Sound Waves

The sound, as physical appearance, can propagate only in material and elastic environment or medium, like air, water, wood or any other firm body. When there is a change in a stationary state of the particles that medium consists of, then that's called a acoustic oscillations. This change in state is manifested as change in the pressure or the density of the medium. The source of sound causes that change and it spread it with proper speed in the space as a sound waves.

The sound waves can be longitudinal or transverse waves. For example, if small ball changes its volume, then it will suppress the molecules of the air around it. That way, the density of the air molecules is increased and they start to suppress the adjacent layers of the air. This process generates changes in pressure in the space, and that's called sound field. The pressure in one point of the space is changing depending on time and space coordinate of that point. The air molecules are not displaced, but their movement is transmitted around the stationary state. As the sound wave is spreading, its intensity decreases, because the energy also decreases by unit volume of the space in which sound wave propagates.

The range of sound that man can hear is from 20 Hz to 20 kHz. The waves at frequency lower than 20 Hz are known as infra-sound, while the waves at frequencies higher than 20 kHz belong to the so called ultra-sound. The propagation of all these sound waves is under the same laws.

Depending on the frequency spectrum of the sound wave, we can distinguish three types of sound:

>> Simple sound - single frequency wave;
>> Complex sound - multiple frequencies wave;
>> Non-periodic sound - or noise;

The pressure intensity of each type of sound depending on frequency looks like the plots shown on Picture 1.

Picture 1: Pressure of sound in frequency domain: A. Simple sound; B. Complex Sound; C. Noise;


The pressure intensity of each type of sound depending on frequency, looks like the plots shown on Picture 1. The simple sound consists of one single frequency, so the plot has just one amplitude shown on current wave frequency, plot A. The complex sound shown on plot B, consists of three different frequencies, the major part is on f1, the next two are for one octave higher, 2*f1 and 3*f1, respectively. Finally, on plot C is shown the pressure of the noise sound wave depending on frequency. As it can be seen, it has some frequency range where its pressure amplitudes varies with different values, randomly.