to determine tJie Direction of Sound. 
319 
I realized how hopeless it was to try such experiments in a 
small laboratory where perhaps 90 per cent, or more of the 
sound had already suffered one or more reflexions ; the 
experiment merely suggested a possible method o£ studying 
the position of nodes and antinodes in such a closed space. 
Later an apparatus to determine the direction of sound by 
magnifying the intensity effects by means of two sound- 
receivers on opposite sides of a large flat board was surpris- 
ingly inadequate except in the case of the higher notes, and 
led to the conclusion that it was very probable that phase had 
a good deal to do with our judgment of the direction of 
sound. 
I postponed further experiments on the direction of sound 
until the summer of 1907, when I should have an opportunity 
of trying them in the open air. 
In the meantime I saw Lord Rayleigh's paper in the 
Philosophical Magazine proving conclusively, in an ingenious 
manner in the laboratory, that phase in some cases gave a 
sense of direction. It only remained to show to what extent 
and within what limits these effects were produced. In 
June 1907 therefore I proceeded to try experiments in the 
open air, the apparatus being a set of adjustable organ- 
pipes, made for me by Hutchings & Votey of Cambridge,/ 
Mass., and cylindrical tubes of various lengths applied to Yy^ 
the ears. 
The tubes were of sheet aluminium, and they could have 
their length altered by sliding one within another. 2^ inches 
was chosen as the diameter, because then the tube could be 
readily fitted quite closely round its whole circumference 
against the head. \^ 
The source of sound was placed at a distance of about] 
30 feet from the observer in an open field. It was at once | 
found that with two unequal tubes applied to the ears, and 
with the observer facing the source of sound, the source' | ' Vt 
appeared to move to the opposite side to that on which the 
longer tube was applied. (This, as will be seen later, is only 
true within certain limits.) 
After certain preliminary experiments, the ground was 
pegged out in an arc of a circle 28^ feet in radius. The 
source of sound was placed at a point of this arc, and pegs 
were placed at intervals of 4° along the circumference up to 
60° on each side of the source. The observer was situated at 
the centre of the circle. As the image of the source moved \ I 
the observer continually faced the image and noted its 
apparent position on the arc. This position could be fixed 
with an error of not more than one or two degrees. 
Z2 
