HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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INTENSITY OF MUSICAL SOUNDS.                ob
distances, the two sounds would be separately heard, the latter being termed the echo of the former. Therefore, when a vibrating body emits a sound in any room or hall, the waves which proceed from it in all directions, strike the walls, floor, ceiling, and also the reflecting surfaces of the various objects in the place, and are re­flected again and again from them. Thus the direct and reflected sounds coalesce, and interfere with one another, in the most com­plicated manner, and the simple law of inverse squares is no longer applicable. This is still the case, even in the open air, away from all surrounding objects, for the ground will here present a reflect­ing surface, and other invisible reflectors are found, as Professor Tyndall has shown, in the surfaces which separate bodies of air of different hygrometric states and of different temperatures.
This may be put in another way. It is a condition of the truth of the law of inverse squares, as above shown, that the sound shall be able to spread outwards in all directions ; if this is not the case, the law no longer holds good. Now, in a building, this is not the case; the sound is prevented from spreading by the roof, floor, and walls. If the sound can be entirely prevented from spreading, its intensity will not diminish at all. This is the principle of the speaking tube. In this instrument, the vibrations of the air par­ticles are transmitted undiminished, except by friction against the side of the tube, and by that part of the motion which is given up to the substance of the tube itself; thus sound can be transmitted to great distances in such tubes. Eegnault, experimenting with the sewer conduits of Paris, found that the report of a pistol was audible through them, for a distance of 6 miles.
The bad acoustical properties of a building are generally due to echoes. A sound from the lips of a speaker, in a building, reaches the ear of the listener directly, and also after one or more reflections from the ceiling, walls, floor, and so on. If the building be of con-