How to convert noise (Lwa db) for a particular distance? If default noise level is given, how can I calculate the noise level at a particular distance? 
For example, if a generator says it's noise level is Lwa $99 ~\mathrm{db}$; how much noise level in  $~\mathrm{db}$ will be at $60 ~\mathrm{m}$ distance? 
 A: Db, alias decibel, is a logarithmic unit and can it can measure power or amplitude. This time it measures probably amplitude.
The logarithmicity of the unit means in this sense, that if you give 10 to the dB, you get 3.162 times bigger amplitude. If you give 20 to it, you get 10 times higher amplitude.
Multiplying the dB value with anything doesn't have any sense.
Now you say the generator gives noise level 99dB and you want to know, what will be the noise 60m far away from it.
The problem is that you don't know, how far away gives the generator this 99dB. It is probably some standardized measurement what the writers of the technical documentation suspected well known, while it is not.
But, for example, if this 99dB would be 1m away from the generator, then 60m away you will have $\frac{1}{3600}$ smaller output (only if we are below free sky!) It is because the sound power is distributed on a 60 times larger sphere, which has a 3600 times bigger surface area.
In this case, you will get $log_{10}3600*2$, also around 7.11 dB loss. So, in this case, the result would be around 92 dB.
