146 



BELL SYSTEM TECHNICAL JOURNAL 



Measuring System. — The circuit arrangement of the measuring 

 system used in making the measurements is shown in schematic form on 

 Fig. 1. An oscillator having a suitable frequency range and power 

 output was alternately connected by means of a two-position switch 

 through a transformer to the loud speaker under test and to the input 

 terminals of an attenuator calibrated in TU. The transformer had a 

 ratio such that the loud speaker being measured was always connected 

 to an impedance equal to that for which it was designed to be con- 

 nected. The condenser transmitter in series with the output terminals 

 of the attenuator and located in the medium as will be discussed later, 

 was connected to a voltage indicating system consisting of an amplifier, 

 a thermocouple and a meter. A low-pass filter was included in the 



VARIABLE 

 TRANSFORMER 



LOUD SPEAKER 

 UNDER TEST 



CONDENSER 



TRANSMITTER 



AMPLIFIER 



-MM/W-WW^T-VWW- 



VARIABLE ATTENUATOR 

 0-75 TU_^ 

 500"500 



Fig. 1 — Schematic circuit of loud speaker response measuring system. 



indicating system as shown to insure that only the fundamental 

 frequency of the output from the condenser transmitter or attenuator 

 was indicated. The measuring procedure was as follows : The output 

 or terminal voltage of the oscillator when open-circuited or connected 

 to the attenuator was kept constant at all frequencies by means of a 

 vacuum tube voltmeter. With the loud speakers considered the sound 

 output over a wide magnitude range was proportional to the oscillator 

 voltage so that the magnitude of this voltage was governed entirely by 

 the sound pressure in the medium most suitable for making measure- 

 ments. With the oscillator connected to the loud speaker (through the 

 transformer) the sensitivity of the voltage indicating system was 

 adjusted at each frequency until a mid-scale deflection of the meter was 

 obtained as a result of the voltage generated by the condenser trans- 

 mitter. After each adjustment the oscillator was then switched from 



