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Posted By Steve Fitzpatrick
Can somebody tell me why a fully calibrated noise meter shows a reading of approximately 40Dba when used in the middle of the night when there is no noise and you could literally hear a pin drop?
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Posted By Paul Maddock Steve, the decibel scale is a ratio (on a logarithmic scale) that has been set for (human) threshold of hearing to the threshold of pain which approximates to 0 - 140dB. You not only need to consider the actual sound pressure level, but also the frequency of the sound, some of which you may not be able to hear. Even in the dead of night in a rural area I have seldom measured below 25dB, and in most instances the "noise floor" of a sound level meter is in the region of 15-20dB (where the noise from the electronics of the sound level meter interferes with the measurement). If you want to know more about some of the basics of the decibel and environmental noise, I suggst you take a look at a booklet published by Bruel & Kjaer which can be found at: http://www.nvms.net/envbooklet.htmRegards, Paul.
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