254 DR. LOUIS YESSOT KING ON THE PROPAGATION OF SOUND IN THE FREE 
• 
order to obtain an idea of the extent to which the phonometer readings might be affected by variation of 
pitch, a series of resonance-curves, of the type shown in fig. (ii.), was obtained for a series of operating 
pressures from 11'7 to 30 - 5 lbs./sq. in. The phonometer was mounted on a theodolite tripod at position 2, 
line VII., Chart 1, 100 feet from the diaphone. Here the note of the diaphone was still somewhat 
rough, indicating the existence of harmonics. While one observer (H. H. H.) operated the fog-signal at 
the required pressures, the writer took a number of phonometer observations with the resonator at 
different positions, with a view to determining that giving the maximum reading. As in the tests 
described in (i.), some cases of spurious resonances were observed, probably due to harmonics coinciding 
with an upper partial of the resonator or of the diaphragm. There was no difficulty in picking out the 
particular position of the resonator corresponding to the fundamental frequency of the diaphone note. 
The final results only are given in Table III. below, and are shown graphically in figs, (i.) and (iii.) of 
Appendix III. The mean of the pressures at the beginning and end of each 6-second blast is recorded. 
The position of the resonator corresponding to the maximum phonometer reading was determined from 
curves corresponding to that shown in fig. (ii.). The pitch of the diaphone note was taken to be that of 
the phonometer resonator as given by Table I. and the curve of fig. (i.). 
Table III. 
Mean pressure. 
Maximum phonometer 
reading. 
Position of resonator at 
maximum. 
Pitch. 
lbs./sq. in. 
mm. 
cm. 
11-7 
4-8 
9-0 
169 
15-5 
5-3 
8-5 
171 
19-1 
8-2 
7-7 
174 
26-9 
5-5 
6-5 
■ 182 
30-5 
6-3 
4-0 
200 
In carrying out the acoustic surveys described in Appendix II., it was decided to set the phonometer 
at position 8, corresponding to pitch 175, and to keep the average operating pressure as closely as possible 
to 20 lbs./sq. in., corresponding to this frequency. From the resonance curve of fig. (ii.), it will be seen 
that a variation of 1 cm. in the setting of the phonometer resonator between 7'5 and 8'5 cm. will alter the 
scale-reading by 0'5 mm., from 5 - 5 to 6 - 0 mm., which represents, roughly, the limits of variations due to 
accidental fluctuations in the note emitted by the diaphone. Reference to fig. (i.) shows that this variation 
of resonator position corresponds to a variation of pitch between 171 and 176. From Table III. above, it 
will be seen that the pressure operating the diaphone may vary between 16 and 22 lbs./sq. in. before the 
pitch is altered to this extent. Furthermore, a reference to the results of Appendix III., tests 1 and 3, 
indicates that the acoustic output, as measured in the diaphone trumpet, does not vary much more than 
10 per cent, between 18 and 23 lbs./sq. in. We conclude, finally, that between these limits of pressure 
the phonometer, with the resonator set in position 8, will give relative measurements of pressure amplitude 
from day to day to an accuracy within the accidental fluctuations of the signal sounded by the diaphone. 
Also, provided the phonometer readings are not too large (not greater than 5 mm.), they may be reduced 
to pressure amplitudes in C.G.S. units by formula (iv.). 
(v.) Note on the Quality of the Sound Emitted by the Diaphone. 
In the Father Point tests of 1913, means were not available for studying the quality of the note emitted 
by the diaphone. It is evident from the remarks already made that overtones probably exist to some extent 
in the sound-waves close to the trumpet. Owing to the complex conditions of finite-wave propagation, 
