| Topic: | Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:3rd runway noise in Chiswick | |
| Posted by: | Tom Pike | |
| Date/Time: | 09/02/08 17:41:00 |
| Jeff, taking you second question first, if you were to take the current Heathrow traffic and spread it over three rather than two runways, it would have a larger 57 dB Leq contour area than at present – a third runway with the current aircraft leads to Heathrow contraction as you’d need to scale back the number of flights to avoid breaching the contour-area cap. BAA have to assume future aircraft are less noisy to have any justification for building a third runway. If they build a third runway and the aircraft noise levels have not decreased by as much as they hoped for, BAA will be very tempted to concentrate departures on the central runway to make most efficient use of the extra capacity – Grove Park would fall silent! "If it's only a little maths..." It's this much, taking it a step at a time: If the total acoustic power from planes passing over a location for a time T is P, then the dB Leq at that point will be dBLeq = 10 log (P/T) = 10 log P – 10 log T Adding in an additional plane of acoustic power Q, the new Leq will be dBLeqNew = 10 log (P+Q) - 10 log T The increase in the Leq will be dBLeqInc = dBLeqNew – dBLeq = 10 log(P + Q) – log P = 10 log (1+Q/P) As the power from the one plane is much less than the power from all of the rest, Q/P will be much less than 1. We can therefore make the approximation that for x much less than 1, log (1+x) = x, or dBLeqInc = 10 Q/P Comparing the increase in the dBLeq from two flightpaths of power P1 and P2: dBLeqInc1/ dBLeqInc2 = P2/P1 If the power, p, is expressed in dB, p = 10 log P or P = 10^(p/10) So dBLeqInc1/ dBLeqInc2 = 10^[(p2 – p1)/10] Hence if the two flightpaths have a 4 dB difference in noise, an additional plane will make 10^(4/10) or 2.5 times the contribution to the Leq of the quieter flightpath. Minimising Leq values will tend to concentrate additional noise on already noisy areas, meaning less increase in noise in quieter areas such as Chiswick at the expense of noisier areas such as Cranford. |