ABSTRACT
Sound waves in air can be remarkably difficult to stop. Air is a fluid of considerable substance. It can support 500-ton 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 (de-pressurising). 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 ton by de-pressurising 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.
