ROOM NOISES IN TELEPHONE LOCA TIONS 



657 



adjustable attenuator controlled by a key and a dial is provided between 

 stages of the amplifier so that the noise energy being measured may be 

 brought within the range of the meter over a range of levels of 80 db 

 (corresponding to a power range of 100,000,000 to 1). 



Fig. 3 — Room noise meter and auxiliary equipment. 



Operation of the Room Noise Meter. — -The noise meter is first cali- 

 brated, as described below, so that its sensitivity is set at a predeter- 

 mined value. The condenser transmitter is then placed at the spot 

 where it is desired to measure noise, and the gain of the amplifier is 

 adjusted by means of a key and dial until the needle of the microam- 

 meter in the output circuit fluctuates about a given point. The set- 

 tings of the key and dial then give a measure of the noise. In addition 

 to the average readings obtained in this manner, readings of the fluc- 

 tuations in the noise can be similarly obtained. As an aid in the read- 

 ing, the microammeter scale is calibrated in decibels. 



The calibration of the meter in the field consists of a check on the 

 over-all sensitivity of the instrument. The filament currents and 

 plate voltages are adjusted to the correct values. Then a fixed percent- 

 age of the electrical output of a standard buzzer, the current from which 

 is measured by a thermocouple, is fed into a special receiver which is 

 placed in a prescribed way on the condenser transmitter. The gain of 

 the amplifier is then adjusted until the output microammeter needle 

 reaches a predetermined point. The sensitivity of the meter will then 

 be as shown on Fig. 1. 



