428 
I)R. W. S. TUCKER AND MR. E. T. PARIS ON 
In conclusion, one example may be given of the use of the microphone to measure 
diffraction of sound. 
An interesting example is that of the diffraction effect of a single disc. A large wooden 
disc, 1 inch thick and 10 feet in diameter, is suspended by one edge. The tuning-fork 
described above serves as a steady source of sound and is placed opposite the centre of 
the disc and 30 feet from it. The microphone, in tune with this fork, is mounted with its 
orifice at the centre of the back of the disc—the axis of the microphone being of course 
vertical. The disc, with microphone, is now swung round about its vertical diameter 
and readings are taken with the vibration galvanometer—using the Amplifier method. 
The bearings were observed by means of a sighting-tube passing normally through 
the disc at a point on its vertical diameter about one-quarter of the way from its lower 
edge. As the disc was rotated one could observe through the sighting-tube a number 
of white posts, which were driven into the ground on the circumference of a circle of 
50 yards radius, the sighting-tube being at the centre. The pegs were 10 degrees apart, 
and the zero reading was given when the central post, the source of sound, and the 
sighting tube were in line. 
Bearing 
(degrees). 
Deflection 
(divisions). 
Bearing 
(degrees). 
Deflection 
(divisions). 
90 
45 
0 
43 
80 
31-5 
-10 
16 
70 
17 
-20 
Min. 2 
60 
5-5 
-30 
10 
52 
Min. 1 -4 
-37 
Max. 14-5 
50 
1-5 
-40 
12 
40 
12-5 
-50 
2 
37 
Max. 14-5 
-51 
Min. 2 
30 
10 
-60 
6 
20 
2 
-70 
18-5 
19 
Min. 2-0 
10 
20 
-80 
32 
0 
43 
-90 
45 
If we plot bearing and deflection, which for faint sounds measures the amplitude 
of the vibration, a curve of the form shown in fig. 21 is obtained. 
It is thus seen that the diffraction gives a central maximum equal in intensity 
to the sound which passes the edge of the disc, and this is surrounded by a ring 
maximum. 
A variant of this experiment was performed in which the disc was set to give different 
angles of incidence for the sound, and the microphone was moved along a horizontal 
diameter until a maximum effect was given. One thus obtains an image of the source 
for each angle of incidence, and the distance of this image from the centre of the disc 
gradually increases as the angle of incidence is increased. 
