A very striking instance of this occurred on the occasion of the funeral procession of our late beloved Queen Victoria of blessed memory. The body was conveyed across the Solent on February 1, 1901, between lines of battleships which fired salutes with big guns. Arrangements were made to determine the greatest distance the sound of these guns was heard. In a very interesting article in Knowledge for June, 1901, Dr. C. Davison has collected the results of observation from eighty-four places, some of which are indicated in the map ([see Fig. 47]), taken, by kind permission of the editor of Knowledge, from that journal. Observations were received from places as far distant as Alderton (Suffolk), 139 miles from the Solent. At several places the sound of the guns was loud enough to make windows shake. This occurred at Longfield (56 miles), Sutton (58 miles), and Richmond Hill (61 miles). But whilst there is clear evidence that the sound of the guns was heard even at Peterborough (125 miles), most curious to say, the sound was hardly heard at all in the neighbourhood of the Solent. The nearest place from which any record was received was Horley, in Surrey (50 miles). Hence it appears evident that the sound was lifted up soon after leaving the Solent, and passed right over the heads of observers near, travelling in the higher air for a considerable distance, probably 40 or 50 miles, and was then deflected down again, and reached observers on the earth’s surface at much greater distances. An examination of the wind-charts for that day makes it tolerably clear that this was due to the manner in which the wind was blowing at the time. Dr. Davison, loc. cit., says—
“Now, on February 1, the wind at places to the west of Spithead was generally light, and from the west or nearly so, though near Lyndhurst there was a fresh breeze from about W.N.W. or N.W. At Portsmouth, again, the wind is described as from the shore. On the other hand, many of my correspondents at great distances from Spithead state that the wind, when sensible, was southerly in direction. Thus the sound-rays were first of all refracted by contrary winds over the heads of observers between 10 and 45 miles, and were afterwards brought down again by favourable upper currents, so that the reports were clearly audible beyond 50 miles and up to 140 miles from Spithead, and were so loud at a distance of 84 miles that labourers in the fields put down their spades and listened.”
The same explanation has been given of the extraordinary differences that are found at various times in the distance at which lighthouse fog-horns are heard by ships at sea. There is in this case, however, another possible explanation, due to what is called interference of sound waves, the explanation of which will be given presently. The late Professor Tyndall, who was an authority on this subject, was of opinion that in some states of the atmosphere there existed what he called “acoustic opacity,” the air being non-uniform in temperature and moisture; and through this very irregular medium, sound waves, when passing, lost a great deal of their intensity by internal reflection, or eclipses, just as light is stopped when passing through a non-homogeneous medium like crushed ice or glass. At each surface a little of the light is wasted by irregular reflection, and so the medium, though composed of fragments of a transparent substance, is more or less opaque in the mass.
On the subject of sound-signals as coast-warnings, some exceedingly interesting information has recently been supplied by Mr. E. Price-Edwards (see Journal of the Society of Arts, vol. 50, p. 315, 1902). The Lighthouse Boards of different countries provide the means for making loud warning sounds at various lighthouses, as a substitute for the light when fog comes on. The distance at which these sounds can be heard, and the distance-traversing power of various kinds of sounds, have been the subject of elaborate investigations.
The instrument which has been found to be the most effective in producing very powerful sound waves is called a siren. It consists of a tube or horn, having at the bottom a fixed disc with slits in it. Outside this disc is another movable one which revolves against the first, and which also has slits in it. When the second disc revolves, the passage way into the horn is opened and closed intermittently and suddenly, as the slits in the discs coincide or not. Air or steam under a pressure of 10 to 40 lbs. on the square inch is blown into the horn, and the rapid interruption of this blast by the revolving slits causes it to be cut up into puffs which, when sufficiently frequent, give rise to a very loud sound. The air under pressure is admitted to a back chamber and awaits an opportunity to escape, and this is given to it when the revolving disc moves into such a position that the slits in the fixed and moving disc come opposite each other. In comparative trials of different sound-producing instruments, nothing has yet been found to surpass this siren as a producer of penetrating sounds.
It has been found very important that the frequency of the note given by the siren should coincide with the fundamental tone of the trumpet or horn. As will be explained in the next lecture, every column of air in a tube has a particular natural time-period of oscillation. Suppose, for instance, that for a certain length of trumpet-tube this is ¹⁄₁₀₀ second. Then the siren with that trumpet will be most effective if the interruptions of the air-blast are 100 per second.
Lord Rayleigh has also shown that the shape of the mouth of the trumpet is important, and that this should not be circular as usual, but elliptical or oval, the shortest diameter of the ellipse being one quarter of the longest one. Also that the mouth should occupy such a position that the longer axis is vertical. Moreover, he considers that the short axis of the oval should not exceed half the wave-length of the sound being emitted. With a trumpet-mouth of such a shape, the sound is prevented to some extent from being projected up and down, but diffused better laterally—a result which is desired in coast sound-signals.
The information accumulated as regards the distances at which sounds can be heard is very briefly as follows:—
First as regards wind. The direction of the wind has a most remarkable influence on the distance at which a given loud sound can be heard. In one instance, the noise of a siren was heard 20 miles in calm weather; whereas, with an opposing wind, it was not heard more than 1¹⁄₄ mile away.
It has been found that for calm weather a low-pitched note is better in carrying power than a high note, but in rough weather the opposite is the case.