If you then doubled the number of sources from 10 to 20 the measured level would only increase by a further 3 dB to 103 dB. It follows therefore that if one machine = 90 dB, then 2 = 93 dB and 10 machines = 100 dB. Similarly twice as much power or a factor of 2 gives a log of 0.3 bels or 3 dB. In the first case we had 10 times as much power and the log of 10 is 1 bel or 10 dB. two machines, then the measured increase would only be 3 dB. ![]() Sound level, sometimes called the decibel level or the dB level increases by 10 dB, which is quite logical. How does this work out in the real world? If we put 10 identical noise sources in a room, then there is 10 times as much This meant the 'normal' range would be from 0 to 120 dB, a much more sensible arrangement. This was adopted for a while, but it was soon found that compressing such a wide range down to 12 'units' was going too far the other way, it was therefore agreed to multiply the answer by 10 and call them decibels i.e. Logarithms so the threshold of hearing would be 0 and the threshold of pain would be 12 and call them Bels. So how do you describe these levels and the stages between in meaningful numbers?Įnter Alexander Graham Bell, the Scottish telephone engineer, who suggested simply converting these enormous numbers into metre, which is a range of a million million to 1. metre and the threshold of pain is around 1 watt/sq. Sound power level of about 0.000000000001 watt/sq. Why use decibels, why not stick to the real units that are directly measurable? Hopefully the reason will be clear when we understand how sensitive the human ear is. A decibel is the relationship or ratio between two sound levels, for example the measured sound pressure level and the minimum sound pressure level a person with good hearing can detect. The dB is not a unit in the sense that a metre or a kilogram are well-defined units of distance and weight.
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