detailed examples of microphone and speaker arrangement of a single AFC system, refer to the “Systems” section of the site.

Microphones are fixed to or hung from the ceiling. They must be visible from the source point. They are set at or around the critical distance (Dc) from a representative source point.

Environmental noise, especially from heating and air conditioning systems, has to be considered. The reverberation enhancement of AFC will also increase the SPL of back ground noise. This is not necessarily a problem because it is natural, e.g. if the reverberation time is increased by absorption decrease of absorptiive materials, the noise level will increase. However, it is important to prevent excessive noise caused by a direct wind to the microphones or proximity to other mechanical noises. A length of 1.8 meters would be criteria for a sufficient distance between a microphone and a supply opening.

For Early reflection enhancement, microphones have to receive the direct sound from a sound source, so they should preferably be set at or within the critical distance (Dc).

Microphones must be visible from any point on or around stage area. If the microphone can be set at the Dc, the microphones can be used either for Reverberation enhancement or ER enhancement.

The microphones should be positioned so that the closest microphone-to-microphone distance is no less than half the length of the lowest wavelength you want to enhance. For example, if you want to enhance more than 125 Hz, the distance between the microphones should be greater than or equal to 4.5 feet.

We recommend using Yamaha’s VXS Series surface mount speakers, VXC Series ceiling speakers and Installation Series loudspeakers for AFC. In the US, the S8AFC speaker is also available. For detailed description of products that are used for AFC, please refer to the “Lineup” section of the site.

Speakers should be placed beyond Dc from the listening area to avoid the localization of sound from the speakers. Wall speakers should be facing directly horizontal. The height can be determined by the distance from the nearest seat which should be at or beyond Dc. Ceiling speakers should face directly downward.

The distance between speakers should be between 15 to 20 feet, which corresponds to Dc. Under a balcony, the distance between the seats and the speakers is often less than Dc. In this case, the speaker distance should be the same as or less than the ceiling height.

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.

- For rooms with excessive natural reverberation, AFC cannot deaden the space’s reverberation.

- Acoustical anomalies such as excessive reverberation, focused echoes, slap-back echoes, diaphragmatic surfaces, noisy rooms, vibrating or shaking floors or structures, and standing waves or room modes cannot be corrected by AFC. Traditional acoustical means or solutions must be employed for these issues.

- While AFC can mask flutter echo, actual correction of the echo problem requires traditional acoustical solutions.

- The primary goal of AFC is to create natural sounding reverberation that correlates with the visual stimulus from the room. Thus, AFC will not make one room sound like another such as the YASI Piano Selection Studio cannot be made to sound like Stern Auditorium at Carnegie Hall. AFC’s natural sound is assured because the reverberation tail used to start the AFC process contains the room’s impulse response signature which is then used to create the additional reverberation in the FIR filters which adds to the reverberant level and further lengthens the tail.