656 



BELL S YS TE M TE CHNI CA L JO UR NA L 



general level is not far from the middle of the range of levels of room 

 noise components. The loudness data used were those given by Kings- 

 bury,^ based on experimental data on single-frequency tones. 



The sensitivity of the meter is such that a 1000-cycle tone about 28 db 

 above threshold would give a reading of db on the meter scale. 



5 

 u 



in 



o 



a > 

 oa 

 u. 



.Q ~ 



O 



try 



^^ 



l^ I 



z 



1000 2000 3000 4000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 1 — Response characteristic of room noise meter. 



The room noise meter employed is shown together with its auxiliary 

 equipment in Fig. 3. It consists of a condenser transmitter for convert- 

 ing acoustical energy into electrical energy, six stages of amplification 

 for raising the level of the noise currents sufficiently to operate a ther- 

 mocouple meter indicating device, and a weighting network, as de- 



CONDENSER 

 TRANSMITTER 



TWO 



AMPLIFIER 



STAGES 



_ ATTENUATOR 

 (key 8, DIAL) 



ONE 



AMPLIFIER 



STAGE 



FREQUENCY 

 WEIGHTING 



THREE 



AMPLIFIER 



STAGES 



THERMO- 

 COUPLE 

 TYPE 

 DETECTOR 



MICRO- 

 AMMETER 



^ 



Fig. 2 — Schematic diagram of room noise meter. 



scribed above, as well as certain apparatus not employed in obtaining 

 the results reported here. The general layout of the circuit is indicated 

 in the schematic diagram of Fig. 2. A portable battery supply and 

 means for calibrating form the necessary auxiliary equipment. An 



^Physical Review, Vol. 19, April 1927, pp. 588-600. 



