ACOUSTICS. 



those superficies or pulses of air are dif- 

 fused as far as D every way, it is plain the 

 force of the voice will there be diffused 

 through the whole superficies of a sphere 

 whose radius is BD; but in the trumpet 

 it will be so confined, that at its exit it 

 will be diffused through so much of that 

 spherical surface of air as corresponds to 

 tne orifice of the tube. But since the 

 force is given, its intensity will be always 

 inversely as the number of particles it has 

 to move ; and therefore in the tube it will 

 be to that without, as the superficies of 

 such a sphere to the are a of the large end 

 of the tube nearly. But it is obvious, Dr. 

 M. Young observes, that the confinement 

 of the voice can have little effect in in- 

 creasing the strength of the sound, as this 

 strength depends on the velocity with 

 which the particles move. Were this rea- 

 soning conclusive, the voice should issue 

 through the smallest possible orifice ; cy- 

 lindrical tubes would be preferable to any 

 that increased in diameter; and the less 

 the diameter, the greater would be the 

 effect of the instrument ; because the plate 

 or mass of air to be moved would, in that 

 case, be less, and consequently the effect 

 of the voice the greater; all which is con- 

 tradicted by experience. The cause of 

 the increase of sound in these tubes must 

 therefore be derived from some other 

 principles : and among these we shall pro- 

 bably find, that what the ingenious Kircher 

 has suggested is the most deserving of our 

 attention. He tells us, that " the augmen- 

 tation of the sound depends on its reflec- 

 tion from the tremulous sides of the tube ; 

 which reflections, conspiring in propa- 

 gating the pulses in the same direction, 

 must increase its intensity." Newton also 

 seems to have considered this as the prin- 

 cipal cause, in the scholium of Prop. 50, 

 B. II. Princip. when he says, " We hence 

 see why sounds are so much increased in 

 stentorophonic tubes, for every reciprocal 

 motion is, in each return, increased by the 

 generating cause." Farther, when we 

 speak in the open air, the effect on the 

 tympanum of a distant auditor is produced 

 merely by a single pulse. But when we 

 use a tube, all the pulses propagated from 

 ihe mouth, except those in the direction 

 of the axis, strike against the sides of the 

 tube, and every point of impulse becoming 

 a new centre, from whence the pulses are 

 propagated in all directions, a pulse will 

 arrive at the ear from each of those points. 

 Thus, by the use of a tube, a greater num- 

 ber of pulses are propagated to the ear, 

 and consequently the sound increased. 

 The confinement too of the voice may have 



a little effect, though not such as is ascrib- 

 ed to it by some ; for the condensed pul- 

 ses produced by the naked voice freely 

 expand every way; but in tubes, the late- 

 ral expansion being diminished, the direct 

 expansion will be increased, and conse- 

 quently the velocity of the particles, and 

 the intensity of the sound. The substance 

 also of the tube has its effect; for it is 

 found, by experiment, that the more elas- 

 tic the substance of the tube, and conse- 

 quently the more susceptible it is of these 

 tremulous motions, the stronger is the 

 sound. If the tube be laid on any non- 

 elastic substance, it deadens the sound, 

 because it prevents the vibratory motion 

 of the parts. The sound is increased in 

 speaking-trumpets, if the tube be suspend- 

 ed in the air ; because the agitations are 

 then carried on without interruption. 

 These tubes should increase in diameter 

 from the mouth-piece, because the parts 

 vibrating in directions perpendicular to 

 the surface will conspire in impelling for- 

 ward the particles of air, and consequent- 

 ly, by increasing their velocity, will in- 

 crease the intensity of the sound : and the 

 surface also increasing, the number of 

 points of impulse and of new propaga- 

 tion will increase proportionably. The se- 

 veral causes, therefore, ofthe increase of 

 sound in these tubes, Dr. Young concludes 

 to be, 1. The diminution of the lateral, 

 and consequently the increase of the di- 

 rect expansion and velocity ofthe included 

 air. 2. The increase of the number of 

 pulses by increasing the points of new 

 propagation. 3. The reflections of the 

 pulses from the tremulous sides of the 

 tube, which impel the particles of air for- 

 ward, and thus increase their velocity. 



An umbrella, held in a proper position 

 over the head, may serve to collect the 

 force of a distant sound by reflection, in 

 the manner of a hearing-trumpet ; but its 

 substance is too slight to reflect any sound 

 perfectly, unless the sound fall on it in a 

 very oblique direction. The exhibition 

 ofthe Invisible Girl is said to depend on 

 the reflection of sound ; but the deception 

 is really performedby convey ing the sound 

 through pipes artfully concealed, and 

 opening opposite to the mouth of the 

 trumpet, from which it seems to proceed. 



When a portion of a pulse of a sound 

 is separated by any means from the rest 

 ofthe spherical or hemispherical surface 

 to which it belongs, and proceeds through 

 a wide space, without being supported on 

 either side, there is a certain degree of 

 divergence, by means of which it some- 

 times becomes audible in every part ofthe 



