Figure 20. Sketch showing partitioned and non-partitioned airspace behind perforated metal facing.
It also makes a very important difference whether the airspace behind the sheet is continuous or divided into small cells by means of partitions:
When the airspace is continuous, the behavior of the ab- sorber changes greatly at different angles of incidence of the sound. As the sound direction changes from perpendicular to the surface of the absorber (angle of incidence = 00) to grazing incidence (90), the resonance frequency changes drastically, rising away from the intended frequency to as much as three octaves higher. In addition, the bandwidth of frequencies within which the high values of sound absorption occur gets smaller and smaller as the angle of incidence tends toward grazing.
By contrast, with the partitioned backstructure, not only does the resonance frequency remain the same as the angle of incidence increases, but the bandwidth for high sound absorption actually becomes broader toward grazing incidence.
Finally, there is the effect of the density of the fibrous material used to fill the airspace.
If it is too loose, the sound passes right through the material without being absorbed. But if it is too dense, the sound is reflected and cannot penetrate the material to be absorbed.
More detailed guidance concerning the trade-offs between perforation patterns and depth of airspaces, as well as on the choice of sound absorptive cavity filling, will be presented in PART TWO, below.
