VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

96 CHAPTER 5. SOUND INSULATIONblocked by the material of the screen or panel, but the sound also travelsfrom the source to the receiver in an indirect way because of diffraction (seeFigure 5.6). In order to calculate the total sound reduction one has to takethe diffraction into account. The diffraction is dependent on the wavelengthλ and thedifference d between the direct distance fromsource to receiver andthe indirect distance. The following equation gives an approximation of thesound reduction index for screens outdoors (no reflection from the ceiling) :( )λR = −10log(5.39)3λ+20dFigure 5.6 shows the graph for a frequency of 1000 Hz. Obviously onehas to be careful with these calculations. The noise level cannot be reducedmore than the surrounding noise level. The surrounding noise can also begenerated by a roadway, railway or an airport at a distance, traffic, industry,etc.In practice, in an industrial environment, one has to place the screen insuch a way that an angle of 60 degrees between the source of sound and thereceiver is obtained (see Figure 5.7).