TRUMPET

In Acoustics, an instrument used for the purpose either of conveying articulate sounds to a great distance,orfor applying to the ear, in order to collect the sonorous rays, and render sounds more distinct. In the fonnei case, the instrument is a speaking-trumpet; in the latter, a hearing-trumpet. Speaking-trumpet. — The object of the speaking-trumpet is to increase the intensity of speech, and transmit it to considerable distances in a particular direction. In order to obtain this effect, it is nece ary that the aerial undulations which would disperse themselves in all directions, be confined by the sides of the trumpet, and reflected so as to take, as nearly as po ible, a direction parallel to the axis. This is accomplished by giving the trumpet such a form that its diameter becomes greater towards the extremity furthest from the mouth. A cylindrical or prismatic tube, of equal diameter throughout, a i-ts powerfully In conveying sound from one extremity to the other, but contributes in no degree to give a direction to sound, or transmit it through In The theoretical explanation of the effects of the speaking trumpet is attended with considerable difficulty. A uming that the reflexion of sound takes place according to tho laws of catoptrics, Lambert (Berlin Memoirs for 1703) shows the best form of the instrument is a truncated cone, for the rays succe ively reflected from the interior surface of this figure make smaller angles with the axis after each succe ive reflexion. Ca egrain (Journal des Savant, torn, iii.) recommends the surface formed by the revolution of a hyperbola about its asymptote; and Kircher had previously proposed a truncated parabolic conoid, the mouthpiece occupying the focus. Some authors suppose that the trumpet should be formed of a very elastic material, in order to strengthen the sound by its vibrations. Others are of opinion that it should be without elasticity, to avoid the confusion that minht be caused by the vibrations of the tube. fTinnenfraU (Jtmnui de Physique, torn. 1v,.>. who made o number of experiments on the power of the trumpet, by fixtng a small watch in the mouth-piece, and observing the distance at which the beats ceased to be audible, found that the effects were precisely the same with a trumpet of tinned iron, whether used in its naked form, or tightly bound round with linen to prevent vibration, or when lined with woollen cloth, whereby reflexion was entirely prevented. It would appear, therefore, that neither the shape of the instrument, nor the material of which it is formed, is of much consequence. Leslie (Kiperimental Inquiry into the JVaturc and Propagation of Heat) supposes the effect of the trumpet to be owing to the more condensed and vigorous impulsion given to the air from its lateral flow being checked. "The tube," he observes, " by its length and narrowne , detains the efflux of air, and has the same effect as if it diminished the volubility of that fluid, or increased its density. The organs of articulation strike with concentrated force; and the pulses, so vigorously thus excited, are, from the reflected form of the aperture, finally enabled to escape, and to spread themselves along the atmosphere," p. 225. The invention of the speaking-trumpet is commonly as cribed to Sir Samuel More land, who, about the year 1670, proposed the best form of the speaking-trumpet as a question to the Royal Society, and exhibited some instruments constructed according to his own views. These were, in general, large conical tubes, spreading suddenly, at the extremity,toa greater width. They were tried in St. James's Park; and it is stated that their effects were such that the king (Charles II.), speaking in his ordinary colloquial pitch of voice, through a trumpet of only five feet and a half long, was distinctly heard at the distance of 1000 yards. Another person was perfectly understood at the distance ot four miles and a half. Kircher claims the credit of having made such experiments previously; but his remarks apply rather to hearing than to speaking-trumpets. Hearing-trumpet. — The hearing-trumpet, or ear-trumpet, may be considered as a reversed speaking-trumpet, with which it generally corresponds in form, though, for the sake of portability, it is often made curved or spiral. Lambert recommends the parabolic figure as the most advantageous; but, in order to give the greatest effect, the apex of the paraboloid must be cut off, and the mouth of a small tube Inserted In the focus, to convey the sound concentrated at that point to the auditory organ. Various other forms are adopted in practice; and, of late years, flexible India rubber tubes have been brought into use, having a conical mouth-piece of ivory or silver at one extremity, and n small tube of like material, to be applied to the car. A trumpet of this kind may be used advantageously, not only for remedying the defects of the organ or hearing, but foi a isting the observer to collect feeble and distant1275 sounds [s. 1289]