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The "singing" of telegraph wires and the "whistling " of the wind through tall grass and trees are very familiar sounds. The "thrumming" of a tightly stretched wire in a stream of water is evidently due to a similar cause. Aeolian tones are produced when wind rushes past a stretched wire, their excitation depending on the formation of unstable vortex sheets. When wind of sufficient velocity passes a wire, a double series of vortices are set up immediately behind it, one set re volving in the opposite direction to the other and the flow of air past the wire wavers from side to side. In this manner vibrations are set up in the surrounding air which, under the right condi tions, will be audible. Strouhal investigated the effect by revolv ing a vertical stretched wire about a parallel axis. He found that the frequency was expressed by N= o.185 v/d where v is the velocity and d the diameter of the wire. When the speed is such that N corresponds with one of the natural frequencies of the wire the sound is greatly increased. Rayleigh showed that the vibrations of the wire are transverse to the direction of the wind. This is readily explained on the vortex theory; the oscilla tions of air flow behind the wire reacting on the wire itself. The Aeolian harp consists of a number of wires, all of the same low pitch but of different thicknesses stretched on a sounding-board and exposed to the wind. The varying thicknesses of the strings
results in a series of different notes.
In addition to the various sources of sound to which we have referred there are innumerable others. Almost every material object in motion is a source of sound vibration. In all parts of the world there has been a steady in crease in traffic noise during recent years, the problem of the reduction of such noise becoming increasingly acute. Relative motion and intermittent contact between solid bodies, such as a vehicle on a road, results in a shock excitation of the numerous resonant vibrations which are possible in a complex structure. A heavy vehicle fitted with solid tyres and having many loose parts is an irritating example of this when it runs over a some what irregular road surface. The complex noise issuing from such a vehicle may be analysed into the simpler forms which we have considered above. The prevention of such noises involves many considerations, but obvious improvements may be ob tained by (I) use of pneumatic tyres, (2) tightening of all loose "rattling" contacts, (3) balancing of all parts of machinery in i rapid motion, (4) use of oil to reduce frictional noises, (5) use of I pressure reducers or "silencers " for exhaust gases, etc.
The noise of breaking waves or rushing water may be ascribed to the resonant vibrations of air cavities enclosed momentarily by the water.