from Science and Music
by Sir James Jeans
Our eardrums are sensitive to an almost inconceivable degree. The tiniest ripple in the air sets them into motion; under favorable conditions a sound-wave of such feeble intensity that the air is displaced only through a ten-thousand-millionth part of an inch will send an audible sound to the brain. The change of pressure produced by such a sound-wave is less than a ten-thousand-millionth part of the whole pressure of the atmosphere, so that the human ear is incomparable more sensitive than any barometer which has ever been constructed. The ordinary barometer will record the lowering of atmospheric pressure which we experience as we walk upstairs in our house, or climb a few feet up the mountainside, but the change of pressure just mentioned is that produced by an ascent of only a 30,000th of an inch.