ABSTRACT
Sound is produced when an object (the source) vibrates and causes the air around it to move. Consider the sphere shown in Figure 1.1 . It is a pulsating sphere which could be imagined as something like a squash ball, and it is pulsating regularly so that its size oscillates between being slightly larger than normal and then slightly smaller than normal. As it pulsates it will alternately compress and then rarefy the surrounding air, resulting in
A Vibrating Source 1 Characteristics of a Sound Wave 2 How Sound Travels in Air 4 Simple and Complex Sounds 5 Frequency Spectra of Repetitive Sounds 6 Frequency Spectra of Non-repetitive Sounds 8 Phase 8 Sound in Electrical Form 12 Displaying the Characteristics of a Sound Wave 15 The Decibel 16 Sound Power and Sound Pressure 16 Free and Reverberant Fields 19 Standing Waves 23
a series of compressions and rarefactions traveling away from the sphere, rather like a three-dimensional version of the ripples which travel away from a stone dropped into a pond. These are known as longitudinal waves since the air particles move in the same dimension as the direction of wave travel. The alternative to longitudinal wave motion is transverse wave motion (see Figure 1.2 ), such as is found in vibrating strings, where the motion of the string is at right angles to the direction of apparent wave travel.
