Speech Power

The amount of power in speech sounds is very small. To quote one authority:²¹

By taking the average speech power for a number of individuals talking in their usual conversational manner, it has been found that the average speech power for American speech is approximately 10 microwatts. If the silent intervals during conversation are excluded, this average is increased approximately 50 per cent. To carry this amount of power the air particles near the mouth vibrate through a distance of the order of ¹/₁₀₀ millimeter. When this amount of sound energy is received directly into the ear, it seems rather large due to the large excitation it produces on the auditory sense. However, it is really very small in comparison with the other powers ordinarily encountered. For example, it takes power equivalent to that produced by more than one million voices to light an ordinary incandescent lamp. It is therefore evident that the electrical currents used to transmit speech are of a different order of magnitude than those used to transmit power for lighting and heating purposes. It is only in some of the larger broadcasting stations that electrical speech currents are comparable in size with those used in power work.

When one talks about as loudly as possible the average speech power increases approximately to 1000 microwatts. When one talks in as weak a voice as possible without whispering it drops to 0.1 microwatt. A very soft whisper is about at 0.001 microwatt.⁽¹⁾

The vowels are the most powerful speech sounds, having an average power value of about 100 microwatts, with peak values of about 2000 microwatts. For any individual the ratio of the strongest to the weakest speech sounds is about 35 to 40 db. The energy distribution among the different frequencies²³ is shown in Fig. 10.

Figure 10. Energy distribution ; composite curve for male and female voices. (I. B. Crandall, Bell System Tech. J., October, 1925.)

It is of interest to note that speech can be produced entirely by artificial means using electroacoustic equipment.²⁴ Such a device, called the Voder, was demonstrated at the 1939 World's Fairs in New York and San Francisco. Equipment for translating speech into visible patterns²⁵ has also been developed.

(1)	Reprinted by permission of the publishers, from Speech and Hearing, by Harvey Fletcher, D. Van Nostrand Co.