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

12.14 Enclosures 305to the extent that the field can be considered a superposition of randomly reflectedcomponents, i.e., it is essentially a reverberant field. Equation (12.28) can be usedto eliminate W 2 in favor of W 1 in Equation (12.40) which then becomesp3 2 = ρ 0 c 4W 1 S wτR 1 R 2and because τ = W 2 /W 1 , L p3 = 10 log (p 3 /20 μPa), we will obtain( )SwL p3 = L p1 + 10 log − TL (12.41)R 2where ρ o c = 407 rayls, and L p3 denotes the sound pressure level at a distancesufficiently far from the wall that the direct field is negligible in comparison withthe reverberant field.Example Problem 8Find the sound pressure level of room 2 at a large distance from the wall in theExample Problem 7.SolutionApplying Equation (12.41) yields12.14 EnclosuresL p3 = L p1 + 10 log= 110 + 10 log(Sw)− TLR 2( ) 20 × 8− 32 = 68.7 dB1350Enclosures may be categorized as being either full enclosures or partial enclosures.These structures of varying sizes may enclose people or noise generatingmachinery. It is advisable to never enclose more volume than necessary. For example,an entire machine should not be enclosed if only one of its components(such as a gear box, motor, etc.) constitutes the principal noise source. In somecases, a partial enclosure may suffice to shield a worker from excessive exposureto noise. The walls of an enclosure should be constructed of materials that willprovide isolation, absorption, and damping—all necessary for effective noise reduction.Moreover, any presence of cracks or leaks can radically reduce the noisereduction of an enclosure, so all mechanical, electrical, utility, and piping outletsmust be thoroughly sealed. Access panels should fit tightly, and viewing windowsshould be constructed of double panes and be impervious to acoustical leakages.The interior of the enclosures should be lined with highly absorbing material so