1233Chapter 34
Sound System Design
by Chris Foreman
34.1 Introduction........................................................................... 1239
34.2 Sound Reinforcement System Models...................................................... 1239
34.2.1 The Four Questions (and a Fifth)....................................................... 1239
34.2.2 The Simplified (Outdoor) Sound Reinforcement System Model............................... 1239
34.2.2.1 Definitions..................................................................... 1240
34.2.2.2 Attenuation with Increasing Distance................................................ 1240
34.2.2.3 Acoustic Gain.................................................................. 1240
34.2.2.4 Feedback and Potential Acoustic Gain (PAG)......................................... 1241
34.2.2.5 Number of Open Microphones (NOM)............................................... 1241
34.2.2.6 Feedback Stability Margin (FSM)................................................... 1241
34.2.2.7 Noise......................................................................... 1241
34.2.2.8 Head Room and Electrical Power Required (EPR)...................................... 1241
34.2.2.9 Equivalent Acoustic Distance (EAD)................................................ 1242
34.2.2.10 Needed Acoustic Gain (NAG)..................................................... 1243
34.2.2.11 Will the System Feed Back?...................................................... 1243
34.2.2.12 The Effect of Directional Microphones and Loudspeakers............................... 1243
34.2.2.13 How These Equations Answer the Four Questions..................................... 1244
34.2.2.13.1 Question 1: Is It Loud Enough?...................................................... 1244
34.2.2.13.2 Question 2: Can Everybody Hear?.................................................... 1244
34.2.2.13.3 Question 3: Can Everybody Understand?.............................................. 1244
34.2.2.13.4 Question 4: Will It Feed Back?...................................................... 1244
34.2.3 Indoor Sound Reinforcement System Model.............................................. 1245
34.2.3.1 Echoes and Reverberation......................................................... 1245
34.2.3.1.1 When an Echo Is a Problem.......................................................... 1245
34.2.3.1.2 Can a Reflection Be Useful?......................................................... 1245
34.2.3.1.3 When Reverberation Is Useful........................................................ 1245
34.2.3.1.4 When Reverberation Is a Problem..................................................... 1246
34.2.3.2 Reverberation and the Sound Reinforcement System.................................... 1246
34.2.3.2.1 Direct/Reverberant Ratio............................................................ 1246
34.2.3.2.2 Loudspeaker Q.................................................................... 1246
34.2.3.2.3 Room Constant.................................................................... 1247
34.2.3.2.4 Critical Distance (Dc ).............................................................. 1247
34.2.3.3 Attenuation of Sound Indoors...................................................... 1248
34.2.3.4 The Four Questions Again......................................................... 1249
34.2.3.4.1 Question 1: Is It Loud Enough?....................................................... 1249
34.2.3.4.2 Question 3: Can Everybody Understand?............................................... 1250
34.2.3.4.3 Alcons Modified for Audience Absorption and Talker/Listener Factors........................ 1251
34.2.3.4.4 Alcons in Terms of RT 60 and Direct, Reverberant, and Noise Levels.......................... 1251
34.2.3.4.5 Question 4: Will It Feed Back?....................................................... 1252