474 



BELL SYSTEM TECHNICAL JOURNAL 



12,000 



11,000 



10,000 



9000 



'H 3000 



O, 2000 



2 4 6 6 10 12 14 16 18 20 22 24 



EASTERN STANDARD TIME 



Fig. 10 — Dependence of critical frequencies on season and hour of day. 

 (Smith, Gilliland and Kirby.) 



