SPEAKING TRUMPET. The efficiency of this instrument is due to the repeated reflection of the sound from Bide to side in passing through it, and its ultimate reflection from the mouth of the trumpet, in such a way as either to collect the rays of sound into a focus at a distance, or to project them forward in parallel lines, instead of allow ing them to diverge in all directions. Fig. 1, illustrates this theory, of which there are many modifications, some of them founded on the supposition of a very close analogy between the motion of sound and that of light. These have given rise to many suggestions respecting the best form and degree of curvature of the sides of the trumpet. Some writers recommend a simple cone, the mouth-piece being at the apex : these explain the motion of the rays of sound on the principle shown in fig. 2, in which, as in fig. 1, the dotted lines represent the rays.
In Dr. Young's Lectures on Natural Philosophy,' the following remarks occur : " If an ellipsis be prolonged without limit, it will become a parabola ; hence a parabola is tho proper form of the section of a tube calculated for collecting a sound which proceeds from a great distance into a single point, or for carrying a sound nearly in parallel directions to a very distant place. It appears, therefore, that a para bolic conoid is the best form for a hearing trumpet, and for a speaking trumpet ; but for both purposes the parabola ought to be much elongated, and to consist of a portion of the conoid remote from the vertex ; for it is requisite in order to avoid confusion, that the sound should enter the ear in directions confined within certain limits : the voice proceeds also from the mouth without any very considerable divergence, so that the parte of the curve behind the focus would in both cases be wholly useless. A trumpet of such a shape does not
very materially differ from a part of a cone, and conical instruments Are found to answer sufficiently well for practice; it appears, however, unnecessary to suppose, as M. Lambert has done, that they differ tially in principle from parabolic trumpets. It is not yet perfectly decided whether or no a speaking trumpet has any immediate effect in strengthening the voice independently of the reflection of sound." [EAR TRUMPET.] Speaking Tubes are used in large buildings for conveying messages from one part to another, and sometimes on shipboard from the cap tain's cabin to the topmast. These tubes are cylindrical, and the rays of sound proceeding from the mouth at one end of the tube instead of diverging in the air are confined within the tube, and successively reflected from its sides, so that a much larger number of rays meet the ear at the farther end than if they had travelled without being re flected. The shafts of mines, wells, and chimneys produce a similar effect.