Vibration - Basic Concept

Sounds are variations in pressure that propagate in an "elastic" environment, usually air, which can generally be perceived by human ears. A well-known example of variation in pressure is the one that takes place in the atmosphere, but these air changes are too slow to be heard. However, if these changes occurred at least 20 times per second, we would perceive them as sounds. In fact, man can hear any variation in pressure that takes place with a frequency ranging from 20 to 20,000 times per second. These variations per second are called "frequency" and are measured in Hertz (Hz). Variations below the threshold of 20 per second generate infra sound, and variations above 20,000 per second generate ultrasound. A drum and a whistle are examples of low and high frequency. Sounds are also characterized by their "amplitude": the buzz of a mosquito and the shot of a cannon are examples of small and large amplitude. Noises are unpleasant and undesired sounds. The degree of nuisance is subjective, and can vary in the same person depending on the circumstances. Excessive exposure to noise can cause a series of specific damages to the hearing, as well as more general damage to the entire body. Sound waves propagate in the medium in which they find themselves, (at 340 m/sec in air, at 1480 m/sec in water, at 4880 m/sec in steel) , through a series of compressions and rarefactions. Sound transmission cannot take place in a vacuum.