One thing that has been noticed by all who have experimented with this subject is the curious occurrence of “areas of silence.” That is to say, a certain siren will be well heard close to its position. Then a little farther off the sound will be lost, but on going farther away still it is heard again.
Many theories have been advanced to account for this, but none are completely satisfactory. It is, however, a well-established effect, and one with which it behoves all mariners to be acquainted.
One curious fact is the very great power that can be absorbed in creating a loud siren note. Thus in one case, a siren giving a high note was found to absorb as much as 600 horse-power when the note was sounded continuously. The most striking and in one sense the most disappointing thing about these loud sounds is the small distance which they travel in certain states of the wind. As a general result, it has been found that the most effective sound for coast-warnings is one having a frequency of 100, or a wave-length of about 10 feet. When dealing with the subject of waves in general, it was pointed out that the velocity of a wave depended upon the elasticity and the density of the medium in which it was being propagated. In the case of a sound wave in air or any other gas, the speed of wave-transmission is proportional to the square root of the elasticity of the gas, and inversely proportional to the square root of the density.
At the same temperature the elasticity of a gas may be taken to be the same as its pressure. Hence, at the same pressure, the speed of sound-wave transmission through different gases varies inversely as the square root of their densities. An example will make this clear. If we take the density of hydrogen gas to be unity (= 1), then the density of oxygen is 16. The ratio of the densities is therefore 1 to 16, and the square roots of the densities are as √1 to √16, or as 1 to 4. Accordingly, the velocity of sound waves in hydrogen gas is to that in oxygen gas as 1 is to ¹⁄₄. In other words, sound travels four times faster in hydrogen than it does in oxygen at the same temperature and pressure. The following table shows the velocity of sound in different gases at the melting-point of ice (= 0° C.) and atmospheric pressure (= 760 mm. barometer).
| Gas. | Velocity. | ||
| Hydrogen | 4163 | feet per second | |
| Carbonic oxide | 1106 | ” ” | |
| Air | 1090 | ” ” | |
| Oxygen | 1041 | ” ” | |
| Carbonic acid | 856 | ” ” |
Accordingly, we see that the lighter the gas the faster sound travels in it, pressure and temperature being the same. If the atmosphere we breathe consisted of hydrogen instead of a mixture of oxygen, nitrogen, and many other gases, a clap of thunder would follow a flash of lightning much more quickly than it does in our present air, supposing the storms to be at the same distance. Under present circumstances, if 20 seconds elapse between the flash and the peal, it indicates that the storm is about 4 miles away, but if the atmosphere were of hydrogen, for a storm at the same distance the thunder would follow the lightning in about 5 seconds.
Furnished with these facts about the propagation of air waves, it is now possible to point out some interesting consequences. It will be in your recollection that in the first chapter it was pointed out that a wave on water could be reflected by a hard surface, and that it could be refracted, or bent, when it passed from a region where it was moving quickly to one where it was moving more slowly. It will be necessary now to prove experimentally that the same things can be done with sound, in order that a body of proof may be built up in your minds convincing you that the external cause of sound-sensation must be a wave-motion in the air.
In the first place, I must describe to you, somewhat in detail, the nature of the arrangements we shall employ for producing and detecting the sound waves which will be used in these experiments.
It would not do to rely upon the ear as a detector because you cannot all be so placed as to hear the sounds which will be produced, and we shall, therefore, employ a peculiar kind of flame, called a sensitive flame, to act as a detector.
If ordinary coal-gas stored in a gasometer is burnt at a small jet under considerable pressure, we are able to produce a tall flame about 18 to 24 inches in height. The jet used is one with a steatite top and small pin-hole gas exit about ¹⁄₂₅ inch in diameter. The pressure of gas must be equal to about 10 inches of water, and it cannot be drawn straight off the house gas-pipes, but must be supplied from a special gasometer or gasbag under a pressure sufficient to make a flame 18 inches or so in height. If the pressure is too great, the flame roars; if the pressure is slightly reduced, the flame can be made to burn quietly and form a tall reed-like flame (A, [Fig. 48]). This flame, when properly adjusted, is curiously sensitive to shrill, chirping sounds. You may shout or talk loudly near it, and it takes no notice of your voice, but if you chirrup or whistle in a shrill tone, or clink your keys or a few coins in your hand, the flame at once shortens itself to about 6 or 7 inches in height, and becomes possessed of a peculiarly ragged edge, whilst at the same time it roars (B, [Fig. 48]). When in adjustment, the clink of a couple of coins in the hand will affect this sensitive flame on the other side of the room.[22]