Posted By Dave Daniel
There are some techniques for theoretical calculation.. if you want to get the old Log tables out!

At a practical level, 2 machines of equal noise next to each other will put the noise between them up by 3dB over what one produces, but this effect reduces with distance. A rule of thumb we used in Rover was to base estimates on a 5dB factor in large machine shops - i.e buy machines at 80dB(A) to give an 85dB(A) background.

Posted By David Buchanan
Alan
At the risk of fobbing you off, I would have to say that the type of calculation you are looking to perform is less than straightforward and as Dave pointed out would be theoretical.
The noise data supplied by the manufacturer is the Sound Power Level (SWL or Lw, being the sound energy emitted by the source), whereas the figure you are looking to derive is the Sound Pressure Level (SPL or Lp, which basically is the sound level as experienced at the receiving point).
In a theoretical free field, where there are no acoustic reflections the Lp = Lw-20Logr-11, where r is the distance from the noise source. Over a reflecting plane this would change to Lp=Lw-20logr-8. Therefore in a free field you would normally expect the Lp to be less than the Lw.
However in practice, acoustic reflectors (any surface really - especially in a reverberant industrial environment) including walls, ceilings, or other obstacles can result in an amplification of the Lp due to indirect noise sources (all the reflections) combining with the direct noise source.
By the way, the simple addition of decibels by calculation is via:
Lp=10log(10^(SPL1/10)+10^(SPL2/10)+...+10^(SPLn/10)).
Back to the fobbing off bit though, I would probably recommend that you source an acoustics consultant for this job. There is computer modelling software available that would generate a predictive noise map of the environment based on the Lw's of the equipment, the dimensions of the environment and structures and the size, shape and relative positioning of the obstacles and surfaces. This is the same kind of software used to model concert halls and auditoria or to produce environmental noise maps.
This software would allow you to also predict noise levels that would be experienced at the facades of residential properties. This information would, in turn, determine the measures you would need to take with regard to the acoustic treatment of the building i.e. insulation, damping, isolation / enclosure of equipment etc.
Hope this helps, but if you need further info can I refer you to Tim South at Leeds Metropolitan University who is the senior lecturer in Acoustics there.
Regards

Posted By David Buchanan
Absolutely.

Since your time he's also been given a shiny new acoustics lab to play with. Has also published a book on workplace noise too. I think he's still selling signed copies in plain brown wrapping!