Effects of sound pollution in London - as measured by the shrinking map of Cockneys

I recently spotted this map that I  thought tells a tale. It was originally from the Times Atlas of London and was reproduced in a  Daily Mail Article  

There was a saying that a "true Cockney" was one born within the sound of Bow Bells, that is within the sound of the bells of St Mary le Bow church on Cheapside in the City of London.

The outer margin purports to be the area they heard in 1851, whilst the inner is that of today. The difference accounted for by noise pollution, much of it traffic noise