If you have ever tried to sleep under a flight-path near an airport, you will know how serious the problem of aircraft noise can be. Aircraft noise is amongst the loudest sounds produced by human activities. The noise is over a broad range of frequencies, extending well beyond the range of hearing. The problem of aviation noise has become more severe as aircraft engines have become more powerful [TM180 or search for “thatsmaths” at irishtimes.com].
Engine inlet of a CFM56-3 turbofan engine on a Boeing 737-400 [image Wikimedia Commons].
is the e-folding time. For every period of length 
. Exponential growth was assumed by Thomas Malthus (1798), and he predicted that the population would exhaust the food supply within a half-century. ![African Leopard (Panthera pardus pardus). [Image from Wikimedia Commons]](https://thatsmaths.files.wordpress.com/2013/04/leopard-mid-res.jpg)
