THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

306 12. Walls, Enclosures, and BarriersFigure 12.15. Plan view of a hood with energy densities δ 1 and δ 2 just inside and outsidethe hood.that the sound level does not build up from reflections, thereby decreasing the wallvibration and the resultant radiation of the noise.Hoods constitute a special category of acoustical full enclosures designed specificallyfor the purpose of containing and absorbing excessive noise from a machine.A hood is designed to minimize leakages at its physical input and output, and accessmust be provided to allow periodic servicing. Usually hoods are sized so thatthe enclosed machine component does not occupy more than a third of the internalvolume. The effectiveness of a hood is denoted by the amount of noise reductionand insertion loss.In the equilibrium condition, the total power W 1 radiated by a source is absorbedby the interior walls of the hood. In Figure 12.15 δ 1 and δ 2 , respectively, denotethe energy densities of the interior and the exterior of the hood in the immediatelyvicinity of the enclosure wall. We assume the wall to be thin enough so thatvolumes V 1 and V 2 are nearly equal and denoted by V . As the time t required for anacoustic wave to travel distance L is given by L/c, the energy contained in volumeV 1 may be approximated by E 1 = W 1 L/c, with the energy density now givenbyδ 1 = E 1= W 1LV 1 Vc