Thanks Tim - you ExPlaned that so well I’ve downloaded the app 
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I’m about 2000 feet away from the South Circular, the railway and London City flight path. Following suggestions from above I’ve compared noise levels from different sources in my back garden.
Background noise. Birdsong. Seldom annoying. Green parakeets sometimes go on a bit.
South Circular. Nope. Nada.
Railway. Negligible. Can sometimes hear a train in right wind conditions.
My road. Very occasional bike or siren, hardly ever notice.
City airport. 55-75 db each time every time.
Pre COvid there were about 17 City planes per hour in each the 3-4 peak hours of their day.
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Ah - interesting - so you’re 2000 feed away, planes are 2000 feet up so you’re (quick root of x^2 + y^2) almost 3000 feet away from the planes or 50% further as I’m right underneath. Something something exponential decay, 6db at each distance doubling (according to the inter web - is that right?) means I should be getting about 68-78db - really we should co-ordinate and measure the same planes at the same time. I will need an appropriate activist t-shirt. Any recommendations?
