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Old 17th March 2004, 01:38 AM   #1
Rancid is offline Rancid  Australia
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Join Date: Mar 2004
Location: Mount Crosby, Australia
Default A Scale and C Scale conversion

Hi folks. I have found a thread regarding conversion between these two scales here, but it went a little over my head.

Just for background purposes, I am a legal advocate in Australia and I have a client that has had noise readings taken on his busines in dB(C). The regulatory authoity mistakenly thought that the limitation imposed on the business resstricted noise in this weighting. The restriction was actually imposed in dB(A).

The readings were taken with a medium priced hand held meter that gave a single figure for the reading - it did not provide readings in each octave band. The authority has since commenced action against my client. It was argued that they could not do this as the readings were in the wrong weighting. They responded that they had been converted using a formula, which basically came down to multiplying the A weighted reading by 12 then dividing by 11. The noise readings were measuring music at three metres from the speaker in a nightclub and they read 115 dB(C).

My understanding is that conversion between scales in such a simplistic manner is not possible - but I am no expert. If anyone here has an intimate understanding of noise, and can advise me I would be very appreciative. It should be noted that I have a basic understanding of noise measurement, and have used a Rion in past employment, but any training I received in that was informal so I am unlikely to understand too much technical jargon.

Cheers
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Old 19th March 2004, 01:01 AM   #2
Rancid is offline Rancid  Australia
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Join Date: Mar 2004
Location: Mount Crosby, Australia
OK. After spending hours trying to understand the technical papers on the net, I finally got what I was after. There is no way to look at an overall noise level in dBC and convert it to dBA. There may be some ways to take a guess at it dependent upon the type of noise, but these would not survive technical examination in a court. Yoi need to "deweight" the noise in each octave band in the C reading, then "reweight" it in the A scale. The weightings are easy enough to find on the net. To find the oer all figure you use the following formula

10log(10^L1/10 + 10 ^L2/10 +.....10^Ln/10)

The L1, L2...Ln are the readings in the various octaves. It is a bit hard to type the formula above, but the above means you take the dB reading, divide it by ten to get the reading in Bels, hen raise 10 to the power of it. Do this for each octave reading. Add them. Find the log of it. multiply it by ten and your done.

Pretty simple huh? LOL
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