Industrial Valves

6Noise

6.1 Introduction to Sound

In physics, sound is the result of vibrations and is accompanied by acoustic waves in liquids, gases, and solids. It is true that when an object vibrates, it imparts energy to the particles around it, causing them to move and this movement is known as sound waves. The waves caused by vibrations of molecules follow sine functions characterized by their amplitude and wavelength or frequency. The two most significant characteristics of a sound are its frequency and intensity (amplitude or volume or loudness). A sound can be described by its low or high frequency. The frequency is determined by the speed at which particles vibrate in the medium, which is defined as the number of waves emitted in a given amount of time. Frequency is the number of cycles per second measured in Hertz (Hz) units. For example, a frequency of 1000 Hz means that the waveform is 1000 cycles/s. The human hearing range is between 20 and 20,000 Hz. Each cycle represents one sequence of waves where the intensity changes from zero to the highest value, then to the lowest value, and then back to zero, as shown in Figure 6.1. The length of a wave or wavelength can be measured from one point on the waveform to the exact point at which the next cycle begins (see Figure 6.1). Frequency and wavelength are inversely proportional to each other; therefore, a wave with the greatest frequency will have the shortest wavelength, and twice the frequency will have half the wavelength. Figure ...