The way we describe noise depends on its level and characteristics. If noise is constant then it is easy to say that the noise is at a certain level. For example a fan may produce a steady level of 55dB(A) at a monitoring location. However many noise sources are not constant, but still need to be described in a useful way to allow comparison with noise criteria. Noise parameters have been developed to describe fluctuating noise levels. Common parameters include: Lmax = the maximum noise level, Lmin = the minimum noise level, Leq = the level with the same energy as the actual fluctuating noise level, L1 = the level exceeded for 1% of the time, L10 = the level exceeded for 10% of the time, and L90 = the level exceeded for 90% of the time. Experience has shown that certain parameters are more useful for describing particular noise sources. For example, the L90 is often used to describe the background noise level, the L10 may approximate the average maximum noise level for certain noise sources and is used to describe road traffic noise and music levels, the Leq is used to describe a wide range of noise sources as it captures the energy associated with the noise source. It is common to use a combination of parameters to characterise a noise source. For example, an industry with some peak events may be described using the Lmax levels to measure the peak events, and the Leq to capture the total noise level. The character of the noise may include tonal or impulsive components, and these can make the noise more noticeable.