What is sound

The amplitude of a sound wave determines loudness, although the two are not directly related, as will be explained later. Typically these pressure amplitudes are very small. For the average human being the audible range is from the threshold of hearing at 20 j,Pa up to 200 pascals (Pa) where the pressure becomes painful. This is a ratio of 1 to 10. Tire intensity of noise is proportional to the pressure squared hence the range of intensity covers a ratio of 1 to 10.  

With such a range it becomes more convenient to express the intensity of pressure amplitude on a logarithmic base. The intensity level is proportional to the square of the pressure, thus the sormd pressure level (Lp) can 

Note tiiat, since tire decibel is based on a logarithmic scale, two noise levels cannot be added arithmetically. Hence, the resultant Lp, from adding sources Lpi, Lp2 etc. is obtained thus  

Sound pressure level dB Condition Relative energy intensity General class 

Thus for all practical purposes doubling the sound intensity increases the sound pressure level by 3 dB. For example  

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