Timbre and Loudness on the Localisation of Sounds. 271 



total sound) to be experimentally varied.* Three positions of the slides 

 were adopted : position B, the middle or " normal " position of the slides, 

 which was used for practising the subjects in sound localisation ; position A, 

 in which the slides nearly closed the open ends of the boxes ; and position C, 

 in which the slides were drawn well out so as to produce relatively loud 

 overtones. 



The loudness of the whole sound (fundamental and overtones) was varied 

 by moving the horn nearer or farther from its middle or " normal " position. 

 The subjects were practised in sound localisation with the horn at its 

 " normal " or B position. Subsequently, the loudness of the sound was 

 decreased by moving the horn farther from the tone-variators (the A position 

 of the horn) or increased by moving the horn nearer to the tone-variators (its 

 C position). 



The use of the tone-variators and of varying positions of the slides or 

 horn, just described, necessitated the presence of an assistant in the room in 

 which the sounds were produced. Communication between him and the- 

 experimenter, who sat with the subject in the sound-proof room, was effected 

 by means of loud-speaking telephones and an electric bell, so that by 

 pre-arranged signs the assistant might give the sounds at the desired 

 moment, and vary their timbre or loudness in the desired order. 



Experiments were also carried out in which the sound was produced by 

 the experimenter within the sound-proof room by means of a telephone 

 buzzer or an electric bell placed at the position of the funnel F, at the free 

 end of the rotating arm of the perimeter. 



Except when otherwise stated, the mode of procedure was as follows: 

 The subject, seated in the chair C in the sound-proof room, was blindfolded, 

 and a head rest was adjusted to the back of his head in order to prevent, so 

 far as possible, any movement. The perimeter was then arranged so as to 

 allow of sounds being given in one or other of the two planes (vertical 

 or horizontal), and the slides and horn were set at their respective B positions. 

 Several sittings were given by each subject for practice in localisation, and 

 later the positions of the slides and horn were irregularly varied for the 

 study of the effects of variations in the timbre and loudness of the sound in 

 one of the two planes, before similar experiments were made in the other 

 plane. The production of the sound which the subject was required to 



* In a number of preliminary experiments, I employed resonators at variable distances 

 from the variators, but the tones conducted from the resonators by narrow rubber tubes 

 to the sound-proof room were too weak for my purpose. I also tried a loud-speaking 

 telephone for conducting the tones, but, owing to the unsatisfactory timbre and incon- 

 stancy of the resulting sound, I had to abandon this method likewise. 



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