| Topic: | 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: | 08/02/08 19:53:00 |
| Here's a go at the explanation: First sum the total acoustic power that is measured at a single location - this is just the addition of the noise power heard from all the planes, with nearby planes contributing much more power. The Leq is proportional to the logarithm of the sum of noise power. A 16 hour Leq will sum noise of all the planes from 7 a.m to 11 p.m. If the power from one additional plane is added on, the new Leq depends on the logarithmic sum, which means the increase in the Leq is larger if the original Leq was smaller. For example: log(3) = 0.48 log(3 +1) =0.60 Increase is 0.12 log(2) = 0.3 log(2 +1) = 0.48 Increase is 0.18 So logarithmically adding 1 to 3 increases the sum by 50% more (0.18 compared to 0.12) compared to logarithmically adding 1 to 2. Rather alarmingly, the recent ANASE review on the effects of noise was unable to properly apply the concept of a logarithmic sum! As far as dB is concerned, the effect is simply quantified (though it takes a little maths to derive the general formula): if two flight paths differ by 4 dB, an additional plane will add 10 to the power of 4/10, or 2.5 times the amount to the Leq of the quieter flight path compared to the noisier one. In general to minimise the logarithmic sum, and hence the Leq, you want to concentrate noisy planes on noisy runways. You can see this effect in the noise modelling done by the CAA. The noisiest region for three parallel runways will be the middle runway - it gets noise from planes on either side. So to minimise the logarithmic noise sum it makes sense to put the loudest operations, those are takeoffs, on the middle runway. CAA have had to cut movements below capacity in 2020 due to breaching the 57 dB Leq noise contour, but putting departures on the middle runway (the current north runway) allows for 670,000 movements a year, while putting departures on the southern runway allows for only 615,000. Alternating mode, as it spreads out rather than concentrates the sound reduces the movements down to 605,000. If the three runways were given the same mix of planes, the restriction on Leq contour area would actually mean Heathrow would have to contract! |