Logarithmic scale measuring the intensity of sound (the sound pressure level); a 10 decibel (dB) increase represents in a doubling of sound level. ince the decibel scale is logarithmic, there is not a linear relationship between levels. For example, on a linear scale, 4 is twice is big as 2 and 8 is twice as big as 4 and 4 times as big as 2. When measuring decibels, 20 is twice the sound level of 10 decibels but 30 is twice as loud as 20. That makes 40 decibels twice the sound level of 30 decibels and four times the sound level of 20 decibels (on a linear scale, 40 is two times 20). In an audio system, to produce three decibels more sound output, the amplification power must be doubled. ne decibel is the smallest perceptible change in sound level that the human ear can detect. Some examples of sound pressure levels are: whisper – 20 dB, normal speech – 70 dB, passing subway train – 100 dB, large jet plane – 120 dB. The threshold of pain is around 120 dB. ecibels are most often used to describe sound pressure level (commonly referred to as volume). However, the term is also used in various other measurements such as signal-to-noise ratio, gain and dynamic headroom. In these instances, decibels refer the measurement of signal increase or signal strength instead of sound pressure level, but the scale and basic idea behind it remains the same.