ABSTRACT
Sound waves in air can be remarkably difficult to stop. Air is a fluid of considerable substance.
It can support 500 tonne aeroplanes and blow down buildings. In fact, it is much heavier
than most people think. On Earth, at sea level, air weighs about 1.2 kg per cubic metre, which is
a very good reason for pumping any unnecessary excess of it out of aeroplanes
(depressurising). This process is partially carried out to reduce the pressure differential stresses
on the aircraft fuselage when flying at altitude in air of lower pressure (and density), but the
aircraft also use less fuel by not having to carry excessive quantities of air over their entire
journeys. A jumbo jet can actually reduce its load by around half a tonne by depressurising to
an equivalent pressure altitude of around 8000 feet (2500 m). Air is also a rather springy
substance, which makes it useful in air-pistols, shock absorbers and car tyres.
