ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 253 
with the resonator set at a. position far removed from that giving resonance. On testing this conclusion 
for the purpose of employing the phonometer to measure very loud blasts giving readings far off' the scale 
of the instrument when tuned to the exact frequency of the diaphone note, it was found that the propor¬ 
tional relationship did not hold with any approach to accuracy. Although it was surmised that the reason 
for this discrepancy was connected with the fact that the sound-waves were very much more intense than 
any which had been hitherto dealt with by means of the phonometer, it was some time before a definite 
clue was obtained as to the exact nature of the difficulty. While carrying out observations at the control 
station on a specially calm day, the writer happened to make a slight adjustment of the resonator while a blast 
was being sounded by the diaphone. A current of air could be distinctly felt blowing outwards from the 
aperture of the resonator. The phenomenon was investigated with a lighted match which was easily 
extinguished when held a couple of inches from the resonator. The writer attributes the phenomenon to 
the formation of a rapid succession of vortex rings set up at the edge of the resonator aperture by the 
pulsations of air due to the very intense sound-waves (the scale-reading of the phonometer was about 
7 mm.). A “ vortex stream ” is thus set up on both sides of the aperture, that moving into the resonator 
being broken up and supplying air which issues outwards from the orifice. The phenomenon of jets 
issuing from resonators has been observed by Lord Rayleigh, who has utilized the effect in the design of 
a “ vibration indicator ” to measure the powerful sound-waves generated by portable forms of fog-signal 
apparatus.* It will be evident that for very intense sounds the theory of the absolute phonometer is 
invalidated to an extent which requires further investigation. 
A few tests were carried out to make certain that the readings of the phonometer were unaffected by 
the direction in which the instrument was pointed with respect to the source of sound. Readings were 
taken behind the diaphone trumpet at the control station already mentioned, and also at a position about 
130 feet from the trumpet on Line VIII. (Chart 1, Appendix II.). Successive readings at intervals of 
45 degrees, measured in the direction N.-E.-S.-W., commencing at 0 degrees with the aperture of the 
phonometer resonator pointed at the diaphone trumpet, are given below. 
0°. 
45°. 
90°. 
135°. 
180°. 
225°. 
270°. 
315°. 
Control station .... 
6 '25 
6-3 
6 '5 
6 '5 
6'5 
6'8 
6 5 
6 ‘5 
Position on Line VIII. . 
5 '5 
5'3 
5 '75 
5 *6 
5 5 
— 
— 
The above readings are constant within the limits of accidental fluctuations of amplitude, indicating 
that neither the azimuth of the resonator axis with respect to the wave-front or the effect of the observer’s 
head has a noticeable effect on the phonometer readings. According to acoustic theory this result is to be 
expected, as the wave-length (in this case a little over 6 feet) is large compared to the dimensions of the 
resonator and of the obstacle formed by the observer’s head. The result was also verified by similar tests 
carried out at a distance in a ship’s boat as well as from the deck of the “ Lady Evelyn,” provided in each 
case the fog-signal station could be directly viewed from the phonometer. This conclusion is of consider¬ 
able practical importance in the measurement of sound in the open air, as the phonometer may be turned 
in such a direction as to eliminate the effect of wind which is liable to set up sound vibrations within the 
resonator when blowing into or across the aperture. Except on very windy days (wind greater than 30 
miles an hour) it was found possible to take phonometer readings without difficulty. 
(iv.) Determination of the Pitch Regulation of the Diaphone. 
It has long been known that one of the important features of the diaphone is its extremely good pitch 
regulation—that is, the remarkably small variation of pitch within wide limits of operating air pressure. In 
* Rayleigh, ‘Sound’ (1896), vol. II., pp. 216-217; ‘Phil. Mag.,’ VI., 1908, pp. 289-305; ‘Scientific Papers,’ vol. v., 
p. 132. 
2 L 2 
