Fire Aims

In this practical we will study a common and useful natural sound effect, fire. We will analyse the physical and acoustic properties of fire and combine several contributory signals to create a composite effect. Analysis What is fire? Fire is a complex phenomenon. It is an example of a composite sound effect, having many contributory parts and it is an example of a volumetric extent, coming from more than one location. Fire is an oxidisation reaction that has gone out of control. It starts when fuel gets hot and starts to oxidise. This generates heat in an exothermic reaction. The hotter something gets the better it oxidises and the more it oxidises the hotter it gets, ever more rapidly in a runaway process. This positive feedback causes a reaction that is self sustaining and will increase in size and rate so long as fuel and oxygen are supplied. The following things usually happen. Liquefaction and Boiling As they heat, some solids melt and then boil. In wood, resins and oils are forced to the surface under pressure. In other materials, wax or plastics may melt and flow from the initial fuel. Some of these change to a vapour state causing bubbles. Outgassing Recall Boyle’s law, one of the many gas laws from kinetic theory, which says the product of pressure P and volume V is a constant for a fixed temperature T (written PV = kT). So if temperature increases, either the volume increases or pressure builds up. In the first case gas must escape from the fuel and comes out making a hissing sound. Where the escape path is impeded by trapped liquids we may hear periodic buildup and relaxations of pressure which sound strongly pitched