Threshold shift

The threshold of hearing is defined as a sound pressure level of 0 deciBel on the ear of a healthy, young human being. The tiniest increase in sound intensity exceeding this threshold of hearing can be observed as sound. The sound pressure, corresponding to this threshold, is about 20 microPascal, a miniscule fraction of the static, atmospheric pressure resting on us (1 Pascal is a pressure of 1 Newton per square meter).

The noise levels around us, the levels we are used to, are related to this threshold. The average conversation between two people standing next to each other corresponds to sound pressures that are about a thousand times the threshold pressure, so approximately 20 milliPascal. However, in modern day life, this correspondence is shifting. There is evidence that people living or working in noisy environments talk louder to each other. And there is even more evidence that children, who have to been talked to in a loud voice for a long period of time, will in the end become loud talkers.
Strong evidence, emerging from classical music recordings, shows that the Concertgebouworchestra Amsterdam plays significantly louder today than it did 50 years ago. This is not because the background noise in the Concertgebouw has increased.

Apparently there is a shift in what we define as the threshold of hearing. The human race in noisy cities has become deafer, and adapts to that situation by producing more sound. A redefinition of the threshold of hearing is due.