PROGRAM TRANSMISSION CIRCUITS 



243 



to 20,000 cycles per second. This range is for young people. It is consid- 

 erably less for middle-aged and elderly people, and varies with individuals. 



In addition to the limitation of the ear there is the fact that there is little 

 energy present in most program material in the extremes of this range, 

 particularly in the upper frequencies. The energy versus frequency spectra 

 of music and other forms of program have been published elsewhere.^ 

 Figure 7 shows the frequency range which must be transmitted for a number 

 of instruments, speech, and certain noises, so that competent observers 

 cannot detect any impairment. "^ For whole orchestras, experiment has 



140 



120 

 100 

 80 

 60 

 40 

 20 







Z -20 



10 



50 100 



500 1,000 5,000 10,000 



Frequency in Cycles per Second 



Fig. 6 — Limits of audible sound 



50,000 



shown that the elimination of frequencies below 40 and above 15,000 cycles 

 per second is undetectable.'* It the upper limit of the transmitted frequen- 

 cies is lowered from 15,000 cycles, the impairment is at first barely detectable 

 but increases at an accelerating rate. When the limit is materially lower 

 than 8,000 cycles, the loss is readily apparent to many people. 



Another important consideration is volume range — that is the difference 

 between the maximum and minimum levels of the program. The ordinates 

 of Fig. 6 show that for part of the frequency range, the ear can respond to a 

 range of intensities of more than 120 decibels, with perhaps 100 decibels as a 

 mean. However, the following considerations show that the volume range 



