Book - School of Science and Technology

470 Ductwork designArrows show directionof soundSourceof soundRoomARoomBRoomCRoomARoomBRoomC(a)(b)Figure 16.31 Cross-talk attenuationthe better privacy attainment. The background may be from external sources, such astraffic and general activity, or internal sources from the ducted systems.Noise break-out from ducts into acoustically sensitive spaces may be a problem.Rectangular and oval ducts, particularly of large aspect ratio exhibit poor break outcharacteristics, whereas spirally wound circular ductwork tends to be more rigid and maybe used to reduce break-out. It should be noted however that that sound which does notbreak out of the duct continues to be transmitted down its length and so breakout must bebalanced against the level of noise at the grille or diffuser. A solution may be to insulatethe duct acoustically with a dense material.High-velocity systemsThe high pressures generated within high velocity systems necessitate special attention toacoustical design. The usual procedure is to provide a long silencer in the fan-discharge toabsorb fan noise, especially at the lower frequencies. Noise generated in ducts is dealt withby lining of selected lengths or including secondary silencers. With the induction systemthe jets have an important attenuation effect, but some acoustical treatment of the cabinetis often provided. In the case of the fan coil unit, dual-duct and variable volume systemsthe terminal control device (box) is always treated acoustically but in some cases, wherethe space served has a low NR requirement, further attenuation downstream of thecontrol device may be needed.Commissioning and measurementFollowing completion of the ductwork installation and that of the associated plant,the system has to be commissioned and the performance tested against the designrequirements. Commissioning involves setting the plant items to work and regulatingthe air flow rates within acceptable tolerances. The procedures to be followed in commissioningare given in a CIBSE Code.* A companion to this Code is a BSRIA ApplicationGuide, y which gives practical advice on measurement techniques and tolerances.The following describes some of the air flow measuring instruments in use today.* Air Distribution. CIBSE commissioning Code `A'. 1996.y Manual for Regulating Air Conditioning Installations. BSRIA Application Guide AG 03/89. RegulatingVariable Flow Rate Systems. BSRIA Technical Note AG 01/91.