PHYSICAL FACTORS 249 



obtaining the data shown in Fig. 1 are indicated at the right of the 

 symboHc circuit diagrams. 



Factors Affecting Depth Localization 

 Before attempting to explain the results that have been given in the 

 foregoing, it may be of interest to consider certain additional observa- 

 tions that bear more specifically upon the factors that enter into the 

 "depth" and "angular" localization of sounds. The microphones on 

 the pick-up stage receive both direct and reverberant sound, the 

 latter being sound waves that have been reflected about the room in 

 which the pick-up stage is located. Similarly, the observer receives 

 the reproduced sounds directly and also as reverberant sound caused 

 by reflections about the room in which he listens. To determine the 

 efi^ects of these factors, the following three tests were made: 



1. Caller remained stationary on the pick-up stage and close to 

 microphone, but the loudness of the sound received by the observer 

 was reduced by gain control. This was loudness change without a 

 change in ratio of direct to reverberant sound intensity. 



2. Caller moved back from microphone, but gain was increased to 

 keep constant the loudness of the sound received by the observer. 

 This was a change in the ratio of direct to reverberant sound intensity 

 without a loudness change. 



3. Caller moved back from microphone, but no changes were made 

 in the gain of the reproducing system. This changed both the ratio 

 and the loudness. 



All of the observers agreed that the caller appeared definitely to recede 

 in all three cases. That is, either a reduction in loudness or a decrease 

 in ratio of direct to reverberant sound intensity, or both, caused the 

 sound to appear to move away from the observer. Position tests using 

 variable reverberation with a given pick-up stage outline showed that 

 increasing the reverberation moved the front line of the virtual stage 

 toward the rear, but had slight effect upon the rear line. When the 

 microphones were placed outdoors to eliminate reverberation, reducing 

 the loudness either by changing circuit gains or by increasing the 

 distance between caller and microphone moved the whole virtual 

 stage farther away. It is because of these effects that all center line 

 positions on the pick-up stage appeared at the rear of the virtual stage 

 for 2-channel reproduction. 



It has not been found possible to put these relationships on a quan- 

 titative basis. Probably a given loudness change, or a given change in 

 ratio of direct to reverberant sound intensity, causes different sensa- 

 tions of depth depending upon the character of the reproduced sound 



