HOW IS NOISE MEASURED?
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When people talk about sound levels, they usually talk about decibels (dB) as the unit of measurement, but this is only part of the story. Volume, or more correctly sound pressure level (SPL), is measured on a logarithmic scale, so 120dB is not twice as loud as 60dB β it is a 1,000 times louder. Every 6dB doubles the sound level, so 78dB is twice as loud as 72dB, and four times louder than 66dB.
But two trucks are not twice as noisy as one. Doubling the sound power only increases SPL by 3dB, so it takes four 72dB trucks to be twice as noisy as one 72dB truck βor just as noisy as one 78dB truck.
However, human hearing is complex, and perceived loudness is not quite the same as SPL: typically, an increase of 10dB in SPL is perceived as a doubling in loudness, while a 3dB increase is barely noticeable.
And the quality of sound is just as important: nails on a blackboard, for instance, are unbearable even at low volumes. Engineers often use A-weighting, shown as dB(A) or dBA, which emphasises the audible frequencies between 500Hz and 6kHz β reflecting the human ear's response to noise. Shifting noise away from this range might increase sound levels at higher or lower frequencies.