Environmental noise involves measurement of the total noise (irrespective of its source) at a particular location, not only the noise emitted by a single source. The noise may thus be due to one or more sources and may also include reflections from walls, ceilings and other machines.
Noise at an employees work station is an example of environmental noise. The measurement is made where the person works, without regard to whether they are in the near or far field of their machine or whether other machines are operating nearby. These conditions may be considered in efforts to reduce the noise level, but not when measuring the employee's actual exposure. They may also mean that one ear of the operator receives more noise energy than the other.
Because environmental sounds come from various directions, the sound level meter should be omnidirectional. It must have a uniform response irrespective of where the various sound sources are located.
Other instances where environmental noise measurements are used are in the community (e.g. road traffic noise measurements), at factory boundaries, in offices and in theatres. (from Bruel & Kjar)