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THE NATURE OF SOUND 293
later and later times, this same situation would be found at regions farther and farther to the left.
If we were to measure the speed at which this pressure wave travels we would find it to be about 1100 feet per second; and if an ear were placed at the far end of the pipe it would eventually notice the pressure wave as a sort of "tut."
Now suppose that the plunger is kept oscillating at a regular rate, say 100 complete oscillations per second. Then a series of pressure waves just like the above, would follow each other down the pipe and eventually the ear would receive them at the same rate as they are produced, viz., 100 per second. The ear would judge this action as a tone, in this case of rather low pitch. If the rate of oscillations be increased to several hundred per second, then the ear would give the sensation of a tone of higher pitch, and we might carry this idea to vibrations of several thousand per second.
We find, by experiment, that the speed of travel of these pressure waves is the same no matter what the frequency of oscillation. At first thought we might imagine that the waves of greater frequency would travel faster, but this is not true.
The action of these pressure waves just described is "sound." In most actual cases, of course, the propagation is not inside of a pipe but out in more or less open air. There would be no essential differences in the action, although there would be a very slight difference in speed. We might think that in free air the variations in pressure could not occur on account of lack of confinement, and it certainly is true that great variations would be difficult to produce in free air, but we find that the amount of variation, above and below ordinary air pressure, which is needed for the ear to recognize, is exceedingly small. Even changes as small as a billionth of the ordinary pressure are detected by the ear, if coming at a rate within a range of about 20 per second up to about 25,000 per second.
Returning to the conditions inside the pipe, while the plunger is vibrating we see that in a long pipe there might be many waves of pressure present in the pipe at the same time, and we are accustomed to call the distance from one wave to the corresponding part of the next wave, the "wave-length." For low frequency the wave-length is long and for high frequency it is short. The two are always so related that the product of