“Certainly: supposing, of course, that your ball is perfectly elastic; and in that case, you no doubt remember the direction it will follow.”
“It will always make the angle of reflection equal to the angle of incidence,”[[58]] said Tom.
“Undoubtedly; and so it is with sound, since air, as you know, is perfectly elastic. If, therefore, the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line; if obliquely, the sound returns obliquely in the opposite direction, the angle of reflection being equal to that of incidence. You will, therefore, readily perceive,” continued Mr. Seymour, addressing his conversation more particularly to Miss Villers, “that a person situated at an appropriate angle may hear an echo, as it is returned from the reflecting surface, without hearing the original sound which produced it. M. Genefay has described, as existing near Rouen, a curious oblique echo which is not heard by the person who emits the sound. A person who sings hears only his own voice, while those who listen hear only the echo.”
“As a smooth and concave surface is capable of producing an echo, how does it happen that we so rarely meet with one in a room?” asked Louisa.
“Echoes, my dear, are, in fact, produced in every room, by the reverberation of sound from its walls; but on account of the velocity with which it travels, they are imperceptible in small chambers, because the sound occupies no sensible period of time in moving from the mouth to the walls, and in returning back to the ear again, consequently the original sound and its echo become so blended and incorporated, as to appear but one sound. As the dimensions of the apartment increase, the defect will increase with it; and, in buildings for music or public speaking, it is often highly inconvenient, and difficult of prevention. Breaking the surface, or rendering it uneven by mouldings and ornaments, appears to be the most effectual method of curing the evil.”
“I perceive then, papa, that in order to produce a perfect echo, the person who speaks must be at a considerable distance from the obstacle that reflects the sound,” said Louisa.
“It cannot be otherwise,” replied her father; “and if you will only consider the rate at which sound travels, you will readily understand the necessity of such an arrangement. In order to produce a distinct echo of one syllable, or of a single sound, the reflecting obstacle must be at least 70 feet from the sound, so that it may have to pass through a distance of 70 feet to get to the reflector, and 70 more to return to the ear, making a total passage of 140 feet, which it will accomplish in rather less than one-eighth of a second; a period of time so small, that, if it were diminished, it is evident the echo must be assimilated with the sound itself.”
“But the echo in the valley,” observed Mrs. Seymour, “will repeat four or five syllables.”
“Undoubtedly. If we make the experiment at a sufficient distance from the sandstone rocks which act as the reflector.”
“It would appear, then, that the farther the reflecting object is off, the greater number of syllables will the echo repeat; and I should think that this fact might enable us to compute the distance of the reflector,” said Mrs. Seymour.