Noise is much more complex than a simple IDW interpolation. Sound propagation depends on many factors and distance is just one of them. Air density, temperature, humidity, terrain, wind direction and ground attenuation should all play their part in even the simplest of models. In addition to these simple factors there are issues relating to tonality of the noise source and octave spreading. Add to this the fact that noise is effected by refraction and reflection, so frequently does not travel in a straight line.
Also remember that perception of noise is highly subjective and non-linear (the latter point being why the dB scale is logarithmic and not linear - that's the way the human ear works). So perception at night and in the day are two very different things for the same sound power level. So, even having derived a number on a map, it must still be interpreted to have any meaning.
That said, you still need a pragmatic approach to noise calculation. So, I would strongly advise you to read up on noise calculations. You can start by having a look at the Institute of Acoustics Best Practice Guide to the Application of ESTSU-R-97 For the Assessment And Rating Of Wind Turbine Noise. Of course, this is noise from a wind turbine perspective but it is a starting point. I would then go on to read some more about the controversy surrounding the ESTSU-R-97 guidance. The Wind turbine noise propagation model is based on the ISO-9613-2 standard, which describes a general method for attenuation of sound during outdoor propagation. As such this is essential reading and following the ISO standard will at least give you credibility and a rationale for your calculations.
The wind industry has specialist tools for doing even simple (simplistic?) calculations of noise propagation. Some of these could take the sting out of what you are trying to do. If you are doing the noise calculation for wind turbines, then I would recommend you use one of these tools (e.g. OpenWind - which has a free version) and then, which ever side of the debate you are one, your results will be acceptable (or at least undissmissably in line with best practice, which is a different thing). If you are not calculating for wind turbines but for some other point source, then you may be able to bend the wind farm software to your will.