Sound measurements
Sound intensity is the amount of sound power per a given area. Sound intensity is measured in decibels (dB), and it is directly related to how our ears perceive loudness. Decibels are logarithmic units that are used to express the ratio of two values. For example, in electronics, decibels are often used to express the ratio of output power to input power of an amplifier. For sound intensity, the ratio is between a measured sound intensity and a reference threshold sound intensity. In order to more accurately reflect human hearing, a condition known as an A-contour filter can be applied to the measurements and the resulting values are expressed as dB(A).The logarithmic values of sound intensity translate into a general rule of thumb that states that a sound will be twice as loud for every 10 dB increase and half as loud for every 10 dB decrease. Unfortunately, as explained by this excellent Hewlett-Packard document (click on the "hp PCs and acoustic noise" link), manufacturers of low-noise components and PCs do not always use standard methods to measure the sound levels of their products. This means that comparing components or systems directly is not always possible and that if a manufacturer does not state how it obtained its measurement, that information is not very useful. Therefore, in such cases, you should use decibel ratings to give you a basic idea of how loud a component is and not as a value to be used in comparison with another product. For further reference, see this Common Noise Levels and Typical Reactions chart (scroll down the page to see the chart).