Index Absorbent screens behind consoles, 477, 522 Absorbers, 117 active, 371 effect of angle of incidence, 150 fibrous, 98, 102 in floor, 330 Helmholtz, 105­6 membrane, 103, 108, 589 panel, 105­6, 126 porous, 98, 107 quarter wavelength, 17 rear wall, 590 resonant, 105, 107 Absorption, 98, 117 coefficients, 109, 282, 297, 324 effects of thickness and density, 99 frequency dependence, 78 interrelation with reflexion and pressure relative to altitude, 155 propagation distortion, 491 weight of, 31 Air conditioning, 254­8, 527 split type, 255 Air-conditioned rooms, 257 Allen, Ioan, 567, 570 Alphabet response, 321 Altec, 584 Ambisonics, 25 Amplifiers: for active monitors, 506 choice of, 511­12 Class `A', 494, 503 as a constant voltage source, 606 instantaneous current capability, 487, 506 sonic differences, 501 valve (tube), 403, 494, 506