Encyclopædia Britannica, Ninth Edition/Trumpet, Speaking and Hearing

TRUMPET, Speaking and Hearing.The speaking trumpet, though some instrument of the kind appears to have been in earlier use in more than one part of the world, is connected in its modern form with the name of Athanasius Kircher and that of Sir Samuel Morland, who in 1670 proposed to the Royal Society of London the question of the best form for a speaking trumpet. Lambert, in the Berlin Memoirs for 1763, seems to have been the first to give a theory of the action of this instrument, based on an altogether imaginary analogy with the behaviour of light. In this theory, which is still commonly put forward, it is assumed that sound, like light, can be propagated in rays. This, however, is possible only when the aperture through which the wave-disturbance passes into free air is large compared with the wave-length. If the fusiform mouth of the speaking trumpet were half a mile or so in radius, Lambert's theory might give an approximation to the truth. But with trumpets whose aperture is only a foot in diameter at most the problem is one of diffraction; and it has not yet been seriously studied from this point of view.

In the case of the hearing trumpet, the disturbance is propagated along the converging tube much in the same way as the tide-wave is propagated up the estuary of a tidal river.

Until the theory has been rigorously worked out the only safe course to adopt in manufacturing either class of instruments is to be guided by the results of varied trials.

The theoretical foundations of the subject will be found in Lord Rayleigh's Sound and in Sir G. Airy's Tides and Waves, respectively. In speaking and hearing trumpets alike all reverberation of the instrument should be avoided by making it thick and of the least elastic materials, and by covering it externally with cloth.