316 BELL SYSTEM TECHNICAL JOURNAL 



device called the compandor has been developed. This device which 

 works automatically makes a further reduction of one-half in the resid- 

 ual db range so that the range transmitted over the circuit is then 

 only 15 db, an energy ratio of about 32 to one. 



Speech Energy 



Quantitative designation of speech intensity and hence of a range of 

 intensities is rendered difficult by the rapidly varying amplitude char- 

 acteristics of the various speech sounds. Devices called volume indi- 

 cators are used fairly extensively to indicate the so-called "electrical 

 volume" * of speech waves. A volume indicator is essentially a 

 rectifier combined with a damped d-c. indicating meter on which are 

 read in a specified manner the standard ballistic throws due to partly 

 averaged syllables at a particular speech intensity. These devices are 

 so designed and adjusted that they are insensitive to extremely high 

 peak voltages of short duration, but their maximum deflection is ap- 

 proximately proportional to the mean power in the syllable. It has 

 been found that, if commercial telephone instruments are used, the ear 

 does not detect amplifier overloading of the extremely high peaks of 

 short duration. Consequently, the volume indicator is a useful device 

 for indicating the noticeable repeater overloading effect of a voice wave. 

 These devices do not tell us much about the effect of the weaker volt- 

 ages in overriding interference or operating voice-operated devices 

 but they give a fairly satisfactory indication of loudness and possibil- 

 ities of interference into other circuits. 



The sound energy that the telephone transmits consists of compli- 

 cated waves made up of tones of different pitch and amplitude. The 

 local lines and trunks connecting the telephone to the subscribers' toll 

 switchboard have little effect in changing the fundamental characteris- 

 tics of these waves but, on account of various amounts of dissipation, 

 the waves received at the toll switchboard are always weaker than those 

 transmitted by the telephone. Furthermore, the strength of signals 

 varies with the method of using the telephone, loudness of talking, 

 battery supply, and transmitter efficiency. The subscriber may be 

 talking over a long distance circuit from a distant city, in which case 

 the loss of the toll line further attenuates the received waves. Figure 

 1 t shows that the range of outgoing speech volumes as measured by a 

 volume indicator at the transatlantic switchboard at New York is 

 nearly 40 db for terminal calls. When via calls and variation in volume 



* The term volume will be used through the rest of this paper to designate this 

 quantity and not as synonymous with loudness.- 



t This curve is plotted on so-called probability paper, in which the scale is such 

 that data distributed in accordance with the normal law will produce a straight line. 



