226 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



associating a probe tube with the condenser transmitter, absolute measure- 

 ments of sound intensity could be made in the ear canal. 



This method of sound measurement led also to the closed-coupler artificial 

 ear^' ^'* means of measuring the acoustic output of a telephone receiver under 

 specified conditions approximating those of a typical human ear. 



Another important measuring device derived from this work was the 

 volume indicator^^' ^^ whose rate of response to the fluctuations of speech 

 sound energy was made to approximate the ear in this respect. This indicator 

 when connected through a suitable amplifier to a telephone circuit could be 

 used to measure the level of speech currents at that point. When a volume 

 indicator was associated through an amplifier with a suitable pickup micro- 

 phone it became a sound level meter for giving a measurement of the level 

 of the complex tones of speech, music and noise. 



Another device made practicable by the availability of the vacuum tube 

 amplifier was the loudspeaker. This permitted suitable sound levels to be 

 delivered by loudspeakers^^- ^^' ^^ which were progressively freed from dis- 

 tortion as the theory and technique of electroacoustic devices was advanced. 

 The loudspeaker was employed in the artificial mouth^'* as a means of pro- 

 ducing speech sounds of prescribed character and level for the testing of 

 transmitters. 



This combination of sound and electrical level meters, artificial mouth 

 and ear, provided for the measurement of the physical performance of trans- 

 mitters and receivers over the frequency range involved in telephony, for 

 the levels at which they were operated and with both speech sounds and 

 single frequency tones. The overall physical performance of these devices 

 were thereby brought to quantitative determination. 



With this situation the "standard cable system" was replaced as a refer- 

 ence system in the latter part of the twenties by what wafe termed the 

 "Master Reference System for Telephone Transmission. "^ 9 This system — 

 an outgrowth of Arnold's "perfect" transmission system — with the thermo- 

 phone means for absolute calibration of the transmitter and the closed 

 coupler arrangement for absolute calibration of the receiver, provided a 

 telephone reproducing system, the performance of which was specifiable in 

 absolute physical terms. Means were furnished in this Master System for 

 including distortion networks to make the idealized instruments of the 

 reference system approximate the characteristics of the instruments used 

 commerically; this distortion facilitated loudness balances with commercial 

 instruments and circuits. This reference system became the reference for 

 expressing loudness reproducing efficiency of commercial circuits and their 

 components. It was adopted as a standard by the Bell System and by the 

 CCLF.t 



t Comite Consultatif International Telephoniquc. 



