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Natural Science Forum / Physics / Acoustics / August 2005Typical C-weighted sound levels
Hey all, Does anyone have or know of reference to a table of C-weighted values for typical noise sources (car @50mph/50ft= xx dBC, vacuum cleaner at 5ft= yy dBC, etc)? Thanks Herb  SignatureHerb Singleton usenet3@ross-specrtrum.com Sound & Vibration Measurements http://www.cross-spectrum.com > Does anyone have or know of reference to a table of C-weighted > values for typical noise sources (car @50mph/50ft= xx dBC, > vacuum cleaner at 5ft= yy dBC, etc)? The C-rated reference that is nearly the linear frequency response http://en.wikipedia.org/wiki/DB(C) is never shown, because the values look to high. What mostly is forgotten, when they tell the dBA value, is the distance of the measuring microphone to the sound source. Then that data is nonsense and useless. There is no standard of the distance for measuring noise. Cheers Jens > Hey all, > > Does anyone have or know of reference to a table of C-weighted values > for typical noise sources (car @50mph/50ft= xx dBC, vacuum cleaner at > 5ft= yy dBC, etc)? No, but... Typically a car accelerating is around 72 dBA (I think, it's been a long time) at the standard measurement distance. About half of that noise, in dBA, is from engine firing order, say 2200 rpm 3rd order, 110 Hz. The rest (69 dBA) is high frequency stuff that to a first estimate is unaffected by the weighting. Since you know the shape of the dBA curve it won't take long to estimate the overall dBC value. That comes to about 90 dBC in my head, which agrees roughly with the setting on the sound level meter - you'd use the 60-100 range on a digital one. Cheers Greg Locock >> Typically a car accelerating is around 72 dBA (I think, it's been a long >> time) at the standard measurement distance. > About half of that noise, in dBA, is from engine firing order, say 2200 rpm > 3rd order, 110 Hz. The rest (69 dBA) is high frequency stuff that to a > first estimate is unaffected by the weighting. Since you know the shape of > the dBA curve it won't take long to estimate the overall dBC value. > That comes to about 90 dBC in my head, The C-weighted level for many domestic sounds is often 10 to 15 dB greater than the A-weighted level. For more accurate results, you need actual broad band C-weighted measurements. It is NOT necessarily accurate to use calculated C-levels synthesized from laboratory data taken over a limited frequency range; e.g. published architectural test data are often limited to the 100 Hz-4,000 Hz range. Many sound survey systems include frequencies only down to 25 Hz. C-weighting slopes gradually below 30 Hz and above 10 kHz, so only meters with C-weighting pass bands can produce an acceptable "C-weighted" value. Then there are "linear" pass band meters that are "flat" from 20 kHz down to the lowest sound frequency that the microphone will purvey, typically about 5 Hz, though electronic circuits purveying down to 1 Hz are common (e.g. the LD2800-3200 series). Data from these meters must be corrected at their low and high end for the appropriate C-response, or their "C" pass band setting must be used. > setting on the sound level meter - you'd use the 60-100 range on a digital > one. The Radio Shack 33-2055 sound level meter can be used to make this measurement, but you must use a wind screen ("nerf ball") on its microphone when measuring such sound outdoors if there is any perceptible wind whatsoever, and follow 7.1 as well as other parts of ASTM Standard Guide E1014. Angelo Campanella |
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