BASICS OF SOUND INSULATION AND ATTENUATION
One has certainly experienced sound attenuation within a building. For example, one may have been waiting in a public open space and discovered that the discussions held in various parts of this open space were heard, if not even understood, while others were not, either because they were too far away or because they were masked by the background noise.
Let us make a distinction: We will talk of attenuation when the sound source and the receiver are contained within the same volume. This attenuation will then simply be expressed as the difference between the sound pressure level generated at the source and the sound pressure level measured at the receiver.
Incidentally, a particular case of volume is the exterior space. One can also talk of attenuation there (cf. Sections 2.4.1 to 2.4.3).
Vegetation is supposed to bring extra attenuation when located along the path of propagation outdoors. When asked about its effect, most residents enjoying the view of vegetation hiding a transportation corridor or some machinery will eagerly remark that it reduces the noise annoyance (though similar tests performed under laboratory conditions may yield a somewhat mitigated result ).
However, physical studies  show that the additional noise reduction brought by a curtain of trees is 0 ± 3 dB in the frequency range of interest! The possible adverse effect comes from reflections on branches and leaves.
Absorptive Treatment in a Room
As sound propagation in a room is highly dependent on reflections on the inner envelope, absorptive treatment is an essential part of noise control measures. Experiments have shown that an absorptive treatment can easily reduce the noise levels in a workshop by at least 3 dB(A) due to the elimination of reflections propagating sound energy throughout the workshop.