56 HAND-BOOK OF ACOUSTICS.
siderable dimensions, these echoes may reach the listener's ear at an appreciable interval of time after the direct sound, or after one another, and will then so combine with the succeeding direct sound from the speaker as to make his words quite indistinguishable. The roof is often the chief culprit in this matter, especially when lofty, and constructed of wood, this latter affording an excellent reflecting surface. An obvious remedy is to cover such a surface with some badly-reflecting substance, such as a textile fabric. A sound-board over the speaker's head, will also prevent the sound from passing directly to the roof. The bodies and clothes of the persons forming an audience, are also valuable in preventing echoes. Professor Tyndall, to whose work on sound the student is referred for further information on this subject, says that, having to deliver a lecture in a certain hall, he tried its acoustical properties beforehand, and was startled to find that when he spoke from the platform, a friend he had with him, seated in the body of the empty hall, could not distinguish a word, in consequence of the echoes. Subsequently, when the hall was filled with people, the Professor had no difficulty in making himself distinctly heard in every part. Again, everyone must have noticed the difference between speaking in an empty and uncarpeted room, in which the echoes reinforce the direct sound, and speaking in the same room carpeted, and furnished, the echoes in this case being deadened by the carpets, curtains, &c.
The Intensity of the sound produced by a vibratory body, depends upon the amplitude of its vibrations.
The Intensity of a sound varies inversely as the square of the distance from its origin, only when the sound waves can radiate freely in all directions without interruption.
Sound is reflected from elastic surfaces in the same way as light, thereby producing echoes.
Sound is well reflected from such surfaces as wood, iron, stone, &c, while cloths, carpets, curtains, and textile fabrics in general, scarcely reflect at all.