546 BELL SYSTEM TECHNICAL JOURNAL 



resulting alternating potentials, faithfully representing the sound 

 pressure waves, are amplified by the associated amplifier. Between 

 the first and second stages of this amplifier, provision has been made 

 for the introduction of distortion networks to simulate the distortion 

 of any transmitting system. Between the last two stages are attenu- 

 ating networks permitting adjustment of the relation between trans- 

 mitter input and transmitter output over a range of 22 db in steps of 



Fig. 6 — -Thermophone in chamber for calibrating transmitter. 



0.1 db. The output impedance of the amplifier is 600 ohms with 

 negligible phase angle. 



The reference line consists of a series of balanced resistance net- 

 works. This line has a characteristic impedance of 600 ohms matching 

 the output impedance of the reference transmitter and the input 

 impedance of the reference receiver, thereby eliminating any reflection 

 efi"ects at these junctions. By means of suitable controls the attenu- 

 ation may be varied over a range of 101 db in steps of 0.2 db. 



The reference receiver consists of a moving coil receiver and associ- 

 ated three-stage amplifier. Means are provided for adjusting the 

 relation between the input to the receiver and its output over a range 



