736 
Proceedings of the Royal Society 
7. On certain Effects of Periodic Variation of Intensity of a 
Musical Note. By Professor Crum Brown and Pro- 
fessor Tait. 
Recent discussions as to the nature of vowel-sounds have led us 
to make experiments (partly with an apparatus constructed some 
years ago for the same purpose) upon the effect of a periodic varia- 
tion of intensity of a simple tone. 
It is obviously impossible to secure a simple harmonic variation, 
so we endeavoured to produce a displacement varying as 
1 - cos mt. 
An organ-pipe, giving a tone very free from harmonics, was 
sounded on one side of a partition in which were cut a series of 
large holes. These were opened and shut periodically by a revolving 
disc cut into separate sectors. The form of the holes was calculated 
on the rough assumption that the intensity of the sound passing 
through them was at each instant proportional to the uncovered area 
of the openings. This may be approximated to in many ways, most 
simply by making the holes approximately square or of rhombus 
form, with one diagonal radial, and the corners at the ends of that 
diagonal somewhat rounded off. 
Supposing the adj ustment perfect, the result should have been the 
disturbance 
(1 - cos mt) cos nt, 
or 
cos nt cos (m + n)t - J cos (in - n)t. 
Thus, in addition to the tone given by the pipe, there should be two 
others of the order of summation and difference tones. The result 
was tested very easily by the help of resonators. Standing in 
front of the openings in the partition, the observer applied a 
resonator to each ear, and the pipe giving the tone whose number 
of vibrations was the arithmetical mean of those of the tones of the 
resonators w r as sounded on a small organ, and the disc made to 
rotate with gradually increasing velocity. The resonators were 
found to be affected simultaneously. 
The experiments, as we made them, succeed much better with 
