Reflection of Sound-Echoes. 325 reflecting surfaces on a large scale are often found among rocks and hills; and hence probably arose the beautiful fiction of the ancient poets, that Echo was a nymph who dwelt concealed among the rocks; a fiction which has its counterpart in the wonder and delight with which a child listens to his own shrill call responded to within some wood or from some bold precipice by an unseen imitator. It does not require a hard surface for the reflection of sound. It is easily reflected from the smooth surface of water, whether as a liquid or in vapour. Clouds reflect sound and to this, probably, is partly due the reverberation of thunder. Even when the rays of sound pass into air of a different density they undergo reflection. 496. The quickness with which an echo is returned to the place where the sound originates, depends of course upon the distance of the reflecting surface; and, as sound travels 1120 feet in a second, a rock at half that distance, or 560 feet, returns a sound exactly in one second. If five syllables can be pronounced in a second, the whole five may, in such a case, be echoed distinctly, but the end of a longer phrase would mix with the commencement of the echo. If the echoing surface be only 4th of 560 feet, that is 112 feet, distant, then only the last syllable will be echoed; if 224 feet, two syllables, and so on. It of course follows that when the ear is unable to distinguish the original sound from its reflection, there will be no echo. The sound is simply prolonged and rendered louder. It is stated on reliable authority that a good ear is able to perceive clearly nine sounds in a second of time; i. e., the sounds to be heard singly, must succeed each other at intervals of th of a second. The least distance at which it is possible for the sound and echo to be perceived distinctly, will be that which the wave of sound can traverse so as to impinge on the reflecting surface and return thence as a reflected wave in th of a second of time. As the average velocity at a medium temperature is 1120 feet in a second, then 1120÷9 will give 125 feet for th of a second. Hence it follows that half of this distance, 62-64 feet, will be the least distance at which the reflecting surface must be placed in order to produce a perfect echo. Where there are many reflecting surfaces placed at proper angles he should be directly in front of the reflecting surface. If placed obliquely to it, the sound will be reflected in the opposite direction, and may be then heard by another. It follows the same law as a ball thrown obliquely against a wall rebounds from the wall. (Art. 484.) 326 Acoustic States of the Atmosphere. to each other the sound may be reflected and the echo repeated many times. At the Villa Simonetta, near Milan, the sound of a pistol is repeated from forty to fifty times, and a building at Pavia was so constructed that in answer to a question, the last syllable was repeated thirty times. 497. Acoustic transparency and opacity of the Atmosphere. These terms have been employed by Professor Tyndall to denote remarkable conditions of the air with respect to the transmissibility of sound. This subject has an important bearing upon the employment of sonorous or fog signals at sea, or along a dangerous line of coast. The recent accident off the coast of Ireland, in which, during a fog, the Iron Duke ran into and sank the Vanguard, has imparted an additional interest to this subject. Professor Tyndall, in the course of numerous experiments on fogsignals made for the Trinity House in 1873, found that the permeability of the atmosphere to sounds underwent frequent and rapid changes for which it was not always easy to account. The instruments used by him were chiefly trumpets blown by powerfully compressed air, as well as guns. On some days the horns were not heard at a distance of two or three miles, while on other days, under an improved condition of the atmosphere, they were heard at six and eight miles from the coast; and on one day, in June, in which the sky was loaded with dark and threatening clouds, they were well heard beyond nine miles. On another occasion the direct or axial blast of the horn was heard at ten and a half miles, and even at the Varne light-ship, which is nearly thirteen miles from the Foreland, where the experiments were made. It was noticed on this occasion that the atmosphere had become decidedly clearer acoustically, but not so optically, for a thick haze obscured the white cliffs. On days of far greater optical purity, the sound had failed to reach one-third of the distance, and the conclusion drawn from repeated observations is, that it is a delusion to make optical purity or clearness of the atmosphere, a measure of acoustic transparency or permeability to sound. This remarkable result, which is opposed to general belief (see Art. 490) is ascribed by this experimentalist to the production of acoustic clouds of invisible vapour impervious to sound. The soundwaves, in fact, are thrown back or reflected from these clouds as the waves of light are from an ordinary cloud. The effect of this reflection is to produce audible echoes of great strength and duration, Reversibility of Sound. 327 the direction in which the sounds returned, being always, that in which the axis of the horn was pointed. This, probably, is the first occasion on which audible echoes have been proved to be reflected from an optically transparent atmosphere. The costacle which reflects the sound-waves is quite invisible, but appears to be as impermeable to them as if a solid wall had intervened. Thus it was found that at mid-day in July neither guns nor trumpets could pierce the transparent air to a depth of three, and hardly to a depth of two, miles. It has been established by experiment that layers of dried air alternating with layers of air saturated with the transparent vapour of a volatile liquid have the property of powerfully intercepting sound. It has been hitherto received as an established fact (Art. 490) that fogs arrest sound, and by producing a number of reflections among the aqueous particles which constitute them, either deaden it or rapidly extinguish it. In very dense fogs this may occur, but in a hazy state of the atmosphere sufficient to conceal distant objects from vision, sound, according to Tyndall, is capable of traversing the medium for greater distances than in clear weather. The usual statement that fogs always deaden sound, is thus proved to require some qualification.* It would also appear from these experiments that heavy rain does not obstruct the passage of sound; and from some observations made in the Alps, Professor Tyndall states that a fall of snow does not interfere with its transmission. 498. Reversibility of Sound.—Acoustic Reversibility.—The fact that sound is thus reflected or turned back in a clear or transparent atmosphere, may explain why at a given point the sound produced by cannon may be heard at some places but not at others which are equidistant from the spot. Thus a sound might be heard at a distance of fifteen miles in one direction, but at not more than five miles in another. Again, it has been found as a still more remarkable fact that cannon may be fired in two places at regular intervals, but the number of reports heard at one place will be fewer than those heard at the other. In June, 1822, experiments were made with cannon near Paris by a Commission of scientific men appointed for this purpose, with a view of determining the * Mr. Douglas stated at a meeting of the Institution of Civil Engineers, that he had distinctly heard in a fog at the Smalls Rock in the Bristo Channel, guns fired at Milford Haven, twenty-five miles away, and Mr. Beazely had heard the Lundy Island gun at Hartland Point, a distance of twelve miles, during a dense fog. 328 Refraction of Sound. velocity of sound. The two stations were about twelve miles apart. So different was the transmissive power of the atmosphere, that on one occasion, while every shot fired at M. was heard at V., only one shot out of twelve fired at V., was heard at M. There was no wind to account for this, and the movement of translation, such as it was, was against the direction in which the sound was best heard. In the absence of wind, these facts appear to be difficult of explanation. Professors Stokes and Reynolds assign the loss of sound under these circumstances to refraction and not to reflection. They affirm that the rays of sound, like those of light and heat, are subject to refraction when they pass from one medium into another of different density.* The sound, under these circumstances, is lifted from the ground, and may be heard at a high but not at a low level. This may be either the effect of wind or variations in temperature. The wind moves with different velocities on the ground and at an elevation above it, and sounds proceeding against the wind are lifted up off the ground; hence the range of sound is diminished at a low elevation. A difference of temperature affects the velocity, and a difference of velocity will cause the sound to be lifted. Balloon-observations have shown that when the sun is shining with a clear sky, the variation of temperature is one degree for every hundred feet, and with a cloudy sky, half a degree. Every degree of temperature adds nearly one foot per second to the velocity of sound (Art. 491). This difference is sufficient to cause the rays of sound to be refracted upwards and lost to a person placed at a low level. But on a cloudy day, in which the difference of temperature and refracting power of the air were less, they might be heard to a greater distance at the lower level.t 499. The breadth of a river might be roughly ascertained if there were an echoing rock on the farther shore. A perpendicular mountain side, or lofty cliffs, such as in many parts skirt the British coasts, return a loud echo of artillery, or of thunder, to a distance of several miles. If two bold faces of rock or wall be parallel to each other, a * The refraction of sound has been experimentally proved by M. Sondhauss. Biconvex lenses of large size, constructed of collodion tissue, were filled with carbonic acid gas. Sounds transmitted through these transparent gaseous lenses were refracted to a focus on the other side. They were more distinctly heard at the focal points than at other parts equidistant. (Ganot.) Proceedings of the Royal Society, April, 1874. Reverberation of Sound. 329 sound produced between them is re-echoed several times, playing like a shuttlecock from one to the other, but becoming fainter each time until heard no more. This kind of echo or reverberation may be heard on a grand scale during a thunder-storm in the long valleys of the Alps. In some situations, particularly when the sound travels thus above the smooth surface of water, a pistol-shot may be counted forty times. Sound is always reflected from the walls of every room; but in rooms of ordinary size the time occupied in the impulse and reflection is so short that the interval is inappreciable to the ear. The original and reflected sounds are heard simultaneously; but in an unfurnished room, with bare walls, the effect of this reflection is observed to intensify the sound, and produce what is called resonance, a subject which will be considered hereafter. In rooms of large size the blending of the reflected with the original sound, forms a great drawback to their use for music or public speaking. The remarkable resonance of narrow inclosed spaces depends on a continued reverberation.* In wider spaces it may modify the effect of music by converting a simple melody, which is a succession of notes, into a harmonized piece, where companion notes are heard. Resonance injures the distinctness of speech, so as even in some ill-contrived halls of assembly, or theatres, to render the articulation of a speaker unintelligible. It is worthy of remark that a small apartment or confined space with parallel walls has a certain musical note proper to it, heard * In the Speedwell Mine at Castleton, in Derbyshire, the galleries are flooded with water. In one spot there is a cavern in the limestone rock reaching to an enormous depth below and to a great height above. The effect of turning a portion of the water into this cavern is extraordinary. The sound exceeds that of thunder, and causes the slender wooden bridge, upon which the spectator stands, to tremble beneath his feet. Some seconds elapse before the sound of the falling water is heard, and this pause is succeeded by a continued reverberation from the sides of the cavern and the arched vault above. Certain natural caverns opening to the sea permit the phenomena of reflected and reverberated sound in a most remarkable degree. The cave of Nero in the island of Capri, Fingal's Cave in Staffa, and Dolor Hugo on the coast of Cornwall are examples of this kind. When the sea is calm, and a sound is made in the centre of the cavern, which is generally domed, there is a complete reflection from the surface of the water to the dome, and a powerful reverberation from all parts of the dome to the centre. |