of Instruments and Voices
b. Because of the greater range of frequencies contained in musical sounds--voice and instrument--telephone
circuits designed for their transmission must have more rigid specifications to prevent distortion. This increases both the
initial cost of the equipment and the expense of maintaining it. For transmission or ordinary conversation, however, it has
been found that a sufficiently high degree of intelligibility can be achieved if the frequencies transmitted are limited to
those between approximately 200 and 2,700 Hertz. This is the range of frequencies with which the various circuits and
equipment to be discussed in this manual are concerned.
The power contained in the sounds of speech depends on the power furnished by the lungs. It varies
considerably during ordinary conversation, with the inflections given to the voice. The average power contained in
speech at a normal conversational level is about 1/100,000 watt, or 10 microwatts. By comparison, the average power of
speech conducted as loudly as possible is about 1,000 microwatts. Words spoken in as weak a voice as possible, without
whispering, have an average power of about 1/10 microwatt; words whispered may have an average power as low as
1/1000 microwatt. In ordinary speech, the vowels contribute the greatest power, reaching a maximum of about 2,000
microwatts. The power of speech in sounds is an important factor in the design and operation of telephone equipment,
because the equipment must be able to respond to the differences in power delivered by the voice.